trackfactory

My apologies if this is long winded and redundant, I have a 2008 Q7 Premium 3.6. A couple of weeks ago she started acting erratic and I noticed the temperature gauge acting erratically. The car started stumbling and missing but straightened itself out after a few seconds. The engine sounded loud and I noticed the cooling fans were on hi and the temp gauge stayed at cold. After I parked her, I noticed the cooling fans stayed on. I looked up the symptoms and most pointed to the temperature sensor. I ran a scan using my VAG and found quite a few errors, including P3081, mis or no communication with the ECU, and ECU power relay. I inspected the ECU and sure enough, there were signs of water on the casing.

Here is what I have done so far:

1. Replaced the ECU power relay with updated relay as per TSB
2. Sent my ECU out for repair. Shop said there was a SW failure on the board and no signs of water or physical damage so there was no reason to open the unit.
3. Replaced the front coolant temp sensor.
4. Went into VAG and cleared the codes. MIL wouldn't clear unless I went into the specific module (Engine) and cleared it. Noticed as soon as I cleared the code, temp gauge went up to the middle and the engine revved up over 1000 RPM and went back down. Gauge went right back down as well and stayed at cold.

Since reinstalling the ECU and the front temp gauge, the car runs fine, but the temp gauge still stays at cold and the fans run all the time. Also have a P3081 code that will not clear. I searched the forum and found that there is a rear temp sensor that almost always causes these symptoms, but I could only find on A6 or other models. Couldn't find anything on where I could locate the rear coolant sensor on my engine.

I know this is a long-winded way of asking if anyone could provide me with a diagram or pics that could point me in the vicinity, but I wanted to provide some background first.

I want to replace that rear coolant sensor next, but can't find anywhere that gives me a diagram or pic that shows where it is on a 3.6.

PS, all ECU communication errors and codes have disappeared after having the ECU repaired, just wondering if a faulty component could be the culprit in taking out the ECU.

Thanks in advance!

trackfactory

Update:

Went to the dealer this morning to see if I could get some info.

Parts guy said the temp gauge/low temp code is probably due to aftermarket part. He spoke to one of the techs who also confirmed the problem still exist due to aftermarket part. Purchased the OEM sensor for $70! Also printed me out a diagram and confirmed that on the 3.6 there is only 1 coolant temp sensor. Got home, popped it in, and...

Problem still exists! Code/MIL won't clear, MIL on, fans on all the time at high speed.

Anyone???

Ryan Paz

Have you been able to fix the issue, I'm having this same issue right now. I replaced the radiator, waterpump, thermostat and temp sensor and my gauge still bounces to overheating. BTW I have a 2009 audi q7 3.6l.

moe akra

My q7 is doing the same thing did u figure it out temp on high fan on high but not overheating replaced thermostat. Temp on scanner reads 124

Steelcoil

I have replace the temp sensor on the lower radiator pipe on the passenger side, as well as the one near the center top of the engine by the firewall. That one back there was hard to see (try a mirror with a flashlight), you follow the "crack pipe" (coolant cross over pipe from the water pump) all the way back there. The sensor is on the plastic pipe on the firewall end. I used inexpensive Meyle sensors from a local store, no problems at all.

The problem with fan running on high for me was the dealer removed the radiator cap to check coolant level while the engine was still warm. That resulted coolant boiling and then air getting trapped in the system. The solution was a vacuum fill of the cooling system and that solved it for me anyway. I do not know if this is your case.

There is supposedly an electric water pump back there too, and I do not know if that one is causing your problem.

Richard Harding

Thanks for sharing about the 2nd coolant temp sensor and it's location. It helped me tremendously. Much appreciated.

'10Q7TDI_Prestige'

Sadly, the dealer parts counter sold you a bill of goods on that temp sensor, as well as misinforming you on how many coolant temp sensors are involved with engine temp regulation/management.

Audi/VW don't make any parts...they make huge money off of selling owners their OE branded parts their other parts suppliers make/package for them. Good news is same suppliers sell exact same part under their own brand name for a lot less $$$ than Audi/VW does, and generally there may be at least a few aftermarket suppliers on offer as well.

FWIW, I generally order parts online; paid $8.12 for coolant temp sensor (sourced from FCP Euro; Hella branded packaging) to swap out proactively during a T-stat ($35) replacement, and FCP also lists/stocks several other well-reputed brand names on that coolant sensor; all priced within a few dollars of each other. You also get a lifetime warranty on what they sell, so if in it for the long-term might wanna look them up.