“Service Park Assist”, “Park Assist Blocked See Owner’s Manual” and “Park Assist Off” are three different statements from the same module, and on the 2010 to 2013 GM trucks and SUVs covered by PI0608A the module writes down why it said each one. GM’s Preliminary Information PI0608A for the 2010 to 2013 Escalade, Avalanche, Silverado, Suburban, Tahoe, Sierra and Yukon with rear park assist (RPO UD7) says the disable reason goes into a Park Assist Disable History that holds the seven previous reasons the system shut itself off[1]. That list, read with a scan tool, is where GM’s own diagnosis starts: it says why the system shut off, and the technician matches the entry to the symptom before any part is ordered.
On those trucks the rear system uses four ultrasonic sensors in the bumper, works only in Reverse below 5 mph (8 km/h), sees objects up to 8 ft (2.5 m) back and only if they stand at least 10 in (25.4 cm) off the ground and below tailgate level, and beeps continuously inside 12 in (30 cm)[1]. Much of what follows comes from that geometry: a trailer, a bike rack, a lowered tailgate or a bent bumper corner can sit inside the beams, and PI0608A lists each one as an attached-object condition that shuts the system off[1]. The rest comes from software, lost communication and power faults that have nothing to do with the bumper. This page lists what each message means on the vehicles GM’s documents cover, which causes GM calls normal operation, which ones end in a new bezel, insulator ring or sensor, and the cases where the sensors were not the problem at all.
Quick answer
Read the exact wording on the Driver Information Center, clear the bumper of ice, mud and anything hanging off the back, then have the codes and, on trucks that keep one, the Park Assist Disable History read before any sensor is replaced.
| What you see | Where to look | What to do first |
|---|---|---|
| “Park Assist Blocked See Owner’s Manual” after a winter car wash or a muddy road | Sensor faces and the gap between each sensor and its bezel | Clean the bumper and let the ice melt; GM says the message may not clear until it has[1] |
| “Park Assist Blocked” with a trailer, hitch receiver, bike rack or lowered tailgate | Whatever is attached behind the bumper | Remove it and raise the tailgate; on the 2010 to 2013 trucks GM’s recheck drive is above 25 mph (40 km/h)[1] |
| “Park Assist Blocked” on a dry, clean, empty bumper | Bezels, silicone insulator rings, sensor paint, bumper alignment | Inspect the bumper and sensor retainers before normal diagnosis; a sensor that sees the bumper itself is logged as an attached object[1] |
| “Park Assist Off” after a fast reverse | Your own speed in Reverse | Nothing: above 5 mph in Reverse the system turns off and comes back below 5 mph[1] |
| “Service Park Assist” at start-up that clears on the next key cycle, 2016 full-size truck | DTC U0159 in the cluster or body control module | PIT5507E’s fix on the trucks it covers is a calibration for the Parking Assist Control Module; have the bulletin’s applicability and the U0159 record confirmed before anything is reprogrammed[2] |
| False alerts with nothing there, then “Park Assist Off”, 2019 to 2025 Silverado or Sierra, no codes stored | Bezel locking tabs, burrs in the bumper holes, the isolator rings, paint on the sensor body | GM’s 22-NA-068 inspection: a sensor grounding out or squeezed in its hole reports normal vibration as an object, and no DTC sets because the sensor itself works[7] |
| “Service Park Assist” together with Service StabiliTrak, dimming lamps or a stall | Battery cables, fuse block, BCM ground G218 | Have the battery cables and grounds checked to 18-NA-161 before anyone touches the park assist system[6] |
A guide, not a diagnosis: every row has more than one cause.
What each message means on the trucks in GM’s bulletin
PI0608A gives the three messages their meanings for the 2010 to 2013 trucks it covers, and the wording on the screen is the first piece of evidence[1].
Service Park Assist
GM’s instruction for this message is two steps: check for DTCs and review the Park Assist Disable History[1]. The message reports a fault in the park assist system; it does not name the failed part. On the 2016 full-size trucks in PIT5507E the same message appeared at random on start-up with U0159 stored in the instrument cluster or body control module, cleared on the next ignition cycle, and was fixed by a software calibration[2]. On the 2014 to 2020 trucks and SUVs in bulletin 18-NA-161 it sits in a list of complaints that runs from Service StabiliTrak to a stall, and that GM traces to battery cables, the battery fuse block, a poor BCM ground at G218 or a discharged battery[6]. On the trucks that bulletin covers, a Service Park Assist message that arrives with those other symptoms puts the power and ground checks early in the diagnosis.
