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Glow plugs of the wrong specification for the engine — a cheaper set, a superseded part number, or a fitment guide followed instead of the engine code.

The reason this matters more than it used to is that a modern glow plug is not a 12 volt heater. Many systems use low-voltage plugs — 5 or 7 volts is common — which the control module supplies by pulsing the battery voltage. A plug of the wrong rating either barely warms or runs far hotter than intended, and the second case damages the plug and the module driving it.

The module also uses the plugs as measuring instruments, and that is what turns this into a diagnostic trap. It watches the current each channel draws to decide whether a plug is open, shorted, or healthy. Plugs whose resistance is not what the module expects therefore report faults on brand new parts, and a set with too low a resistance overloads the driver and overheats the module.

They can also be physically wrong in ways that are not obvious. Reach, thread and tip length differ between variants of the same engine family: a plug that sits too proud intrudes into the combustion chamber, one that sits too deep never heats the right place, and ceramic and metal plugs are not interchangeable even where they fit.

It is worth suspecting on any vehicle whose recent history is unknown, since the plugs are a common cheap fix by a previous owner. Check the part numbers against the engine code rather than the model, which is where the mistake is usually made.