Audi Diesel Glow Plug Failure Symptoms Decoded

Had the exact same symptoms on my 2004 Audi A6 diesel engine last winter. Having some experience with diesel engines, I first noticed similar issues during cold starts, rough idle and the permanent preheating light. The workshop ran diagnostics and confirmed a faulty glow plug was causing engine misfires, particularly noticeable during cold weather. They tested all glow plugs and found one completely failed, with another showing signs of wear. The whole diagnostic and replacement process took about 3 hours. Total repair cost was 440 Euro, including parts and labor. Since the fix, cold starts have been smooth, and the check engine light has stayed off. Given the age of these engines, glow plug issues are fairly common. To better assist with your situation, could you share: Current mileage on your A4, How long has the preheating light been on, Any other warning lights showing, Does the engine misfire improve once warmed up, Have you noticed decreased fuel efficiency These details would help determine if your symptoms match what I experienced with my A6.

Thanks for sharing your experience. My A4 has 146624 KM and last service was 2 months ago. The preheating light started flashing about a week ago. After checking system codes, one glow plug shows as faulty, affecting engine combustion during cold starts. The diesel engine runs much smoother once warmed up, which supports the glow plug theory. No other warning lights are active and fuel consumption seems normal. The cold start issues are most noticeable in the morning when the diesel car has been sitting overnight. Would replacing just the faulty glow plug be sufficient, or should I consider changing the whole set given the age of the engine?

Thanks for those details. Looking back at my Audi A6 experience, I strongly recommend replacing all glow plugs as a set. While my initial issue showed just one faulty plug, the second worn plug failed shortly after, causing more engine misfires and requiring another repair visit. Replacing the complete set makes sense considering: Your mileage is similar to when I had the issue, Glow plugs typically wear at a similar rate, Labor costs are nearly the same whether replacing one or all, Prevents multiple repair visits The improved cold start performance after my complete glow plug replacement was noticeable. The diesel engine fired up immediately, even in cold conditions, and the rough idle completely disappeared. Fuel efficiency actually improved slightly due to better engine combustion during warm-up. The full set replacement cost me 440 Euro, but saved money long-term by avoiding repeated repair visits. Given the age of our engines, this preventive approach helps maintain reliable starting and proper engine combustion. Have you noticed if the engine misfire is worse on particularly cold mornings? This could indicate if other glow plugs are also beginning to fail.

Thanks for the detailed advice. You make a good point about replacing all glow plugs at once. Yes, the diesel engine misfires are definitely worse on colder mornings, which suggests other glow plugs might be degrading too. I will schedule an appointment to have the complete set replaced. The labor cost difference is minimal, and I want to avoid multiple repair visits. My check engine light has started appearing intermittently now, so better to address all glow plugs before winter arrives. 440 Euro sounds reasonable for peace of mind and proper cold start performance. Given the improved engine combustion and smoother idle you experienced, this seems like the right approach for my A4. I appreciate you sharing your repair experience, it helped confirm my suspicions about the glow plug being the root cause. Will report back once the repair is done to let others know if replacing the full set resolved my cold start issues completely.