Park Assist Off
Three things produce it on the PI0608A trucks: the driver pressed the disable button next to the radio, the truck went faster than 5 mph (8 km/h) in Reverse, or the module lost or received invalid GMLAN signals, which GM calls Inhibit[1]. The speed case fixes itself: once the truck drops below 5 mph the system turns itself back on and the message leaves the display[1]. GM’s bulletin describes the trap that follows: a driver who sees “Park Assist Off” after a fast reverse assumes the system is broken, presses the disable button, and has now switched off a system that was already back on. The history buffer shows that sequence as Reverse Over Speed followed by Manual Disable, and the fix is to stop and press the button once more[1]. The system defaults to on at every start[1].
On the 2014 to 2018 trucks in PIT5507E, “Park Assist Off” also follows a run of false front alerts: repeated false front alerts can end with the system disabled and the switch LED dark[2]. That version of the message is covered under false alerts below.
Park Assist Blocked See Owner’s Manual
This is the message with the longest list. GM’s conditions for the 2010 to 2013 trucks are: sensors not clean; frost or ice around and behind the sensors, which can form after a cold-weather wash and may not clear until it melts; a hitch receiver, trailer, tow bar, bike rack or an object hanging out of the tailgate; a lowered tailgate; damage to the rear of the truck or a misaligned sensor, such that the sensor detects the bumper itself; sensor disturbance from heavy pounding like a nearby jackhammer or a pressure change like a large truck’s air brakes; and Sensor Ring Time, where a sensor fails its own initialisation check[1]. The ring-time failures have their own sub-list: a silicone insulator ring that is missing, cut or twisted; a sensor sitting crooked because the harness is tight; a scratched or chipped sensor; and paint thicker than 6 mils on a repainted sensor[1].
Two more rules sit in the same document. While the blocking condition is present, the system is disabled; pressing the disable button with a trailer attached makes the Blocked message flash five times to say so[1]. And after the cause is removed, PI0608A has the truck driven above 25 mph (40 km/h) so the system can recheck the sensors[1]. A driver who cleans the bumper in the driveway and reverses straight out again may still see the message. If it stays after that drive, have the disable history and the codes recorded and continue the diagnosis from there.
Symptoms and what else produces them
| What you notice | Competing sources | Clues that shift suspicion |
|---|---|---|
| “Park Assist Blocked” on cold mornings only | Ice between sensor and bezel; a cracked bezel holding water; a rolled insulator ring | Clears once visible frost or ice has melted: that fits the icing case GM calls normal operation. Returns on dry warm days: inspect the bezel, the ring and the sensor[1] |
| “Park Assist Blocked” every drive, bumper clean | Hitch receiver or tow bar left on; license plate bent toward a sensor; bumper pushed in at one corner; sensor repainted | History buffer says “Attached Object” with nothing attached: look for the bumper itself inside a beam. “Ring Down Time too long” on one sensor: look at that sensor’s paint and ring[1] |
| False alerts in rain or snow, 2014 to 2016 full-size trucks | Old module calibration; sensor tip painted so the gap to the body is bridged; non-scalloped front rings on a 2016 to 2017 Silverado or Sierra 1500 | Ask which corner the display showed the phantom object in; GM uses that to narrow down which sensors to inspect[2] |
| “Service Park Assist” at start-up, gone after a restart | Software (2016 trucks, U0159); a power-supply or ground problem that drops several modules at once | On a 2016 truck covered by PIT5507E, a random start-up message with U0159 stored: check the installed calibration first. Arrives with Service StabiliTrak, a dim cluster or a stall: battery cables, G218 ground and the network[2][6] |
| “Service Park Assist” with U0159 in one or more modules, 2020 to 2021 Cadillac XT5 or XT6 | Lost communication that still needs a diagnosis: GM’s 2021 engineering document replaces the module only when B101D is also stored, and otherwise asks for sleep-mode voltage readings at fuse F25DA while it looked for the cause | GM’s instruction: do not remove power from the park assist module during testing, note whether U0159 is current or history before clearing[5] |
| “Park Assist Sensor Blocked” on a new 2016 Volt | Factory tape left on the sensors during the initial learn | GM says not to replace sensors: a technician runs the GDS2 sensor learn with the bulletin’s clearance, temperature, drive and module-sleep conditions[4] |
Sensor fault, or something else?
