Audi EGT Sensor Repair

There is a car in my driveway that I have been struggling with for about six months now. I am at the tail end of it now, and it has taken an exuberant amount of time, energy, and spilled fluids, but we won’t get into that just yet. Keeping it light, here is a fix for faulty EGT sensors, common on 2.7T-engined Audi A6’s.

The Exhaust Gas Temperature sensors measure the temperature of the exhaust gas, so the turbos and cats don’t melt. But there is a catch. For some reason or other, Audi decided that they only want to read temperature between 945C and 1035C.That’s pretty freakin hot, and the car shouldn’t be seeing those temperatures very often. If the ECU reads temperatures it deems too hot, it can cut ignition and timing and put the car into limp mode. When these sensors fail, they will either display 945 or 1035, in my case, 1035. The car gives an error code ‘EGT short to plus’, with the corresponding sensor reading showing up as that max value of 1035. When the ECU sees this, it throws a fit, and performance is effected quite a bit.

 

Mounted to the back of the intake, the EGT’s consist of two modules with sensors (Shown below).

With the y-pipe removed (not shown), rag is in place to keep small animals out of the engine.

The black and red boxes middle left contain the suspect malfunctioning chips. The actual sensor probe itself threads into the exhaust manifold (lower rear of passenger side engine).

The passenger-side sensor and box came out really easily for me, but the driver side is nearly impossible to get out, so I decided to only remove the module box, and cut the sensor wire. I attach an audi two-wire connector to the ends of the module and sensor wires. This is not 100% kosher, but it allows me to repair the sensor box, as the temperature probe itself is rarely defective.

Tools needed to remove the two sensors- 17 & 10mm wrenches, flat screwdriver.

Tools and supplies for repairing the sensors- soldering iron, flux, solder, RTV silicone, wire cutters/ strippers.

The process to replace the circuit board is fairly straightforward:

Open the box(es) up, and scrape out whatever sealant is in there. I attempted to repair the red (passenger side) sensor already by resoldering the joints, however this didn’t work, as the board itself was fried.

Unsolder the 3 pin legs and 2 sensor legs from the housing; the legs bend up pretty easy once the solder is melted.

The new chip is a little smaller than the OEM one, and uses short lengths of wire rather than rigid lets to attach to the housing. Before installing the new board, I wanted to make sure the sensor probe was working properly. The probe is simply a resistor that changes resistance proportional to its temperature. Using an ohm-meter, I measure a room-temperature resistance of 2.2 ohms. Heating it up, the resistance increases to 32.1 ohms, and as it cools, I watch the resistance steadily drop back to single digits. That seems normal. I foolishly grab the probe and burn myself.

After verifying that the temperature probe is still working, it is time to solder the new board in.

Instructions supplied with the new boards indicated the board should be held in place with double sided tape, and soldered to the connection points. The new board is sealed with an epoxy, so it is only necessary to protect the connection points and seal the outer perimeter of the box. This sealing requirement was accomplished using an RTV silicon around the wires and solder joints, and a bead on the rim of the cap.

Now just need to put the cap on, and squeeze it in the vice for an hour. Lets look at the other sensor.

This is an original factory sensor (see date stamp, ‘99), so it is potted in silicon to prevent moisture from fouling the electronics. Removing all silicone is made easy by using an old toothbrush (OBVIOUSLY DON’T REUSE THE TOOTHBRUSH). Replacement of the board is the same as for the passenger side.

Here are the two finished sensors with new boards and sealed with silicone:

Installing them back into the car is the reverse of removal, and went fairly quickly thanks to my extra connector on the driver side sensor. A quick test fire, and checking the EGT measuring block with the code reader verifies that the sensor are repaired.

Note: the idea for this repair was developed by some innovative Polish and American Audi enthusiasts. Full credit to the Audiworld post for the idea to replace the boards, and for much of the instructions on how to repair them.

LINKS: http://s4sid.pl/index.php (guy I emailed for the circuit boards- google-translated english to polish)

http://sidmotorsport.pl/pl/ store that has EGT boards (show as out of stock)

http://forums.audiworld.com/showthread.php?t=2850845 Forum post with info about replacement boards.