Amber, flashing, and what to do
| Behaviour | What it means | What to do |
|---|---|---|
| Steady amber | A fault is stored, the car is running in a compromised state | Drive gently, get it read within days |
| Flashing amber | Active misfire, unburnt fuel entering the exhaust | Stop as soon as it is safe, do not continue |
| Amber with limp mode | The ECU has restricted power to protect something | Get it read before driving distance |
| Light on, car drives normally | Common with sensor and emissions faults | Still needs reading, it will not clear itself |
What usually causes it
In rough order of frequency on UK cars:
- Oxygen and lambda sensors. Cheap part, common failure, immediate fuel economy penalty.
- Mass airflow sensor. Often just contaminated rather than dead.
- Ignition coils and spark plugs. The usual cause of a flashing light.
- EGR valve. Carbon build-up, very common on short-journey diesels.
- DPF. Blocked filters on diesels, covered in our DPF guide.
- Loose or failed fuel cap seal. Genuinely a cause of evaporative system codes, and the cheapest fix in motoring.
Reading the code properly
A code is a starting point, not a diagnosis. P0171 means the engine is running lean; it does not tell you whether that is a vacuum leak, a dirty airflow sensor, a failing fuel pump or an exhaust leak upstream of the sensor. What separates a real diagnosis from a parts swap is reading the live data alongside the code: fuel trims, sensor voltages, airflow readings against expected values.
A mobile diagnostic health check takes 30 to 45 minutes at your address and produces a written summary: the codes, what they mean, what is likely to have caused them and what fixing it involves.
Why clearing the light is not a fix
Clearing codes resets the memory. It does not repair anything. If the fault is still present, the light returns within a few drive cycles, and in the meantime you have deleted the freeze-frame data that would have helped diagnose it.
There is one legitimate use: clearing codes after a repair, to confirm the fault does not return. Anything else is hiding a warning from yourself.
When to stop driving
Stop as soon as it is safe if the light is flashing, if it comes on with a temperature or oil pressure warning, if the car has lost significant power, or if there is a new noise, smell or smoke. Those combinations mean something is actively being damaged.
Otherwise, drive gently and get it read within a few days. A stored code is a car telling you something in plain language. It is worth listening before it gets louder.