Several items on this list are not faults of any part. GM’s history table marks each disable reason “Operating as Designed: Yes” or “No”, and the yes column is long: ice, mud, snow, a trailer, a tow bar, a bike rack, a lowered tailgate, a plate hanging past the fascia[1]. The corrective action for those is “explain normal operation to customer”. The part replacements come from the other column.
Bezel, insulator ring and paint
For the 2010 to 2013 trucks, GM replaces the bezel or retainer when it is damaged, replaces the silicone insulator ring when it is missing, cut or twisted, and replaces the sensor when it is scratched, the paint is chipped, or paint on the sensor exceeds 6 mils[1]. The sensor needs an even gap all round inside the bezel: GM’s own photo pair in the bulletin is “Sensor Contacting Bezel” against “Sensor Centered in Bezel”[1]. A sensor that has been pushed against the bezel by a tight harness, a jagged fascia hole or a bumper knock rings longer than it should, and the module reads that as a block.
The 2014 to 2018 trucks add a second failure of the same kind. The tip of the sensor is isolated from its body and has a small amount of free movement; paint that bridges that gap gives the tip a transmission path into the body, and the sensor reports objects that are not there[2]. The 2016 and 2017 Silverado 1500 and Sierra 1500 were the first full-size trucks with scalloped front sensor rings, and GM reports replacement front sensors arriving with the old non-scalloped ring; fitted to those bezels the old ring can cause false front alerts and can end in “Park Assist Off”, and PIT5507E names the scalloped ring, GM part number 84075328, as the correct part; confirm current fitment for the exact truck before ordering[2]. A ring rolled during installation can cause the same false alerts[2]. On the 2019 to 2025 Silverado and Sierra, GM traces the same false alerts to burrs in the bumper holes and bezel tabs not fully engaged, with no code stored because the sensor is working[7].
The module and its software
The start-up “Service Parking Assist” with U0159 on 2016 full-size trucks, and the false alerts in rain or snow on 2014 to 2016 trucks, both ended in a calibration update to the Parking Assist Control Module (K41/K86) through TIS2WEB[2]. Before that calibration existed, GM’s earlier version of the same bulletin told dealers to advise the customer to cycle the ignition[3]. On the 2020 to 2021 XT5 and XT6, GM’s 2021 engineering information document for a Service Park Assist lamp with U0159 gives one conditional instruction, replace the park assist module when B101D is also present, and otherwise asks for sleep-mode voltage readings at fuse F25DA and a call to the engineer[5]. That was an investigation, and a technician checks the current service information before acting on it. U0159 is set by other modules that lost communication with the park assist module; it does not name the failed part.
Power and ground
Bulletin 18-NA-161 lists “Service park assist message” among the complaints on 2014 to 2020 Escalade, Silverado, Sierra, Suburban, Tahoe and Yukon that it traces to high resistance or loose connections at the battery cables or fuse block, a poor BCM ground at G218, a shorted battery cable at the starter solenoid, or a discharged battery[6]. Its test is a loaded voltage drop while cranking, at the test points the bulletin shows: above 100 mV on the short positive cable, or above 200 mV on the negative cable, and the bulletin replaces that cable[6]. GM also asks for history and current codes from all modules to be recorded before anything is cleared[6].
By model: what GM’s bulletins say
2010 to 2013 Escalade, Avalanche, Silverado, Suburban, Tahoe, Sierra, Yukon (RPO UD7)
PI0608A is the reference for the three messages, the seven-entry Disable History, the normal-operation list and the bezel, ring and sensor replacements described above[1]. Two details are easy to miss. With a trailer attached and the truck above 30 km/h, which the bulletin prints as 26 mph (the metric figure converts to about 19 mph; the document’s own units disagree), the display shows “Park Assist Blocked”, and the system re-enables after the trailer and hitch are removed and the truck again passes that speed[1]. And even when the driver switched the system off at start-up, the module still runs its check at speed and still posts “Park Assist Blocked” if a trailer is on[1].
