S5 Diesel Filter Heater Causing Power Loss and Poor MPG

Had the same symptoms on an A6 3.0 TDI last winter. The fuel efficiency dropped significantly and the engine started showing power loss, especially during cold starts. The fuel filter heater malfunction causes poor fuel system performance and engine stalling in cold conditions. The issue was a damaged wiring harness connecting to the fuel filter heating element. Corrosion had gotten into the plug connection, causing intermittent electrical contact. This is a known weak point in these diesel engine setups. The repair involved replacing the complete wiring harness and connector assembly for the fuel filter heater, plus installing a new fuel filter as preventive maintenance. It was a moderate repair, not engine-threatening if caught early, but definitely needs proper attention to prevent further fuel system issues. Make sure your shop checks both the heater element and the wiring/connections. Sometimes mechanics only replace the filter without addressing the electrical fault, leading to recurring problems. A proper diagnostic scan should show if its the heater element or just the wiring causing the fault code.

Thanks for the detailed explanation about the wiring harness and connection issues. Your cold start and rough idle problems sound exactly like what I experienced. I ended up getting mine fixed right away since I noticed the engine stalling was getting worse, especially after sitting overnight. Would you mind sharing what the total repair cost was? My fuel filter assembly and related parts are ready for replacement next week, but Id like to know what to expect cost-wise. Also curious if youve had any other fuel line problems since getting it fixed.

Got the repair done and the fuel filter heater is working perfectly now. The mechanic found a corroded plug connection that was causing the intermittent electrical issues, similar to what was causing your engine stalling problems. Total cost was 95 Euro for fixing the cable and connections, which was much less than I initially feared. Since the repair, the diesel engine has been running smoothly with no cold start issues or fuel system problems. Fuel consumption returned to normal levels and the rough idle completely disappeared. The repair shop did a thorough job checking all related components while they were at it, which probably helped prevent any recurring issues. No other fuel line problems in the past year since getting it fixed. Just make sure they properly check all electrical connections during the repair, thats where the real problem usually lies, not just with the filter replacement itself.

I took my car to a new mechanic last week and got everything sorted. The entire diagnosis process matched exactly what you described, they found corroded wiring going to the fuel filter heater plus some damaged connectors. Total repair came to 320 Euro including parts and labor. They replaced the complete fuel filter assembly, all associated wiring, and cleaned out the surrounding fuel lines while they were at it. More expensive than your repair, but they did quite a bit of preventive work. The difference in performance is amazing. Cold starts are smooth now, power delivery is back to normal, and fuel consumption has dropped back to what it should be. Really glad I got it fixed before winter hits. The engine runs so much quieter too, guess I had gotten used to that rough idle without realizing how bad it was. Haven't had any fuel system issues since the repair. The new mechanic actually showed me all the corroded parts they removed, which helped explain why the engine was struggling so much. Much better experience than my previous shop visits.