U0174: Lost Communication With Restraints System Sensor E
What to do
The airbag warning light is on and the car's crash sensing is not working properly. It drives normally; it should not be left.
A car with this many crash sensors usually has a system most owners have never heard of: pedestrian protection. A row of sensors across the front bumper works out whether the car has struck a person rather than an object, and if it decides it has, it lifts the back of the bonnet by several centimetres to put a crushable gap between the panel and the engine underneath — or on some cars fires an airbag across the bottom of the windscreen.
With this fault, that will not happen. It is protection for somebody outside your car rather than inside it, which is exactly why it gets put off and should not be.
Two things worth knowing. Those bonnet lifters fire at speeds low enough that the impact felt minor, and once fired they are single-use devices that must be replaced — the bonnet does not simply go back down. If anyone ever suggests pushing it back into place, do not let them: it leaves the bonnet insecure at speed.
For this fault, the thing to mention is the front bumper. These sensors live in it and bumpers get replaced for parking knocks all the time. A bumper refitted with the sensor wiring left unplugged, trapped or routed wrongly is the commonest cause — so any front-end work, however cosmetic, is worth telling the garage about.
What this code means
A restraints system sensor has stopped being heard from. See U0170 for the one-versus-several rule and U0475 for pedestrian protection, which is what a vehicle with this many sensors is likely to be doing.
The short version, because most owners have never heard of it: a row of sensors along the front bumper beam distinguishes a person from a bollard, and when it decides the object struck was a pedestrian, the vehicle fires actuators that lift the rear of the bonnet by several centimetres — putting a crushable gap between the panel and the hard engine components underneath — or deploys an airbag across the base of the windscreen.
A silent sensor in that row means the system cannot make that decision, so it will not fire. That is a protection for somebody outside the vehicle rather than inside it, which is precisely why it gets deferred and should not be.
Two facts follow that are worth knowing before any repair is discussed.
- The bonnet lifters are single-use pyrotechnic devices, exactly like an airbag. A vehicle whose lifters have fired is not repaired by pushing the bonnet back down — that is occasionally attempted and leaves the bonnet unlatched at speed.
- They fire at speeds low enough that the impact felt minor, so a vehicle bought second-hand may have had them fired and replaced, or fired and not.
And the practical cause is the bumper, which is the most frequently replaced panel on any vehicle. Bumper covers, beams and foam are changed for parking damage constantly, and the sensor tube or its loom must be transferred, refitted correctly and not pinched. A bumper fitted with the sensor wiring trapped, mis-routed or left unplugged is the commonest reason for this family of codes at the front of a vehicle — and unlike a mis-mounted sensor, an unplugged one produces exactly this: silence.
Likely causes
Start here
- Impact sensor disturbed during body repair · moderate
- Corroded or loose connector · easy
- Physical damage from road debris · moderate
- Water ingress into connector · 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
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 U0174.