ericrayl

Car is a 2002 A6 3.0, with around 100k miles. I do all maintenance on it, nothing has been touched recently. Yesterday it failed to start several times. If it does start it starts immediately and runs perfectly. When it doesn't start, engine turns over normally, but no sputtering or partial ignition, nothing.

Specifics-the 3 times it didn't start were all after it had been driven, and then parked for 1/2 to 2 hours, so it was still warm. Daytime temps here are in the 80's.

It has not failed to start when cold.
If I shut it off and immediately try to re-start it has always started.

A couple times I came back a few minutes after it wouldn't start and it started normally and immediately-possibly because I opened the hood and something cooled off?

No codes except after a no-start it throws a bunch of cylinder misfire codes.

Once I sprayed starting fluid in the air cleaner box during a time when it didn't start, and it started, then after shutoff it immediately re-started every time. I wouldn't make to much of this though because whatever condition causes the no start could have gone away just coincidentally at that time.

I suspect some critical sensor is on the way to dying, and doesn't like sitting under a hot hood, but works OK when it cools off. What are the likely culprits?

Thanks!

ericrayl

Alright, I'll answer my own thread. It was a crank position sensor aka engine speed sensor starting to fail. When it would be heat soaked after engine shutoff it would fail. Any cooling including opening the hood for a few minutes and it recovered.

The first replacement unit I bought was a Myele-it was no good. The motor ran but it immediately gave codes P0321 and P0322, and at 4000 RPM the engine would start missing so it couldn't climb abive 4000 RPM, additionally the tach. dropped to zero a few times with the engine running, and eventually the CEL and ETK lights came on.

The second replacement unit was a Duralast #SU4342 from Autozone, which was actually a Bosch unit re-branded. Problem solved.

Here's a few tips on how to change the sensor on a 2002 A6 Avant 3.0 quattro:
The plug is a grey plug on the firewall. In the engine bay loosen the coolant reservoir, unplug it's connector, and flip it over on top of the motor. Remove the coil wire harness plugs and get the harness out way, remove the wires to both cam position sensors, and pull the lower sensor, then you'll have decent access to the plug and for routing the new sensor wire. With the new sensor in hand you'll understand how the plug is clipped into it's retainer-sort of roll the plug counterclockwise to disengage it from it's retainer

Jack the car up, and put a light inside the drivers side wheel well shining through through to the sensor. The sensor is on the right side of the bell housing about 6" above the bottom of the tranny, and just behind the block.

Remove the 3 Dzus screws at the back of the belly pan, and a sheet metal bracket retaining some other plug with a single allen screw into the tranny. Then you can reach the single retaining bolt on the sensor with a socket.

From the sensor the harness runs up through a fabric heat shield which is about 5" tall, and curled around 3 wires including the sensor harness, and retained to itself with 2 snaps. The unsnapping and re-snapping is best done from above, a tricky blind one handed operation. The sensor harness also runs around a tube near the top of the motor to keep it away from the exhaust.