Faulty Glow Plug Control Causing Subaru Smoke & Warning Light

Had similar symptoms on a 2005 Subaru Legacy 2.0 Diesel. The electrical fault turned out to be more than just the glowing plug control unit. The ignition system required a complete diagnosis because multiple car electronics were affected. Main symptoms matched yours, constant glow plug light, engine warning, and excessive smoke under load. The defective part was actually a damaged wiring harness near the glow plugs, which caused incorrect signals to the control unit. The repair involved: Testing all glow plugs for resistance, Checking the preheating control unit connections, Replacing damaged wiring harness, Reprogramming the engine control module This was classified as a serious electrical issue since it affected starting reliability and engine performance. If left unfixed, it could have led to starting problems in cold weather and potential engine damage from incomplete combustion. The diagnostic process is crucial here, skipping steps or replacing parts without proper testing often leads to recurring issues with the preheating system.

Thanks for sharing those details about the Legacy repair. I had a similar experience with my 2008 Impreza and the defective part turned out to be exactly what you described. The preheating unit issues were causing all sorts of trouble with cold starts and the electrical fault kept triggering the check engine light. Just wondering, do you remember roughly how much the full repair cost you? My fix was about 850€ including diagnosis. Also, has everything been working properly since the repair? No recurring problems with the engine preheat system?

The repair solved all preheating control unit problems completely, no more electrical faults or cold start issues since then. My final bill was much lower at 95€, which covered fixing a faulty cable connection to the control unit. The mechanic found the issue quickly without needing extensive diagnosis or parts replacement. Your 850€ repair sounds like it involved more complex work, similar to what I initially feared. In my case, the vehicle heater and ignition system fortunately just needed a connection fix rather than full component replacement or rewiring. The control unit now works perfectly, with no warning lights or smoke issues during the past 6 months. The engine preheat cycle operates normally in all weather conditions, which has improved overall starting reliability and performance. Pro tip: regular checks of the preheating system connections can prevent these problems from developing into more expensive repairs like yours.

Just got back from the mechanic after dealing with my preheating control issues on the Impreza. The total repair came to 850€ which included replacing the heating module and fixing some damaged wiring. The electrical fault was exactly as suspected, a combination of worn wiring harness and a failing control unit that was causing the engine preheat cycle to malfunction. The mechanic did a thorough check of the ignition system and found several connection points that needed attention. Better to fix everything properly now than risk more expensive repairs later. They also cleaned all the glow plug connections while they were at it. First cold start this morning worked perfectly, no more warning lights or smoke issues. The heating indicator now functions normally instead of staying permanently lit. Definitely worth getting it sorted by a qualified technician despite the cost. Will be keeping a closer eye on the preheating system maintenance going forward to avoid similar issues.