Why the engine light comes on, and what we do about it

A fault code names a circuit, not a part. P0420 does not mean 'buy a catalytic converter', it means the sensor after the cat is reading more like the one before it, and there are five reasons that happens. Fitting the expensive part first is how people end up paying twice.

Here is what that means in practice:

1. Read the codes, and the freeze frame with them

The freeze frame records what the engine was doing the moment the fault logged: temperature, load, speed. A misfire that only logs cold is a different problem from one that logs under load.

2. Test the cheap things the code implicates

Split hoses, a loose connector, a tired sensor, a sticking valve. Most of what sets a light is worth less than the diagnostic hour.

3. Only then, condemn a part

If a part has to go, we tell you why the test pointed at it and what we expect afterwards. If it was the wrong call, that is on us, not on your bill.

4. Clear it and drive it

A code that does not come back after a proper drive cycle was a symptom, not a fault. We would rather hand a car back with an empty fault memory than an empty wallet.

A flashing engine light is different: stop driving and ring us. A steady one usually means book it in this week.