2004 Chevrolet Tahoe, Compressor Will Not Turn On

This 2004 Chevrolet Tahoe came in with the complaint that the a/c was not cold due to the compressor not coming on. Whenever the a/c was turned on the snowflake on the digital display would flash. This Tahoe has auto a/c controls. I checked codes with my Tech 2 scan tool and there was a code B0159 stored for an issue with the ambient temperature sensor. I looked at system data stream and could see that both the raw and filtered outside air temperature readings were at -40 °F and that a/c permission was not granted. Time to go find the outside air temperature (OAT) sensor. The sensor is located behind the passenger side daytime running lamp socket.

The first step is to rotate and pull the pins for the passenger headlight assembly.


Then remove the headlight assembly. The light assembly does not need to be electrically disconnected and fully removed from the vehicle unless you just choose to.


With the headlight assembly positioned out of the way the locking tab for the DRL assembly can be depressed…


…. and the assembly pulled loose from the vehicle.


The OAT position can be seen in the next picture. It is merely snapped into a retaining hole just to the inside of the DRL socket.


The OAT and it’s harness assembly. Note that it is a new sensor that I had just installed.


The old/original OAT in my hand and the new one hanging below.


The new sensor installed.


Testing the sensor wiring is as follows.

The light green/black wire should have a nominal 5 volt signal which is supplied from the HVAC control head assembly on terminal B.

The brown wire is a system low reference signal (return ground) and is supplied from the HVAC control head assembly on terminal K.

The higher the resistance the colder the reading. An open connection between the two circuits will yield a -40°F reading on the scan tool.

The lower the resistance the higher the reading. A shorted connection between the two circuits will yield a 100°F + reading. I did not take an actual reading during this test as it was not relevant to the current issue.

The data will be reported on the scan tool as Outside Air Temperature Raw.

After replacing the sensor the data reading changed to actual ambient temperature which was 95°F. There was also about 90% relative humidity. Those conditions coupled with the pressure to get the job done are why I do not have some pictures. In particular scan tool screen shots.

There will be another data line in the scan tool data listed as Outside Air Temperature Filtered. That data line appears to refer to the sensor used for the thermometer built into the rear view mirror. I also had a reading of -40°F on this circuit as well. The displayed temperature in the rear view mirror fluctuated between -40°F and -26°F. The compressor was still not coming on.

Time to move on to the other OAT sensor. It is located in front of the condenser assembly and is accessed by removing the cover over the top of the radiator and condenser area. That panel is held in place by push pin retainers. The center pin has to be lifted most or all of the way out before the main pin can be pulled free.


With the cover removed and looking down on the driver’s side between the condenser and the grill the OAT sensor can be seen.



Now you would think that by simply changing the faulty sensor everything would go back to work and both the raw and filtered data would be correct with actual conditions. Not so. I suspect that they would have adjusted if I would have had test driven the vehicle but since I was still working on the refrigerant system that was not going to happen anytime soon. Under “Special Functions” there is a command function to perform and “Instant Update” for the OAT. Performing that task updated the data and the compressor came on as it should have. There was no code for the -40°F reading of the Filtered OAT. I had tried using that function before to try and rectify the condition with the faulty sensor and it did not work. It only worked with a good sensor installed. Well kind of. While I was waiting on a new sensor I connected my decade resistance box to the circuits and played around with the values. I knew the signal that was being generated by the suspected faulty sensor was about 1.45 volts and that correlated with a -40°F to -27°F thermometer reading. I found that a .4 to .6 volt reading resulted in a 95°F reading on the thermometer. Disconnecting the sensor from the harness resulted in a reading of OC. Jumping the terminals together with a jumper wire resulted in a reading of SC.  My assumption is that OC means open circuit and SC means shorted circuit. By the way, the above tests that resulted in the OC and SC readings was enough to tell me that the sensor was the problem.

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