Audi Glow Plug Issues: Smoke and Warning Lights

I experienced something very similar with my 2013 Audi A4 diesel. I have some basic experience working on cars, but with electrical issues like this, I needed professional help. The symptoms you describe match what I encountered, constant preheating light, engine warning, and smoke during acceleration. After vehicle stalling became an issue during cold starts, I took it to a workshop for a proper diagnostic test. Turned out there was a faulty connection between the glow plug control unit and the wiring harness. The mechanic found a damaged cable that was causing poor contact. The repair involved replacing the connection and checking all glow plugs, total cost was 95 Euro. Before heading to the workshop, could you share: Are you experiencing any cold start problems? Does the smoke color appear black or blue? How long has the preheating light been staying on? Have you noticed any changes in fuel consumption? This information would help determine if your issue is similar to what I encountered and whether the control unit itself needs replacement or if its just a connection problem.

Had my vehicle at 107135 KM and just took it for a diagnostic test last week. Yes, having cold start problems and black smoke. The diesel engine struggles especially in colder weather. The preheating light has been on for about 2 weeks now, and fuel consumption increased by roughly 15%. The heating element seems to be working intermittently. Recently noticed the vehicle stalling when idling, which wasnt happening before. The glow plug system definitely needs attention, my last service was 3 months ago but this wasnt an issue then. Would you recommend checking individual glow plugs first or going straight for the control unit replacement?

Thanks for those details, they actually mirror what I dealt with on my A4 almost exactly. The rising fuel consumption and black smoke during cold starts were key indicators in my case too. After my experience, I would strongly recommend checking the individual glow plugs first. In my case, doing this helped avoid unnecessarily replacing the entire control unit. The engine misfire and stalling issues you describe turned out to be caused by two failed glow plugs in my diesel engine, not the control unit itself. The diagnostic test from my mechanic showed the glow plugs were not receiving proper voltage due to both faulty plugs and connection issues. Car maintenance cost breakdown in my case: Individual glow plug testing: 45 Euro, Replacement of 2 glow plugs: 120 Euro, New wiring connection: 35 Euro This systematic approach saved me from spending 400 Euro+ on a new control unit that wasnt actually needed. The improved cold start performance and normalized fuel consumption confirmed we found the real issue. Based on your kilometer reading and symptoms, your glow plugs might simply be at their service limit. A proper diagnostic test focusing on individual plug resistance values would be the most cost-effective first step.

Appreciate the detailed response and sharing your experience. After reading about your similar case, I feel more confident about the repair approach. Your suggestion to test individual glow plugs first makes sense given the age and mileage of my A5. Just scheduled a diagnostic test focusing on glow plug resistance testing next week. You make a good point about checking the basic components before replacing the entire control unit. The increasing cold start problems and engine misfire symptoms do suggest individual glow plug issues rather than total system failure. The cost breakdown you provided is helpful for budgeting. If the heating element testing shows similar issues to what you encountered, I might be looking at around 200 Euro total for the repair, which is much better than replacing the whole control unit. Will update once I get the results from the car maintenance check. Really hoping its just a couple of faulty plugs or wiring issues like in your case. Thanks for helping me avoid potentially unnecessary expensive repairs.