U0172: Lost Communication With Restraints System Sensor C
What to do
The airbag warning light means the airbags may not deploy correctly — and where a side sensor is involved, the side airbags and curtains have the least time to react of any on the car.
These sensors often live inside the pillar between the doors, behind the trim, at about the height of the seat belt mounting.
Two things are worth mentioning, and the first is the commonest cause by a distance.
Any water inside the car — a damp carpet, a wet sill, condensation, a leaking windscreen seal or a sunroof that drips. Water runs down inside that pillar and collects exactly where the sensor's plug is. If you have a sunroof, mention that too: a blocked sunroof drain empties down inside the front pillar and along the sill, which is precisely the wrong place.
And any trim that has been off: seat belt work, sill trims, carpets lifted or dried, speakers or amplifiers installed. A connector left not fully clicked home after a job that had nothing to do with airbags is very common — and it looks perfectly connected.
As always: do not have the light cleared, and refuse any offer to fit a resistor in place of a sensor.
What this code means
A restraints system sensor has stopped being heard from. See U0170 for the one-versus-several rule and U0473 for what a side sensor's location implies.
Where the sensor is a side one — B-pillar, sill or door — the causes reorder, and the top of the list is water.
A B-pillar sensor sits inside the pillar behind the trim, at the height of the seat belt anchor and just above the sill. Water running down inside that pillar — from a door seal, a roof drain, a sunroof tube or a windscreen surround — collects exactly there, and a connector sitting in it goes open circuit long before anything looks wrong from the cabin.
So the free check is the carpets and the sill, and a damp one is worth more than any measurement. U0169's sunroof drains are a specific and commonly missed source, because a blocked front drain empties down the inside of the A-pillar and along the sill.
The second cause is trim. This area comes apart for seat belt work, sill trim replacement, carpet drying and audio installation — jobs with nothing to do with restraints that put hands on a restraints connector. U0152's shorting bar applies: a connector pushed back on but not latched reads as a fault while looking perfectly seated.
And the safety framing from U0453 is worth keeping in view. The side of the vehicle has no crumple zone to measure, so the decision has to be made from a smaller and faster signal. A side sensor that cannot be heard from is the input for the airbags with the least time to work with.
Likely causes
Start here
- Water ingress into connector · moderate
- Corroded or loose connector · easy
- Damaged wiring under the front seats · moderate
- Impact sensor disturbed during body repair · moderate
Less common
- Damaged wiring harness · moderate
- Faulty restraints control module · specialist
Symptoms
Components involved
Information, not instruction
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