Warning lights: which ones mean stop now
Red means stop at the first safe place; amber means get it looked at but you can normally reach somewhere sensible first. Four warnings override that rule and mean stop now, wherever you are: air pressure, brakes, oil pressure and steering. What they have in common is that the thing about to fail is the thing keeping the vehicle on the road, and none of them gets better by driving further. Everything else — engine, emissions, AdBlue, DPF — is a reason to pick a good place to stop rather than to stop instantly.
The four that override the colour
**Air pressure.** A dropping system will eventually apply the spring brakes by itself, and it will do so wherever the vehicle happens to be when the pressure reaches the threshold. That might be a hard shoulder. It might be a roundabout. Stop while you still choose where.
**Brakes.** Any brake warning that is not a handbrake reminder. A truck that has lost circuit pressure on one axle still stops — until the load, the gradient and the distance line up badly.
**Oil pressure.** Minutes, sometimes less. Every second of running with no pressure is damage that was avoidable a second earlier. Switch off rather than coast to the next exit.
**Steering.** Power assistance that has failed is heavy but drivable at low speed and effectively undrivable at motorway speed. Reduce speed first, then stop.
The ones that buy you a few kilometres
Engine and emissions warnings, AdBlue, DPF regeneration, and most sensor faults will normally let you reach a rest area, a lay-by, or an exit with a hard standing. That is worth using. A van can work on a truck at a rest area in a way it cannot on a hard shoulder with traffic passing a metre away, and the same job takes half the time and carries a fraction of the risk.
The exception inside this group is anything that has already put the vehicle into limp mode. Limp mode is not a warning about the future; it is the system already protecting itself, and continuing usually means the next stop is chosen for you.
Thirty seconds before you call
None of this needs tools, and all of it changes what gets loaded into the van:
- Read the message, not just the symbol. The text line underneath is what a mechanic actually needs.
- Note the order things happened. A light that came on after a noise is a different fault from one that came on before it.
- Listen with the engine off. An air leak is audible in a way it never is at speed.
- Look underneath from the verge side — a puddle tells you which system, and its colour tells you which fluid.
- Check whether the warning clears on a restart, and say so either way. A fault that stays through a power cycle is stored, and a stored fault is one a reader can pull straight off.
Related
The light came on and then went off by itself. Is it fine?
The fault is almost certainly still stored even though the light is out, and an intermittent fault is worth more attention than a permanent one, not less — it means something is on the edge rather than already gone. Sensors, connectors and wiring that flex with the chassis are the usual cause, and they fail properly at the least convenient moment. Have it read at the next stop rather than waiting for it to become permanent.
Trailer ABS light is on. Can I keep going?
The trailer still brakes — ABS failing does not remove braking, it removes the anti-lock function — but you are now driving a combination that can jackknife under heavy braking on a wet road, and in most of the EU it is also a defect that will be recorded at a roadside inspection. Treat it as a reason to get it fixed at the next stop rather than a reason to stop instantly, and drive to the weather you actually have rather than the one you had this morning.
Can I clear it by disconnecting the battery?
You can sometimes make the light go out, which is not the same as fixing anything, and on a modern truck it can also lose adaptation values, tachograph power records and stored data a mechanic was going to use to find the fault in five minutes. It turns a diagnosable problem into a guessing game. If the light is out and the fault is not, it will be back — usually further from help.
Truck stopped right now?
A mechanic answers, day or night, and you get an arrival time before you hang up.