C1 Preheating Lamp and Smoke

Sounds like a classic glow plug control unit issue. The ECU monitors the glow plugs to ensure they're heating correctly for cold starts. If the ECU detects a fault, like a bad connection or a failing control unit, it will keep the preheating lamp on and trigger the check engine light. The smoke on acceleration could be related, as unburnt fuel can be present due to insufficient preheating, leading to incomplete combustion. Have you checked the wiring and plug connectors to the glow plug control unit itself? A loose or corroded connection is often the culprit before the unit completely fails.

Yeah, I gave the connectors a quick look, but honestly, I'm not 100% sure what I'm looking for. Is there a specific way to test the control unit or the connections to rule them out definitively?

Okay, a quick look might not cut it. You need to be methodical. Check for any signs of corrosion (green or white powdery stuff) on the terminals. Give the wires a gentle tug to see if any are loose or broken inside the insulation. If you have a multimeter, you could check for voltage at the glow plugs when the preheating cycle is active (ignition on, but engine not started). No voltage means the control unit isn't sending power. Testing the control unit itself usually requires specific diagnostic equipment. Honestly, for this, unless you're very comfortable with electrical testing and have the right tools, I'd recommend taking it to a workshop. They can pinpoint the issue quickly and accurately. Mucking around with the ECU and electrical system can lead to more problems!

Thanks for the advice! I took it to a local shop, and you were spot on – it was a corroded connector on the glow plug control unit. They cleaned it up and replaced a short section of damaged wire. Cost me 95€, but it's running perfectly now.