U0171: Lost Communication With Restraints System Sensor B
What to do
The airbag warning light means the airbags may not deploy correctly. Safe to drive to a garage; it should not be left.
The front crash sensors work as a pair, one on each side of the car's nose. They are not spares for each other — the difference between what the two of them feel is how the car works out whether it has been hit head-on, at an angle, or on one corner, and that decides which airbags fire.
One question is worth asking, because it changes the size of the job: has the other one gone quiet too? Both at once is usually a single break in the wiring they share on the way forward from the airbag computer — one repair, not two. Only one means the problem is on that sensor's own branch, and the search is local.
Mention any front-end work, however minor and however long ago — a bumper off, a headlight, a radiator, a wing. That is where these live.
And if a connector is found corroded, ask for it to be replaced rather than cleaned up. The front of the car is the worst place on it for a connector, and an airbag circuit is not somewhere to accept a patched-up contact.
What this code means
A restraints system sensor has stopped being heard from. See U0170 for the one-versus-several rule and U0471 for what these sensors do.
Front crash sensors come in pairs, one on each side of the vehicle's nose, and the pairing is a measurement rather than redundancy — the difference between what the left and right sensors feel is how the module works out whether an impact is full-width, offset or a pole strike, which changes which airbags fire and how hard.
For a lost-communication code that produces a specific and useful question: has the other one gone quiet too?
Both silent is not two failures. They share the leg of the loop that runs forward from the restraints module, and often a common connector behind the bumper or at the front crossmember. A single break in that shared run takes both, and it is one repair.
One silent and one reporting is that sensor's own branch, and the search is local.
Either way the front of the vehicle is the worst environment on it for a connector — spray, salt, stone damage, and heat from the engine bay a few centimetres behind. The corpus's standing advice applies: check the connectors before the sensors, and where one is corroded, replace it rather than cleaning it. A restraints circuit is not somewhere to accept a repaired contact.
Likely causes
Start here
- Corroded or loose connector · easy
- Water ingress into connector · moderate
- Impact sensor disturbed during body repair · moderate
- Damaged wiring harness · moderate
Less common
- Physical damage from road debris · moderate
- Faulty restraints control module · specialist
Symptoms
Components involved
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.
Community experience
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- What you measured, not only what you replaced.
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