Why did my airbag light come on after moving the seat?

Because the side airbag's deployment circuit is wired through a connector under the seat, on a loom that has to flex every time the seat slides. Over years and thousands of adjustments, individual conductors fatigue and break inside insulation that still looks perfect, and the fault often appears at one particular seat position. A connector knocked loose by a foot or by something rolling under the seat does the same thing. Sliding the seat slowly with the ignition on and watching the lamp flicker will often confirm it.

Can seat covers cause an airbag fault?

Yes, and there is a more serious issue behind the fault code. Seats with side airbags have a specially stitched seam in the outer bolster designed to split as the bag deploys. A cover that is not approved for airbag seats can restrain that deployment or send the bag in the wrong direction, and fitting one can also snag or disturb the wiring, which is what sets the code. If you want covers on a car with side airbags, use ones explicitly certified as airbag compatible.

Will B0020 fail an MOT?

Yes. An airbag warning lamp indicating a malfunction is a major defect with 84,147 recorded failures in the DVSA data behind this site, and the tester does not distinguish between which airbag circuit caused it. The lamp must illuminate when the ignition is switched on and then go out; staying on is a failure, and never coming on at all is also a failure because it suggests the warning has been disabled.

Do side airbags matter as much as front ones?

Arguably more in the crashes they are designed for. A frontal impact gives the car a substantial crumple zone to absorb energy; a side impact has only the door and a few inches between the striking vehicle and the occupant. Side and curtain airbags are what fill that gap, protecting the chest and head. That is why an SRS fault involving a side circuit is worth treating as seriously as one involving the steering wheel, even though the lamp looks identical.

Body code · standard

B0020Left side airbag circuit fault

Standard definition: Left Side Deployment Control

Last updated 27 July 2026

Quick answer

The airbag control unit cannot verify the deployment circuit for the left side airbag. These live in the seat or the roof rail, and the connector under a seat that slides back and forth all day is the usual suspect.

How the car works this out

The measurement is the same as for the frontal airbags: a small current through the deployment loop, a resistance reading, and a fault if it is open, shorted or out of range. What makes the side circuits distinctive is where they live. A side airbag is normally built into the outer bolster of the seat, and a curtain airbag runs along the roof rail above the doors. The seat-mounted ones are wired through a connector beneath the seat, on a loom that flexes every time the seat is moved, which is a mechanically harder life than any other airbag circuit on the car.

What you might notice

Airbag or SRS warning lamp on
Lamp appearing shortly after the seat has been moved, a seat cover fitted, or the car cleaned
Lamp coming and going as the seat is adjusted
Heated seat or seat position memory not working, where those share the same connector
No other symptom

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.

1The connector under the seat unplugged, partly latched or disturbedCommon
How to check: Slide the seat fully forward and look underneath for a yellow connector. Airbag plugs are yellow by convention, and one hanging loose or sitting at an angle is the answer.
2A fatigued or broken wire in the seat loom, from thousands of fore-and-aft movementsCommon
How to check: Move the seat slowly through its full travel with the ignition on and see whether the lamp flickers. A fault that tracks seat position is a broken conductor inside intact insulation.
3Water or damp under the seat corroding the connector, from a leaking door seal, a blocked sunroof drain or a spillagePossible
How to check: Press the carpet under the seat firmly with your hand and see whether it is damp. Water finds its way under seats and stays there.
4A non-approved seat cover fitted over a seat with a side airbag, damaging or fouling the circuitPossible
5A failed side airbag module or a curtain airbag connector at the A-pillar disturbed during trim workPossible
6Accident damage, or an airbag that deployed previously and was not correctly replacedLess common

What to do about it

1Look under the seat first, with the ignition off. A yellow connector that has been kicked loose or caught by a foot is the commonest cause and costs nothing to put right.
2Do not test the circuit with a multimeter. Side airbags fire sideways out of a seat bolster at very close range, and setting one off while working under the seat would be extremely dangerous.
3Disconnect the battery and observe the manufacturer's waiting period before doing anything under the seat, so the system's reserve power discharges.
4Check for damp carpet. Water under a seat corrodes connectors and will undo any repair if the leak is not found.
5Remove any seat cover that is not approved for a seat with a side airbag. Beyond causing this code, a cover over an airbag bolster can stop the bag deploying properly, which is a safety problem in its own right.

Can you keep driving?

Briefly, gently

Nothing about the way the car drives changes, but with the lamp on the restraint system is out of service and on most cars that includes every airbag rather than just the one at fault. Side and curtain airbags do a great deal of work in side impacts, which are among the least survivable crashes for the occupant nearest the strike, so this is not one to leave for months because it feels like nothing is wrong.

Will it fail an MOT?

An airbag warning lamp indicating a malfunction is a major defect, recorded 84,147 times in the DVSA data behind this site, and one faulty side circuit lights the same lamp as a faulty driver's airbag. There is a second thing a tester will look at on this side of the car: seat belts and their anchorages are inspected directly for condition, operation and security, and seats must be secure. A car that has had trim or seat work done badly enough to disturb an airbag connector is worth checking over for those too.

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

Why did my airbag light come on after moving the seat?: Because the side airbag's deployment circuit is wired through a connector under the seat, on a loom that has to flex every time the seat slides. Over years and thousands of adjustments, individual conductors fatigue and break inside insulation that still looks perfect, and the fault often appears at one particular seat position. A connector knocked loose by a foot or by something rolling under the seat does the same thing. Sliding the seat slowly with the ignition on and watching the lamp flicker will often confirm it.
Can seat covers cause an airbag fault?: Yes, and there is a more serious issue behind the fault code. Seats with side airbags have a specially stitched seam in the outer bolster designed to split as the bag deploys. A cover that is not approved for airbag seats can restrain that deployment or send the bag in the wrong direction, and fitting one can also snag or disturb the wiring, which is what sets the code. If you want covers on a car with side airbags, use ones explicitly certified as airbag compatible.
Will B0020 fail an MOT?: Yes. An airbag warning lamp indicating a malfunction is a major defect with 84,147 recorded failures in the DVSA data behind this site, and the tester does not distinguish between which airbag circuit caused it. The lamp must illuminate when the ignition is switched on and then go out; staying on is a failure, and never coming on at all is also a failure because it suggests the warning has been disabled.
Do side airbags matter as much as front ones?: Arguably more in the crashes they are designed for. A frontal impact gives the car a substantial crumple zone to absorb energy; a side impact has only the door and a few inches between the striking vehicle and the occupant. Side and curtain airbags are what fill that gap, protecting the chest and head. That is why an SRS fault involving a side circuit is worth treating as seriously as one involving the steering wheel, even though the lamp looks identical.

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