This timeless luxury sedan definitely turned heads by when they started making these models from 1997-2001 equipped with the V6 2.8l engine.
This vehicle was loaded with every luxury car amenity that you could ask for including power-everything, thick leather seats, voice guided navigation, Bose 8-way speaker system with a Bose subwoofer, active motorized headlight projectors (some models), and even a motorized headlight washer. Talk about a car that was way beyond it's time!
However being beyond it's time and squeezing in every bit of tech and engineering in this luxury sedan may have had its drawbacks as the years went on being past down from owner to owner.
Common problems:
Clogged Breather Hose (Crankcase ventilation) Piping
The piping that allows the pressure in the crankcase to be relieved often has a tendency to be filled around the walls with carbon. As an effect the pressure in the crankcase can build up which therefore leads to the next problem.
Valve Seal Oil Leaks
This model along with the 2.7t models are known for having valve seal leaks which allows small amounts of the oil in the engine to leak into the cylinder and be burned up therefore leading to excess carbon build-up
Coolant Leaks
Be on the lookout for small puddles of coolant and keep and eye on the expansion tank level because coolant leaks are notorious for most of the A6 c5 models. There have not been many issues of head-gasket failures for these models so the coolant just tends to leak out the vehicle rather than in the engine. The leaks tend to come from coolant piping and connections especially the ones with plastic parts.
Electrical Problems
There is so much tech and computers on board this vehicle it becomes a headache to pin point certain electrical issues. Which is why this is one job that requires skilled technicians that have experience working on European luxury vehicles.
Vacuum Line Leaks
You may often get a code that says the vehicle is running too lean. If this is the case I would recommend double checking the vacuum lines because after some years the do get extremely brittle. So while working on you car make sure to be gently around the vacuum lines
MAF Sensor Issue
These mass air flow sensors are known to cause a "running too rich" problem as the car is not reading the correct flow of air. It can also cause a "lean" situation as well. If you have tried to clean it with the MAF sensor cleaner and there still isn't any difference. Try unplugging it and then turning on the car. If the car doesn't seem to run and worse than it has been then your culprit is most likely a faulty sensor. I would recommend to NOT go cheap on this sensor. Go with a reputable brand so that you just need to pay once and be done with the issue.
Front Axle CV Boot Torn
For some reason these boots are was more susceptible to tearing than others. Even the re-manufactured axles have the same problem. The CV boot will tear first and then allow dirt and debris to enter the CV joint furthermore making the axle click while turning. However, if you have this issue you can follow this guide on how to replace it yourself.
https://youtu.be/91S5ZIulO2Y?si=fqgmGIg6mObuvaJl
Brittle Interior Plastic
If you own a c5 model after this many years, be careful while remove plastic trim pieces because after this many hot & cold cycles the interior plastic can be broken to pieces with minimal effort
Not so common but not uncommon... (Personal Experience)
Clogged AC evaporator ducts leading to water inside
There has been an instance where the AC evap ducts were clogged during a road trip which therefore led to water being built up in the passenger side area. Mind you the weather was unbearable hot & humid so there was no choice but to leave the AC running during the entire trip while occasionally soaking up the water on the carpet with a towel and wringing it out.
As an effect of the water moving around, the water actually fried the TCM (Transmission Control Module) which made the vehicle run on "limp" mode which locked the gear into 3rd gear.
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