Audi Q7 Diesel Power Loss Points to Fuel Heater Fix

About two years ago, I faced nearly identical symptoms with my 2006 Audi Q5 diesel. The power loss and rough idle during cold starts were particularly frustrating. I have some experience working on diesels, mainly basic maintenance and troubleshooting. After checking the stored error codes myself using a basic scanner, I took it to my regular workshop. They diagnosed a faulty connection in the wiring harness leading to the fuel filter heater. The cable insulation had degraded over time, causing intermittent connection issues. The repair involved replacing the damaged section of the wiring and ensuring proper connection to the fuel filter heater unit. Total cost was 95 Euro for parts and labor. The fix completely resolved the power loss issues and improved fuel efficiency back to normal levels. The rough idle disappeared, especially during those tricky cold starts. Before suggesting more specific solutions, could you share: Have you noticed any other warning lights besides the check engine light? What are the exact error codes your scanner showed? Has the fuel filter been replaced according to maintenance schedule? Does the rough idle improve once the engine reaches operating temperature? This information would help determine if your Q7 is experiencing the same electrical issue or if there might be other factors involved in the fuel system.

Thanks for the insights shared about the wiring harness. As requested, my Q7 had its last service about 4 months ago at 71212 KM. The only warning light showing is the check engine light. The rough idle seems to improve after the engine warms up, but the power loss remains consistent throughout. The stored codes are P0380 and P0671, pointing to glow plug circuit issues. The fuel filter was changed during the last service, so I do not think that is the problem. The cold start issues and engine stalling are most noticeable in the morning, which seems consistent with a glow plug system problem rather than just the fuel filter heater. Could definitely use more opinions on this before taking it to a shop.

Based on those codes and symptoms, particularly the P0380 and P0671, this does sound more like a glow plug system issue rather than just the fuel filter heater problem I initially encountered. When my Q5 had similar issues, I actually ended up having both the wiring harness and glow plugs replaced, as the diagnostic revealed multiple failing components. The cold start issues you are experiencing are very typical of glow plug failures, and the engine stalling matches what I dealt with. The fact that your fuel filter was recently replaced rules out one potential cause, though fuel system issues can sometimes mimic glow plug symptoms. In my case, after fixing the initial wiring problem, we discovered two failed glow plugs during a follow-up check when some symptoms persisted. The full repair included: Replacing all glow plugs as a set (recommended practice), Testing the glow plug control module, Checking fuel lines for any restrictions, Verifying proper fuel filter installation from the recent service Total cost came to around 320 Euro, but it completely resolved all starting and performance issues. The engine stalling disappeared, and cold start performance returned to normal. Would recommend having the glow plug control module tested, this can cause similar symptoms and trigger those specific codes even with good glow plugs.

Thanks for the feedback. After reading through the responses, I am more confident that this is primarily a glow plug system issue rather than just the fuel filter heater I initially suspected. The cold start problems and engine stalling symptoms match what others experienced, and the error codes P0380 and P0671 strongly point to the glow plug system. My fuel filter is relatively new from the service 4 months ago, so I will focus on having the glow plugs and control module checked first. The consistent power loss even after warm-up is concerning, but it makes sense if multiple glow plugs are failing. Will schedule an appointment to have the full glow plug system diagnosed, including testing the control module and checking all electrical connections. While the potential 320 Euro repair cost is not ideal, having the entire system properly checked and potentially replacing all glow plugs as a set seems like the right approach to resolve these cold start issues and restore normal engine performance. Will update once I have more information from the diagnostic tests.