Peugeot 1007 preheating lamp stays on

I have the same model. Did you check the resistance of the glow plugs? A deviation from the standard value often points to that. How does the engine behave upon starting – any roughness or misfires?

Yes, the glow plug resistance was outside the acceptable range. But the engine has no discernible roughness or misfires upon starting. Could a faulty glow plug relay also cause these symptoms, or am I missing something?

A faulty relay could contribute, but the error code and abnormal glow plug resistance strongly suggest the glow plugs themselves. The absence of rough idling may mean the other cylinders are compensating. Replacing the defective glow plug(s) seems best. If the engine light persists, get a workshop to check if the issue is fixed.

Following your recommendation and a workshop diagnosis, a defective glow plug was indeed the problem. The repair, parts and labor, cost 440€. The preheating lamp now works, and the error code is cleared. Thanks for your help.