7 repairs you can do yourself after an OBD2 diagnosis

Basic rules

Engine cold. Handbrake on. For any electrical work, disconnect the negative battery terminal (note the radio code if needed). Work methodically: one thing at a time, then clear the code and test (method); the app clears engine codes after confirmation, without reprogramming anything. Then read the codes again with DiagAuto AI to check that the repair was enough. Never touch the high-pressure circuit of a common rail diesel or the airbags.

1. Fuel cap (P0455, P0442, P0457)

Difficulty: none. €15 – €40. Replace it with a good-quality original-equipment cap, tighten until it clicks, clear the code, drive for a few days. Details.

2. Air filter (P0101, black smoke, fuel consumption)

Difficulty: very easy. €10 – €30. A few clips or screws on the air box. Check which way round it fits and the condition of the intake hose while you're there.

3. Cleaning the mass airflow sensor (P0101, P0102)

Difficulty: easy. €10 of dedicated MAF cleaner. Unplug the connector, unscrew the sensor (often 2 Torx screws), spray without touching the sensing element, let it dry for 15 minutes. Details.

4. Spark plugs (P0300 to P0306, rough idle)

Difficulty: easy to moderate. €20 – €80 for a set. Spark plug socket, moderate tightening torque (around 20 – 30 Nm, check the manual), gap checked. On engines with pencil coils, remove each coil first (1 screw). Never force a plug that isn't going in straight.

5. Ignition coil (P0301 to P0306 on one specific cylinder)

Difficulty: easy. €30 – €120. One screw, one connector. Before buying, do the swap test described in our misfire guide to be sure.

6. Cleaning the throttle body (rough idle, stalling)

Difficulty: moderate. €10 of cleaner. Remove the intake hose, clean the throttle plate and its housing with a cloth, without forcing the plate on electronic throttle bodies. Some cars then need a relearn (ignition on for 1 minute without touching the pedal, then start and idle for 5 minutes). Background.

7. Glow plugs (P0670 to P0674, hard starting when cold)

Difficulty: moderate to hard. €15 – €40 per plug. Test them with a multimeter first. When removing them, if a plug resists, don't force it: a glow plug broken off in the cylinder head is very expensive to extract. Penetrating oil the day before, engine warm, the right socket.

What to leave to the garage

Injectors, high-pressure pump, turbo, an EGR valve buried under the manifold, catalytic converter or DPF, head gasket, anything involving the timing or the brakes. You'll arrive with a precise diagnosis, which is already a lot: how to present it.