2014 to 2018 full-size trucks and SUVs (PIT5507E)
Three complaints: false alerts in rain or snow on 2014 to 2016 models, the random start-up “Service Parking Assist” with U0159 on 2016 models, and false front alerts ending in “Park Assist Off” on all listed models[2]. For a confirmed match, PIT5507E gives the first two a calibration update and the third an inspection of sensor paint, ring type and ring seating[2]. GM’s own caution in the same document: even with the new calibration and the new rings, the system can still give false alerts and disable when a brush guard, cattle guard, plow, push bar, added lights, hitch or bike rack is on the truck, when the sensors are under snow, mud, ice or heavy rain, when the bumper is damaged, when a replacement sensor carries too much paint, or when a large vehicle, a jackhammer or an aftermarket horn shakes the sensors[2].
2019 to 2025 Silverado and Sierra: false alerts and “Park Assist Off” with no codes (22-NA-068)
GM’s bulletin 22-NA-068, released in 2022 and revised in February 2025 to add the 2023 to 2025 model years, covers the 2019 to 2025 Silverado 1500, the 2020 to 2025 Silverado 2500HD and 3500HD, and the matching Sierra 1500, 2500HD and 3500HD[7]. The complaint is unwanted or false alerts with nothing in front of or behind the truck, which can end with the system disabling itself, “Park Assist Off” on the DIC and a dark LED in the switch[7]. The cause GM gives is mechanical: burrs at the sensor openings in the bumper or incorrectly installed isolator rings, so that a sensor grounds out against the bumper or gets squeezed and picks up normal vehicle vibration as an object[7]. Because the sensor itself is working normally, no DTC sets[7].
The diagnosis starts in the same Disable History. “Trailer or Other Object Attached” with nothing attached is GM’s pointer to a grounded or squeezed sensor, and the bulletin warns that “Manual Disable” next to it may only be the driver cycling the switch to get the system back[7]. The Parking Assist Display in the DIC shows which corner the false alert comes from, and on trucks with front and rear systems the “Attached Object Detected Front” and “Attached Object Detected Rear” parameters in GDS2 say which end is reporting, because gear position alone does not: in Reverse both ends are active[7]. The inspection, which GM says in most cases needs the bumper off, checks that all four bezel locking tabs are fully engaged, then looks for burrs on the back of the bumper hole, excess paint or clear coat around it, plastic flashing on the bezel and a hole slightly too small; the correction is to file or grind the burrs or flashing, enlarge the hole if needed, treat bare metal, and refit the same bezel and sensor, which GM says need not be replaced unless damaged[7]. For repainted sensors the bulletin adds one rule: only the tip is painted and clear-coated, because paint running into the gap between the tip and the body can ground the sensor out[7]. The bulletin’s labour allowance for inspecting and removing burrs from the bumper beam is 1.6 hours, a bulletin-only operation rather than a workshop quote[7].
2016 Volt
A new 2016 Volt showing “Park Assist Sensor Blocked” could still be wearing the tape the plant put over the sensors during the initial learn. GM’s 16-NA-067 says not to replace sensors: with the car at room temperature and nothing within 50 cm of the sensors, run “Parking Assist Sensor Learn” in GDS2, hold more than 25 mph (40 km/h) for one minute, then cycle the ignition and let the module go to sleep[4]. GM’s labour allowance for that operation is 0.4 hour[4].
2020 to 2021 Cadillac XT5 and XT6
GM’s engineering information for a Service Park Assist lamp with U0159 in one or more modules is an investigation, not a repair procedure: it asks the technician to record the exact message (Service Park Assist, Park Assist Off, APA unavailable), note whether U0159 is current or in history, identify which modules set it against the park assist module, and measure the voltage at the 5 A fuse F25DA after a ten-minute sleep timer[5]. The one firm instruction is that power must not be removed from the park assist module during testing[5].
