THE 10-STEP DIAGNOSTIC PROCESS

Is there a fault?

Establish if both you and the car think it has a fault. An illuminated warning light (as shown at right) is an obvious sign of a fuel or engine electrical/emissions problem, but it does not always light up.

Interrogate the car

If you possess a fault code reader, extract the fault codes and make a note of them. However, be wary that the fault code description might not point you to the exact cause, especially if additional faults have resulted from a component being disconnected and reconnected randomly.

Inspect the wiring

Check the wiring surrounding the suspected component by sight and feel (including wiggling the connectors by hand). Check that the wires are routed correctly and are undamaged.

Check your tools

Ensure that any tools used are calibrated and reset correctly, and that any power supply is in good order.

Is the whole electrical system sound?

Check that the circuit is in good order, including the earth supply, and that the battery voltage measures at least 12 volts at the terminals and that its connectors are making a good contact. Never assume anything.

Research and pinpoint

Spend some time narrowing down the potential location of the fault. Track down any credible data that will allow you to establish the precise function of any electrical socket pins and individual component values, such as resistance, output voltage or signal pulses.

Extract and record live data

With a multimeter and/ or oscilloscope, test the component’s output signals and verify it against supplied data. If none is available, establish any patterns in performance or check if the readings react as expected to any changes (such as engine speed, throttle position, etc). Record any results.

Record and rest the codes

Resetting the fault codes should be one of the last operations, but you can consider recording and erasing them earlier to see if any of them return as a permanent fault. If you do not have a code reader, it is possible that the car will recognise that a faulty component has been replaced and will extinguish the lamp automatically, although this might not happen immediately.

Replace any faulty parts

Only at this stage should you replace parts if you are certain they are defective. Compare the new part’s live data readings against those taken from the original component – they should be different.

Ask further questions

Always question why the original component failed in the first place and make additional checks.