Peugeot 406 Glow Plug Issue

The symptoms suggest a problem in the glow plug control system. The preheating lamp and check engine light, with smoking, are classic signs. Before replacing the control unit, do a diagnostic test. These Peugeot systems are sensitive; a fault code should help. It could be a faulty glow plug, a wiring issue, or the control unit. Have you checked the glow plugs for continuity?

Thanks for the quick reply! I haven't checked the glow plugs directly yet. I was hoping to avoid getting dirty if it was a control unit issue. Am I right that going straight to the control unit is like assuming the lightbulb is broken before checking the switch?

Exactly. While a defective glow control unit is possible, diagnosing it without ruling out other causes would be unwise. Modern diesels use many sensors and actuators; a small fault can cause many errors. Given your symptoms and the car's age, I advise taking it to a workshop for a diagnostic test. They can find the fault and save you time and money.

I took it to the shop. It WAS the glow control unit, but the connections were the problem. The cable was corroded. A new unit and some cleaning, and it's running well. It cost 95€.