U0173: Lost Communication With Restraints System Sensor D
What to do
The airbag warning light means the car's crash protection may not work correctly. Here the sensor is most likely at the back of the vehicle.
Rear crash protection is a real thing that few people know about. A rear-end shunt does not usually fire an airbag — it fires the seat belt pretensioners and, on many cars, the active head restraints, which throw the headrest forward to catch your head before your neck snaps back. Whiplash is the commonest serious injury in a rear-end crash, and that is what the car does about it.
One question matters more than any other here: has a tow bar been fitted? Towing electrics are installed in exactly the area these sensors and their wiring live, and an installation that cuts into the harness rather than using the proper vehicle-specific loom is a very common cause of this fault. If a tow bar has gone on in the last few months, start there.
Otherwise: mention any water in the boot, any reversing knock however slight, and any work involving the rear trim or boot lining. And if the fault comes and goes, open and close the tailgate slowly and watch — the wiring runs through the hinge and flexes every time.
Do not have the light cleared, and do not accept 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 U0474 for what rear crash sensing actually protects against — the belt pretensioners and the active head restraints that catch the head before the neck extends, in what is the commonest kind of collision there is.
One cause dominates the rear of the vehicle, and naming it is most of the value of this page: towing electrics.
A tow bar loom is fitted into the rear harness, very often by splicing rather than through the vehicle's dedicated connector, and it is fitted in exactly the area these sensors and their wiring occupy. A rear restraints fault appearing within weeks of a tow bar being fitted is that installation until proven otherwise.
The proper way is a vehicle-specific loom plugged into the manufacturer's provision and coded to the vehicle. The cheap way is cutting into the harness, and the cheap way is what produces this.
After that the rear's ordinary list applies: water in the boot floor and rear quarters, reversing damage however slight, and the connectors behind rear trim that come off for boot liners, load-space fittings and audio installations. The harness through a tailgate hinge flexes like a door boot and fails the same way, with the same free test — does the fault change as the tailgate is moved.
Likely causes
Start here
- Poor wiring splice in an aftermarket installation · moderate
- Corroded or loose connector · easy
- Water ingress into connector · 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
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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