What to check first
- The words on the screen, and what was behind the bumper. “Blocked” with a hitch, rack, trailer or open tailgate can be the system working as designed. With the truck parked, remove what can be removed and close the tailgate. On the 2010 to 2013 trucks PI0608A has the truck driven above 25 mph before the result is judged; other years follow their own service information[1].
- The sensor faces and the gap around each one. Wipe them with a soft cloth, look for a sensor sitting against its bezel, a split or twisted insulator ring, chipped paint or a repaint, and a bumper corner pushed toward the sensor[1][2]. Look, do not push: a sensor pressed into place or a harness tugged can move the very alignment being checked. Removing or reseating sensors, rings and retainers follows GM’s service procedure.
- Record the history with the codes. On trucks that keep a Park Assist Disable History, have it recorded along with current and history DTCs from all modules before anything is cleared[1][6]. Clearing can erase the evidence an intermittent cause left behind.
Can you keep driving?
If park assist is the only thing missing, and steering, brakes, engine and the rest of the electrical system behave normally, yes. On the 2010 to 2013 trucks GM’s bulletin says the system is disabled whenever one of these messages is displayed[1], so treat the parking aid as absent and check behind the vehicle yourself. That is the same caution GM gives for a lowered tailgate, where the system may not detect an object behind you even when it is working[1].
Treat it differently when the message is not alone. A Service Park Assist message that arrives with Service StabiliTrak, a blank cluster, dimming lamps, a stall or a no-start calls for the wider diagnosis in 18-NA-161, battery cables and grounds included, on the trucks that bulletin covers, and GM’s list for those trucks includes reduced power steering assist on some models and a steering wheel that jerks[6]. A truck that stalls or loses steering assist is not driven on; it gets recovered and diagnosed, and the cable and ground checks are part of that diagnosis, not a substitute for it.
FAQ
What does “Service Park Assist” mean?
On the GM trucks in PI0608A it means the park assist module has a fault to report, and GM’s first two steps are to check for DTCs and read the Park Assist Disable History[1]. On 2016 full-size trucks a random start-up version with U0159 was software[2]; with other service messages alongside it, GM’s 18-NA-161 points at battery cables and the G218 ground[6].
What does “Park Assist Blocked See Owner’s Manual” mean?
The module has decided a sensor cannot see properly: dirt, ice behind the bezel, an attached trailer, hitch, rack or lowered tailgate, a bumper or sensor out of position, outside vibration, or a sensor that failed its own ring-time check[1]. On the 2010 to 2013 trucks the message can stay until ice melts, and PI0608A has the truck driven above 25 mph before the system rechecks the sensors[1].
What does “Park Assist Off” mean?
On the 2010 to 2013 trucks in PI0608A: someone pressed the disable button, the truck exceeded 5 mph in Reverse, or the module lost its GMLAN signals[1]. For that bulletin’s speed case the message clears by itself below 5 mph. On 2014 to 2018 trucks it can also follow a run of false front alerts from a painted tip or the wrong sensor ring[2].
How do I reset “Service Park Assist”?
There is no reset procedure for this message in the bulletins above. On the 2016 trucks in the original PIT5507, an ignition cycle cleared the start-up message while GM worked on the calibration[3]; that was interim advice for one condition, not a reset. Record the codes and the history first: clearing them can remove useful evidence[6].
How do I fix the “Service Park Assist” system?
Start with the exact message, current and history codes, and the disable history where the truck keeps one. U0159 means another module lost communication with the park assist module, and power, ground, wiring, network, software and the module itself are all candidates[2][5][6]. A physical block ends in a cleaned bumper, a removed accessory, a straightened bumper corner or a new bezel, ring or sensor[1]. The bulletin for the exact model years says which checks come first.
Can a trailer hitch cause “Park Assist Blocked”?
Yes, and GM counts it as normal operation. PI0608A lists hitch receivers, tow bars, trailers and bike racks among its “Attached Object” conditions; with a trailer on, the 2010 to 2013 trucks post the Blocked message once the truck passes the bulletin’s trailer threshold, and the system comes back after the trailer and hitch are removed and the truck passes it again[1].
