[awg]

good post ^^

if a failure to earth is the problem, due to a PCM (ECU) wiring issue

In some cases the problem can be rectified by providing a separtae earth wire

this would not work if the earth is "switched" though

to probe the PCM wires, I would be inclined to use a homemade backprobe, which is a small safety pin, attached to my multimeter probe with an alligator clip.

If you cannot backprobe the cavity, it is acceptable practice to pierce the wire with the pin