Symptom
Blind spot monitoring not working
The blind spot warning in the door mirror stops lighting, or the car reports the system unavailable. The sensors are radar units behind the rear bumper corners, which is why the commonest causes are not electrical at all: road film, snow, or a bumper that has been off and refitted slightly out of position. The danger is specific and worth stating plainly — a system that has failed silently is more dangerous than one that was never fitted, because a driver who has learned to rely on the mirror light will keep trusting it after it stops telling the truth.
Also described as: blind spot not working, blind spot warning off, mirror warning light not working, bsm fault, lane change assist not working, هشدار نقطه کور کار نمیکند, angulo muerto no funciona, detector punto ciego falla, aviso angulo muerto no disponible, مراقبة النقطة العمياء لا تعمل, تحذير المنطقة العمياء معطل, حساس الجنب لا يشتغل
What this usually means
The sensors are radar units behind the rear bumper corners, which is why the commonest causes are not electrical at all: road film, snow, or a bumper that has been off and refitted slightly out of position.
The danger is specific and worth stating plainly — a system that has failed silently is more dangerous than one that was never fitted, because a driver who has learned to rely on the mirror light will keep trusting it after it stops telling the truth.
That is the reason this entry exists, and it is worth saying to any driver who reports it: until it is fixed, check over your shoulder every time. A habit built over months does not switch off because a warning appeared once.
Cleaning is the first action. A radar behind a plastic bumper is blinded by a layer of road film thick enough to be invisible, by packed snow in the corner where spray collects, and by ice.
Then position. A bumper removed for a repair and refitted a few millimetres out changes what the radar sees down the side of the car — so a system unavailable since bodywork is a fitment or calibration question rather than a component one. Metallic paint repairs and stick-on parking sensors in the same area do it too.
A system that works on one side only points at that sensor or its wiring rather than at the module, and the wiring in a rear quarter is exposed to water.
And check what is being towed. Many vehicles disable blind spot monitoring when a trailer is detected, because the trailer is permanently in the zone — which is correct behaviour and not a fault.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Unavailable in bad weather, returns when clean and dry | Road film, snow or ice over the radar behind the bumper corner | Clean the rear bumper corners thoroughly and retest. The film is often invisible. | No repair. Most common cause by a distance. |
| Unavailable since bodywork or a bumper removal | Position or calibration — a few millimetres changes what the radar sees | Establish what was done. Metallic paint repairs and added sensors in that area matter too. | Refit correctly and recalibrate to the manufacturer's procedure. |
| Works on one side only | That sensor or its wiring rather than the module | Read codes for the named side. Rear quarter wiring is exposed to water. | Repair the wiring or renew the sensor. |
| Disabled while towing | Not a fault — the trailer is permanently in the monitored zone | Unplug the trailer and retest. Correct behaviour rather than a failure. | No repair. |
| Mirror lamp never lights but no message appears | The silent failure — the most dangerous version | Test deliberately on a quiet road with a known vehicle alongside. Do not assume it works because nothing complains. | Diagnose properly. A habit built on a working system does not adjust itself. |
| Unavailable with other assistance functions | Supply, network or a shared fault rather than the blind spot sensors | Test the battery and charging first, then read codes across modules. | Repair the supply or the shared fault, then re-scan. |
-
Unavailable in bad weather, returns when clean and dry
- Likely cause
- Road film, snow or ice over the radar behind the bumper corner
- How to confirm it
- Clean the rear bumper corners thoroughly and retest. The film is often invisible.
- What fixes it
- No repair. Most common cause by a distance.
-
Unavailable since bodywork or a bumper removal
- Likely cause
- Position or calibration — a few millimetres changes what the radar sees
- How to confirm it
- Establish what was done. Metallic paint repairs and added sensors in that area matter too.
- What fixes it
- Refit correctly and recalibrate to the manufacturer's procedure.
-
Works on one side only
- Likely cause
- That sensor or its wiring rather than the module
- How to confirm it
- Read codes for the named side. Rear quarter wiring is exposed to water.
- What fixes it
- Repair the wiring or renew the sensor.
-
Disabled while towing
- Likely cause
- Not a fault — the trailer is permanently in the monitored zone
- How to confirm it
- Unplug the trailer and retest. Correct behaviour rather than a failure.
- What fixes it
- No repair.
-
Mirror lamp never lights but no message appears
- Likely cause
- The silent failure — the most dangerous version
- How to confirm it
- Test deliberately on a quiet road with a known vehicle alongside. Do not assume it works because nothing complains.
- What fixes it
- Diagnose properly. A habit built on a working system does not adjust itself.
-
Unavailable with other assistance functions
- Likely cause
- Supply, network or a shared fault rather than the blind spot sensors
- How to confirm it
- Test the battery and charging first, then read codes across modules.
- What fixes it
- Repair the supply or the shared fault, then re-scan.
Codes that produce this symptom (4)
Listed with the codes that most typically present this way first — the likeliest place to start.
- U0232 Lost Communication With Side Obstacle Detection Control Module A Typical of this code Moderate
- U0233 Lost Communication With Side Obstacle Detection Control Module B Typical of this code Moderate
- U0533 Invalid Data Received From Side Obstacle Detection Control Module A Typical of this code Moderate
- U0534 Invalid Data Received From Side Obstacle Detection Control Module B Typical of this code Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Blind spot monitoring not working.