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Tech Article

Audi C5 A6 Quattro 4.2 V8 Engine speed sensor

Last Updated: 09/24/2017
Parts Group: Engine speed sensor
This article discusses the engine speed sensor for the Audi C5 A6 with the 4.2 V8 as made from 1999 to 2004, with the main focus being on how a clueless repair attempt can mess things up. If you're not here for the technical analysis but would rather buy a used unit from us, guaranteed to work and fit, then please select the link below: The information herein is based on our own project car. a 2001 4-door.

The seller had had the timing belt done ... not at the dealer, who charges $2K or so, I've read. Instead, he had it done at a nearby service station. When I heard that, I though that this seemed to be a gamble, but ... maybe they knew what they were doing, and they charged only $800 or so. I can understand the temptation.

Next, the engine speed sensor had failed. The seller had explained that the dealer had wanted $800 or so to fix that, and that had been the straw that broke the camel's back. He decided to sell the car. For $1,000, I bought it and had it transported to my shop. I ran some diagnostics and saw error code 16705, which means: an implausible signal coming from the engine speed sensor. That reconciles to what the seller had said.

He'd also said the car won't start, but on a hunch, I tried to start it anyway. It started, but ran very, very roughly and then stalled. I turned my tech loose on the problem. First of all, we studied an out-of-the-car engine and transmission so we could conveniently see where the engine speed sensor was attached. We saw that it was attached just above the rectangular green-and-silver plate that identifies the transmission, on the driver side, in front of the front drive shaft. From there, we wend back to our project car -- and saw that someone had been messing with the wiring for the engine speed sensor. All three wires had been cut and then reattached. My tech compared how the wires were connected to the wiring on a similar car ... and found that they had been connected incorrectly. He broke the connections, connected the three sets of wires correctly and ... the car started, and idled just fine.

Even so, there was an odd noise coming from behind the engine, from where the engine speed sensor was located. He removed the part (one Allen head bolt, size 6) and found that it had beem hewed up pretty badly from hitting something that was rotating. The reason was obvious: the correct installation uses a spacer plate, and the engine speed sensor had been installed with this spacer plate omitted, so the sensor was extending too far and was making contact with whatever it was supposed to be measuring. The sensor was still working but it was in pretty bad shape, so we replaced it with a good used one that we had. We started the car again ... and again it idled and revved nicely but the alarming noise had also vanished.

Out of morbid curiosity, I just looked into the new prices for this part. Assuming I'm looking at the right part, a new Bosch unit can be had for less than $50, and for someone who'd never done this before, the remove-and-replace work took less than an hour. One what premise someone would quote $800 to do the work, I cannot imagine.

Anyway, for $1,000 plus the trivial price of transporting the car to my shop,and the trivial cost for today's fix, I have a nicely running 2001 Audi A6 Quattro 4.2 V8. Life is good.

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