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Tech Article

Audi C5 A6 Quattro 4.2 V8 Error 17748 - P1340 - Camshaft Position Sensor (G40) / Engine Speed Sensor (G28): Incorrect Correlation

Last Updated: 09/24/2017
Parts Group: Engine speed sensor
I recently bought a 2000 Audi C5 A6 with the 4.2 V8. As I worked on this car, correcting various little issues with it, I formed the opinion that whoever had worked on it probably before hadn't done the work to ... how do I say this politely ... Audi standards. The work that had been done left some not-so-impressive tell-tale signs as such.

The car has more than 170,000 miles on the odometer, and it's reasonable to guess that by now at least a couple of timing belt changes have been performed otherwise the car might be unlikely to still be driving. My guess is that the last one probably wasn't done up to Audi standards either.

This engine has an engine speed sensor mounted in the bell-housing of the transmission, just above the rectangular, green transmission ID plate. Audi refers to this part as G28. As I recall it senses when a particular spot on the engine flex plate is passing by the sensor. Although it senses engine speed, it also senses the engine position, specifically the lower part of the engine including the crankshaft.

This engine also has a sensor for the upper part of the engine, specifically the position of the camshafts. Audi refers to this part as G40.

It is vital that the camshafts be positioned correctly relative to the crankshaft, so that the valves (mechanically linked to the camshaft) open and close in harmony with the pistons going up and down (mechanically linked to the crankshaft).

A bad misalignment can cause a valve to still be open when the piston moves up, thus impacting the valve and damaging the valve, the piston or both.

A not-as-bad misalignment is still not great, since it can mean sub-optimal compression.

On this car, we changed the spark plugs and did a compression test, and one bank of cylinders had consistently lower readings than the other side. This might well fit with the camshafts on that side being misaligned relative to the crankshaft.

The VCDS software from Ross-Tech also reports error code 17748 - P1340. It describes this as incorrect correlation between the Camshaft Position Sensor (G40) and the Engine Speed Sensor (G28).

This means that the Audi's electronic control unit compares the values from these two sensors, and can infer when the camshafts are out of alignment relative to the crankshaft. I'm impressed. One of the nagging concerns I often have when doing a timing belt replacement on BMW M20 engines is: I hope I have the camshaft positioned correctly. I check and double-check because I am always concerned that I'm off by a timing belt tooth as to having the camshaft positioned correctly. With this Audi engine, I needn't worry. If there's a problem, the car will so inform me. I'm impressed.

That's one more reason by, from now on it is tempting to make it a "best practice" in my shop to start the engine and look for this error, before putting the entire front end back together and then having to tear it apart so as to correct the position of the camshafts. However, we do make a point of always renting the timing belt tool kit from Blauparts. Their tools make it, as far as I can tell, impossible to misalign the camshafts.

I did some reading on the Web to see if how others have experienced this error message. In one case, someone's car reported the message, and the person had the timing belt changed, and the camshafts brought back into alignment, and the message went away. Someone else experienced the issue in the opposite direction: didn't have the message, then had the timing belt changed, and the camshafts went out of alignment, and then got the bad-news message as such.

As a result of what I have learned, this car is being scheduled for a timing belt replacement sooner rather than later, and in the process, I expect we'll correct the camshaft alignment too.

We do sell good, guaranteed, used G28 units: However, if you get this message, especially after just having bought the car, or having had work done, then it is probably prudent to not simply assume that it's caused by a bad sensor.
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