U0242: Lost Communication With Headlamp Control Module B
What to do
Check both headlights in the dark. If either is out, the car is not roadworthy at night.
Stand in front of the car with the lights on and compare the two sides. Different brightness, different colour, or beams aimed at visibly different heights tells you which side has lost control — and that is worth reporting.
If both headlights have lost their adaptive or automatic behaviour, that does not mean both have failed. These systems reference the two sides against each other, so one faulty module often disables the clever functions on both. The stored codes tell a workshop which is which, and it may well be one repair rather than two.
As at U0241: look for misting or water inside the lamp units, and mention any front-end accident repair.
What this code means
The second headlamp control module has stopped being heard from — the other lamp of the pair described at U0241, where the roadworthiness and beam-aim arguments are set out.
Having two makes a comparison possible, and on headlamps that comparison is unusually informative because the pair should be identical.
- Both modules silent points at a shared supply, a shared fuse or the network branch feeding the front of the vehicle — not at two lamps failing together. That is the cheaper answer and it is also the commoner one.
- One silent points at that corner: its connector, its loom, or water in that lamp unit.
A visible asymmetry is diagnostic in its own right and available to anybody. Standing in front of the vehicle at night, two headlamps of different brightness, different colour, or aimed at visibly different heights say plainly that one side is not being controlled — and which one.
One thing specific to a pair is worth naming because it costs money: adaptive and levelling systems reference the two sides against each other. Losing one can therefore disable the adaptive behaviour on both, so the good lamp reverts to a plain dipped beam as well. A driver seeing both lamps lose their adaptive function may reasonably conclude both modules have failed, when only one has — and the pattern in the stored codes is what separates those.
Causes are as at U0241, with the same emphasis: water inside the lamp unit, the connector, and front-end repair history.
Likely causes
Start here
- Water ingress into module · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
- Blown fuse or failed supply · easy
Less common
- Failed power or earth supply to the module · moderate
Symptoms
Components involved
Information, not instruction
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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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- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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