What is the difference between C0121 and C0265?

They concern two different power supplies inside the same unit. C0121 is the relay feeding the hydraulic solenoid valves, so it fails with the valves isolated and the anti-lock function disabled while the module itself carries on working and can still be talked to by a scan tool. C0265 is the relay powering the brake control module itself, so when it fails the module can go completely silent — no communication, no road speed being published, and lamps appearing across the dashboard. The intermittent behaviour is far more typical of C0265.

Why does my ABS light come on over bumps?

That pattern usually means a physical connection is marginal rather than broken. Vibration momentarily opens a cracked solder joint on the module's circuit board, or moves a corroded pin in the connector just enough to interrupt the supply. The module loses power, records the fault and lights the lamp, then recovers. It is a genuine fault that will get worse, and the bumps are a clue worth mentioning to the garage rather than an oddity to dismiss.

Will an intermittent ABS light fail an MOT?

It depends whether it is on at the moment the tester looks, which makes this an uncomfortable code to gamble on. The tester switches on the ignition, checks the lamp illuminates as a bulb check and then confirms it goes out. If it stays on, that is a major defect — a brake system warning device indicating a malfunction, with 85,196 recorded failures. A car with an intermittent fault might pass, but if the lamp is on it will fail, and presenting a car in that state is a gamble with a retest at stake.

Can a bad earth cause C0265?

Yes. The module's supply is only as good as its return path, and a corroded earth strap between the engine, body and battery raises the resistance in that path enough for the module's voltage to sag under load. The relay drops out, the module resets, and the code is stored. Measuring the voltage drop across the main earth straps with the engine running and a decent electrical load switched on will reveal it, and cleaning up an earth is one of the cheapest repairs there is.

Chassis code · standard

C0265ABS module main relay fault

Standard definition: EBCM Relay Circuit

Last updated 27 July 2026

Quick answer

The relay that powers up the electronic brake control module itself has a fault. Where C0121 concerns the supply to the hydraulic valves, this one concerns the module's own power, so the symptoms tend to be broader and more intermittent.

How the car works this out

The brake control module is fed through a main relay, generally integrated into the module, that brings the electronics up when the ignition is switched on and holds them up while the car is running. The module monitors that supply and can tell whether the relay has energised, whether it has stuck closed, and whether the voltage on the far side is what it should be. C0265 is set when the relay does not respond to the command, or the voltage on the supply rail does not match the relay's state. Because it is the module's own power that is affected, this fault often behaves intermittently — the module works for a while, drops out, then comes back.

What you might notice

ABS, traction control and stability lamps appearing and disappearing, sometimes clearing after a restart
Lamps that come on over bumps or in cold, damp weather
A scan tool unable to communicate with the ABS module at all when the fault is present
Speedometer dropping out intermittently on cars that take road speed from the ABS module
Normal braking throughout

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.

1Failed internal solder joints or a failed relay on the module's circuit boardCommon
How to check: The intermittent-with-vibration pattern is itself a strong clue. A specialist bench test confirms it.
2Water ingress into the module or its connector, which corrodes the board from underneathCommon
How to check: Check for evidence of water in the area — a blocked scuttle drain, a damp bulkhead, or a wet carpet on cars with the module inside.
3A corroded or loose module connector, or spread pins in the plugCommon
4A failed supply fuse or a corroded fuse box terminalPossible
5Low or unstable system voltage from a failing battery, alternator or earth strapPossible
How to check: Have the charging system checked and measure the voltage drop across the main earth straps.
6Chafed wiring between the fuse box and the moduleLess common

What to do about it

1Note when the fault appears. Cold mornings, damp weather, rough roads or particular bumps are all useful evidence, and an intermittent fault with a pattern is much easier to find than one without.
2Have the battery, charging voltage and earth straps checked. Unstable supply voltage produces this code without anything being broken in the braking system.
3Inspect the module connector for corrosion and the surrounding area for water ingress, including blocked scuttle drains.
4Ask about module repair before replacement. Internal relay and solder failures are among the most commonly refurbished faults on these units.
5Have the fault properly diagnosed rather than clearing the code repeatedly. A module that keeps dropping out is not stable, and stability control that comes and goes is worse than none in terms of knowing what the car will do.

Can you keep driving?

Briefly, gently

The hydraulic brakes are unaffected, so the car stops normally. The problem with this particular fault is its unpredictability: the anti-lock and stability systems may be working one moment and gone the next, which means you cannot rely on them being there. Drive as though they are absent, and get it diagnosed rather than waiting for it to settle down.

Will it fail an MOT?

A brake system warning device indicating a malfunction is a major defect with 85,196 recorded failures. This code brings a particular risk on test day because it is often intermittent: a tester checks that the lamp illuminates with the ignition and then goes out, and the car may pass that check on a good day and fail it on a bad one. It cuts both ways, though — a lamp that does not illuminate at all when the ignition is switched on is also a failure, because the tester cannot verify the warning system is working.

The light this turns on

ABS warning lightThe anti-lock braking system has found a fault and switched itself off. Your brakes still work exactly as they always did in normal driving — what you have lost is the system that stops the wheels locking and the steering going light under a hard, emergency stop.

Common questions

What is the difference between C0121 and C0265?: They concern two different power supplies inside the same unit. C0121 is the relay feeding the hydraulic solenoid valves, so it fails with the valves isolated and the anti-lock function disabled while the module itself carries on working and can still be talked to by a scan tool. C0265 is the relay powering the brake control module itself, so when it fails the module can go completely silent — no communication, no road speed being published, and lamps appearing across the dashboard. The intermittent behaviour is far more typical of C0265.
Why does my ABS light come on over bumps?: That pattern usually means a physical connection is marginal rather than broken. Vibration momentarily opens a cracked solder joint on the module's circuit board, or moves a corroded pin in the connector just enough to interrupt the supply. The module loses power, records the fault and lights the lamp, then recovers. It is a genuine fault that will get worse, and the bumps are a clue worth mentioning to the garage rather than an oddity to dismiss.
Will an intermittent ABS light fail an MOT?: It depends whether it is on at the moment the tester looks, which makes this an uncomfortable code to gamble on. The tester switches on the ignition, checks the lamp illuminates as a bulb check and then confirms it goes out. If it stays on, that is a major defect — a brake system warning device indicating a malfunction, with 85,196 recorded failures. A car with an intermittent fault might pass, but if the lamp is on it will fail, and presenting a car in that state is a gamble with a retest at stake.
Can a bad earth cause C0265?: Yes. The module's supply is only as good as its return path, and a corroded earth strap between the engine, body and battery raises the resistance in that path enough for the module's voltage to sag under load. The relay drops out, the module resets, and the code is stored. Measuring the voltage drop across the main earth straps with the engine running and a decent electrical load switched on will reveal it, and cleaning up an earth is one of the cheapest repairs there is.

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