Pontiac Grand Prix Dual AC Climate Control Problem

Reader Question I have a 1999 Pontiac Grand Prix Daytona Pace car. I have a problem with the dual climate control. When I start the car for the first time in the morning the digital readout shows the outside temperature reading almost in the freezing area when the actual temperature is in the 90’s.

Therefore, my air conditioning will not blow out any cold air until the read out climbs above 60 degrees. The only way it will climb away from the freezing read out is for me to get on the highway and drive at a steady speed (around 65mph) for about 3-5 miles, the readout starts to climb to a temperature that is closer to the real outside temperature. However, if I stay in town and do not get to a steady high speed, (like stop and go traffic) it never changes. It seems like the thermometer is not working any longer.

I bought a repair manual and looked for anything that would lead me to where the problem could be, but all it shows is replacement and the steps I would need to take to replace the complete unit. I want to make sure if I replace this dash control head, that will solve my problem.

On the other hand, is there a thermometer somewhere that may need replaced or just cleaned? Where dose the temperature read out get its data.

Thanks in Advance


Hey there Larry,

My top auto AC guy says this:

I would replace the “ambient outside temperature sensor” first and see what happens. It is located behind the front panel mounted to the radiator air baffle. Has two wires to it, yellow and light green with black tracer. This is a dealer only part…I think, but A/C Delco might also make it available at the auto parts houses.

I hope he is correct, let me know what happens will ya?


Austin Davis

Reader Follow up


You can tell your top AC guy he was right on with that fix. I replaced the Ambient-Outside-Temperature-Sensor located in front of my radiator, on the left hand side, attached to the plastic panel, about midway down.

I noticed the connection pins were a little rusted when I removed the old sensor. Therefore, I clean the pins then applied and little electrical connection lubricant before I connected the plug into the new sensor.

I started the car and the readout on the Climate-Control Head unit displayed the correct outside temperature. The AC system started immediately. This morning everything still is operating as planned.

Note: The part only cost a little over 10 bucks.

Thanks again for your time and the correct analysis.


Posted in: A/C and Heating

14 Comments on "Pontiac Grand Prix Dual AC Climate Control Problem"

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  1. Bri says:

    What if we have replaced the Ambient Outside Air Sensor and the air and outside temp. reading worked for a few days, but then went back to saying it was 20 degrees and ice outside. What would you try next?

    • By Austin Davis says:

      I would test the dash controls and see if there is a problem with the “A/C control head” the part on the dash which has all the A/C and heat controls. they do sometimes go bad, and they are not easy to test unless you have the proper training and equipment. The dealer is probably your best bet for these kinds of complicated diagnosis issues. You can always get another mechanic (or yourself) to do the actual repairs once you are sure of a proper diagnosis. A used control head from a junk yard might be a good option if that is the culprit.

  2. sw says:

    yes, thank you I get that … I have seen a lot of others with these problems… so to answer your questions; the coolant is checked regularly the, dash/control lights up, and has returned to intermittently working and on some controls ie ( 2 or 5 ) work randomly. I think we will try the relay switch and see if that works

    • Austin says:

      if its the BLOWER that is intermittent, I would call the dealer and see if this has a blower motor resistor, when they fail only certain speeds work….usually it defaults to high speed only. Worth a guess, if you just want to guess at something.

  3. sw says:

    2001 Pontiac grand prix, the heater worked great up until about 4 months ago. In the beginning the heater would randomly not come on or would only come on in certain control (feet, hands, both etc.) or controls (1-5)… it gradually got worse until it just quit working… anyone have any ideas as to what is going on or how to fix it? Thanks

    • Austin says:

      My first thought is, do you have enough coolant inside the radiator? Check and top off with coolant as needed, that would eliminate a mechanical issue of not having enough coolant in the system to heat the heater. Now, if you have a full radiator, you might have a “control” issue, like a problem with the dash control unit or a problem inside the dashboard not allowing the control unit to work properly. Unfortunately that is going to require a mechanic to troubleshoot for you further. If there is NO air coming out of the vents at all or is a smaller amount than it used to be, you most definity have an issue inside the dash (like a bad blower motor) or a control issue. If the blower motor is bad, you will not have any air coming out the vents on A/C mode either, so if you have air with A/C mode, but no air on heat mode, then you have a control issue. understand?

  4. John R. says:

    The Ambient Air Temp Sensor is most commonly the issue here guys but it is NOT a dealer only item most parts stores have them on hand. What I can’t find is the dual climate control unit it self the temp sensor didn’t fix my issues with the thermometer readings being way off. Any sugesstions?

  5. billjacko says:

    It looks like you can get the part at http://www.rockauto.com for about $8-$9. I’m going to buy it and see if it solves the problem because it is an annoyance now in the summer, but it will really suck in the winter 🙂

  6. Joe says:

    I should have said 1999 Grand Prix GTP…. Joe

  7. Joe says:

    I am not a car mechanic, but am willing to try and replace the outside air temp sensor, as you mention. Where, on the 1999 Grand Prix can I find it? Can you give me the location in more layman’s terms, please, and also describe what it looks like? Thanks, Joe (FYI, I added R134 freon, and it just barely cooled the passenger side, and not the driver side. so, I wonder, also, if the dual AC vent doors are stuck…) (will this get to my email address? I am not sure I can find this string on the website, again…) Thanks, Joe

  8. Brandon says:

    i have a 2000 GTP that reads 59, 60 outside. today it actually read 90, then when i started it again it went back to 60. ive been searching the net for hours and looking at my car. is there any way you can take a pic of what im looking for and post the website URL so i can order one? Thanks!

  9. Brian Z says:

    I just did the same fix on by 1999 GTP and it was just what it needed. A cheap and easy fix for an otherwise complicated looking problem.

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