What is a side impact sensor and where is it?

It is a small crash sensor mounted out at the edge of the car, usually inside a front door or at the base of the B-pillar, and its job is to detect a side impact fast enough for the side and curtain airbags to inflate before the intruding vehicle reaches the occupant. A sensor in the centre of the car would be too slow, because a side impact gives only a few inches of crush and a few milliseconds of warning. These satellite sensors report continuously to the airbag control unit, which is how a failure is noticed.

Can water in the door cause an airbag fault?

Yes, and it is the leading cause of this particular code. Doors are designed to let water in and drain it out again through holes along the bottom edge, with a plastic vapour barrier keeping it out of the cabin. Block those drains with dirt and leaves, or tear the barrier during a speaker installation, and water sits inside the door where the sensor and its connector live. Corrosion follows. Clearing the drains and repairing the barrier matters as much as replacing the sensor, because otherwise the new one goes the same way.

What is the difference between B0020 and B0092?

They are the two halves of the same side of the system. B0092 concerns an input: the crash sensor that detects a side impact and tells the control unit it has happened. B0020 concerns an output: the circuit that actually fires the side airbag when the control unit decides to. A fault in either lights the same warning lamp, but the parts, their locations and the repairs are entirely different — one is a sensor in a door or pillar, the other is a deployment loop usually running under a seat.

Will B0092 fail an MOT?

Yes, through the lamp. An airbag warning lamp indicating a malfunction is a major defect with 84,147 recorded failures in the DVSA data behind this site. The tester has no way of checking an individual crash sensor and does not attempt to — the MOT is not a diagnostic scan. What is checked is that the SRS lamp comes on with the ignition as a bulb check and then goes out. Staying on fails, and never coming on at all also fails.

Body code · standard

B0092Front left side impact sensor fault

Standard definition: Side Impact Sensor Left Front Malfunction

Last updated 27 July 2026

Quick answer

The airbag control unit has lost contact with, or failed the self-test on, the crash sensor in the front left of the car. This is the sensor that detects a side impact and decides whether to fire the side airbags — it is an input to the system, not an airbag itself.

How the car works this out

Side impacts happen too quickly for a sensor in the middle of the car to react in time, so manufacturers fit satellite crash sensors out at the extremities, usually inside the doors or at the base of the B-pillar. Each one talks to the airbag control unit over a dedicated link and reports in continuously. The control unit checks that each satellite is present, responding and passing its own self-test. B0092 is set when the front left sensor stops answering or fails that test. Understanding which side of the system this sits on is useful: B0020 concerns the circuit that fires a side airbag, while B0092 concerns the sensor that decides whether to fire it.

What you might notice

Airbag or SRS warning lamp on
No other symptom whatsoever, which is the norm
Occasionally an intermittent lamp that appears in wet weather

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1Water ingress into the door corroding the sensor or its connector, usually from a blocked door drain or a failed plastic vapour barrierCommon
How to check: Check the drain holes along the bottom edge of the door are clear, and look for water sitting inside the door skin. This is much the most common cause and it will destroy a replacement sensor too if it is not fixed.
2A connector disturbed during door speaker, window regulator, central locking or door trim workCommon
How to check: Ask whether anything has been done to that door. These sensors sit behind the trim panel and are easy to knock.
3A failed sensorPossible
4A chafed or broken wire in the door loom, which flexes through the rubber boot between the door and the body every time the door opensPossible
How to check: Peel back the concertina boot between the door and the A-pillar and look for broken conductors inside. It is a classic fatigue point on any door circuit.
5A sensor that was triggered in a previous impact and not replaced, since these are single-use once they have firedPossible
How to check: Ask whether the car has any accident history and whether the control unit holds a crash record.
6A fault in the airbag control unit itselfLess common

What to do about it

1Check the door drains. Blocked drains are the leading cause of door electronics failing, and clearing them is free and takes minutes.
2Mention any recent door work — speakers, window motors, locks, trim, or accident repair. These sensors are behind the trim and are frequently disturbed.
3Disconnect the battery and observe the manufacturer's waiting period before removing any door trim on a car with side airbags.
4Ask about accident history if the fault will not clear. Satellite crash sensors are single-use, and one that fired in a previous impact must be replaced rather than reset.
5Fix the water leak before fitting a new sensor. A replacement in a wet door has a short future.

Can you keep driving?

Briefly, gently

The car drives normally and there is nothing to feel. The consequence is entirely about what happens in a crash: without a working side impact sensor the system cannot detect a side impact quickly enough to fire the side and curtain airbags, and with the SRS lamp on the whole restraint system is usually out of service anyway. Get it repaired rather than getting used to the lamp.

Will it fail an MOT?

An airbag warning lamp indicating a malfunction is a major defect, with 84,147 recorded failures in the DVSA data behind this site. It is worth being clear that the tester does not and cannot assess whether an individual crash sensor works — the MOT is a visual and functional inspection, not a diagnostic scan. What is checked is the lamp: it must illuminate with the ignition and then go out. A lamp that stays on fails, and a lamp that never illuminates also fails, because that points to the warning having been disabled rather than to a healthy system.

The light this turns on

Airbag warning lightThe supplementary restraint system has found a fault and has almost certainly disabled itself. The car drives exactly as normal, which is what makes this lamp so easy to ignore — but you should assume the airbags and seat belt pretensioners will not fire in a crash while it is lit.

Common questions

What is a side impact sensor and where is it?: It is a small crash sensor mounted out at the edge of the car, usually inside a front door or at the base of the B-pillar, and its job is to detect a side impact fast enough for the side and curtain airbags to inflate before the intruding vehicle reaches the occupant. A sensor in the centre of the car would be too slow, because a side impact gives only a few inches of crush and a few milliseconds of warning. These satellite sensors report continuously to the airbag control unit, which is how a failure is noticed.
Can water in the door cause an airbag fault?: Yes, and it is the leading cause of this particular code. Doors are designed to let water in and drain it out again through holes along the bottom edge, with a plastic vapour barrier keeping it out of the cabin. Block those drains with dirt and leaves, or tear the barrier during a speaker installation, and water sits inside the door where the sensor and its connector live. Corrosion follows. Clearing the drains and repairing the barrier matters as much as replacing the sensor, because otherwise the new one goes the same way.
What is the difference between B0020 and B0092?: They are the two halves of the same side of the system. B0092 concerns an input: the crash sensor that detects a side impact and tells the control unit it has happened. B0020 concerns an output: the circuit that actually fires the side airbag when the control unit decides to. A fault in either lights the same warning lamp, but the parts, their locations and the repairs are entirely different — one is a sensor in a door or pillar, the other is a deployment loop usually running under a seat.
Will B0092 fail an MOT?: Yes, through the lamp. An airbag warning lamp indicating a malfunction is a major defect with 84,147 recorded failures in the DVSA data behind this site. The tester has no way of checking an individual crash sensor and does not attempt to — the MOT is not a diagnostic scan. What is checked is that the SRS lamp comes on with the ignition as a bulb check and then goes out. Staying on fails, and never coming on at all also fails.

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