A fault that only appears in frost or in rain

A fault that comes with the weather is not a mystery but a hint. It names the place to look.

Some codes do not come all the time but with the weather: only on a frosty morning, only after rain, only when hot. It is maddening — at the garage the car behaves impeccably and «nothing was found».

In fact such a fault is far more talkative than a permanent one. The condition under which it appears points at the cause.

Only when cold, in winter

Cold shrinks metal and plastic. A cracked connector that holds contact in summer by sheer interference loses that grip in frost.

What to look for:

• Connectors and wires where they flex: at the doors, at the hubs, at the vent valve under the floor.
• The battery. In frost a tired battery gives noticeably less, and units start complaining about power and communication. You can check this yourself.
• Thickened oil — if the code is about oil pressure or valve timing.

It is also worth remembering that some winter behaviour is normal rather than a fault: that is covered in a car behaves differently in winter.

Only after rain or a wash

Water. And almost always in the same short list of places.

• Connectors underneath: wheel speed sensors at the hubs, the vent valve by the tank, the crankshaft sensor low on the engine.
• The body control module — on many cars it sits low and gets flooded through a blocked drain.
• A pressure wash. If the code appeared the same day the car was washed, that is not a coincidence. A jet into a connector fills it for a long time.

The signature of a water fault is recognisable: it lasts a day or two, goes away by itself, and comes back with the next rain.

Only when hot, half an hour into the drive

Heat. A warmed component changes its resistance, and the weak spot finally shows.

The classic example is P0335: the car stalls when hot, refuses to start for a minute, then drives as if nothing happened.

Ignition coils behave the same way: fine when cold, missing spark when hot.

Only over bumps

Not weather, but the same kind of hint: it means a contact rather than a part. A wire, a connector or an earth that opens when shaken. Look where the harness is clamped to the body or passes through a hole.

How to catch such a fault

The main tool here is the freeze frame. The unit records the conditions at the moment the code appeared: coolant temperature, revs, speed, load. If every record shows a temperature near zero, there is hardly a question left.

The second trick: do not clear the code straight away. A cleared fault takes its freeze frame with it — and with it the only piece of evidence.

In short

• A weather-related fault is a hint, not a mystery: the condition names the place.
• Frost — contacts, battery, thick oil.
• Rain and washing — connectors underneath and anything water reaches.
• Warm-up — whatever changes resistance with heat: sensors, coils.
• Bumps — a contact, not a part.
• Read the freeze frame and do not rush to clear: the evidence goes with the code.