When my son bought his 96 A4 2.8L Quattro the CEL was on with a P0116 Temperature Sensor - Implausible signal code present (along with a few others). I replaced the original black 4 pin square connector Engine coolant sensor with the supposidly updated blue version, flushed the coolant system and added the correct amount of VW/Audi antifreeze to the system and all was well for about 100 miles and now the code has returned. I have actually seen the temperature value change from something like -47 degrees to +50 degrees using VAG-Com once. The dash gauge seems to indicate correctly, eventually reaching about mid point on the gauge as the car is driven. The question is, is it more likely just another bad sensor or bad wiring back to the ECM? There are no other accompanying codes that are coming back with it, just this one.
I don't know, went and got my last scan the full error message is 16500 - Engine coolant Temp Sensor G62 Implausable Signal P0116 - 92-00 Unknown Error Elaboration.
Does the ECM compare this signal to any other signal, like transmission temp, or oil temp and look for a difference? I can't really find a decent description of the error. I'm used to US cars which have a decent diagnostic tree that says exactly what causes this error to be set, but I can't find any such info for the Audis. I think the 92-00 coding means the output from the sender is not what the ECM is expecting. 35-10 is the code for an intermittant problem so I don't think its a wiring issue but you never know.
Guess I'll pull the cover off the ECM and try cleaning up the contacts this weekend. May have to do a thermostat though.
Heres another possibility, the High pressure switch on the A/C system is open so I can't get the air conditioning to work yet (its 50 degrees in Mass now so thats a spring project). Theres some pressure in the system but I don't know if its actually the low pressure part of the HP switch (its supposed to be both a low and high pressure switch) or if the switch itself is dead.
Ever since he's owned the car the drivers side fan starts to spin as soon as the car is started and it shuts off when the car is stopped. The passenger side fan has never run don't know if thats because the A/C isn't functional, the engine temp never gets high enough, or the fan is dead. Is the fan supposed to run when the car is started, or is it supposed to start at a certain temperature of water in the radiator? If its not supposed to start immediatly could this cause the implausible error code to be set?
Think I found the problem. While the car is idling the temperature gauge will go up to about the middle of the gauge but as soon as you start moving at any speed the temp comes back down to about a tick or two above the C on the gauge. So i figured the thermostat was stuck open. Went out bought a new one and put it in (havent run it yet because i broke the small vacuum line that runs from the black plastic vacuum reservoir next to the ignition coil up under the manifold to the solenoids on top of the throttle body and itas about 30 degrees outside now) Anyways I took the old one and stuck it in boiling water, and it opened fine right around 190 degrees, but then it stayed open as the water cooled down, it was still partially open (maybe 50 percent) when the water was cool enough to reach in and pull it out with my fingers. Finally closed all the way sitting on the kitchen counter at about 70 degrees. So its sort of working but this has got to be whats giving me the implausible signal error and the check engine light. Know more tomorrow when i run it. Hopefully I'm not going to have any leaks.
The heating up issue was a stuck open thermostat, the car now heats up fine. The A/C system was because the wrong HVAC controller was in the car. some previous owner must have replaced the unit before my son bought his and it was the wrong one. There was an 8D0 720 043B unit in there, which is used in 97 and up A4's. The 96 is a unique year in that it seems to be a transition car from the 95 to 97 model years, it has all kinds of one off controllers in it and the HVAC controller is one of those. There are two controllers used based on what type of compressor is in the car. The transistion between the two is right around a cars production date either before or after January 1 1996. The later version (which my sons car is) uses and 8D0 0420 043H controller and only this controller, nothing else will work. The earlier unit part number ends in 043F and I have never seen one of these for sale anywhere. Once I got the right controller, with a usable display the A/C system works fine now.
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