The problem:

  • Mazda 3
  • 2008
  • 125 K miles
  • this is the first major ( > $300) thing to need fixing on this car

The problem originally was that when you turn the AC on you get some cool air for about 20 minutes. Then it slowly gets warmer until the air coming out is the same temperature as the outside.

What the mechanic did to fix the problem:

I took my car in to have the AC fixed recently. The ASE certified mechanic (small shop) did a test and found that the system wasn't leaking. Then he found that the condensor was damaged and so he replaced that.

But then he said the computer wasn't turning on the sytem so he replaced a wiring harness that was part of a recall (as in this wiring harness had a recall but I had never taken the car in to have the recall fixed). THEN he said the computer had to be reset at the dealer (he didn't have the right equipment).

My mechanic is now saying to me "the dealer says the expansion valve is clogged and needs to be replaced and their quote is $1200... but I'm not sure if replacing the expansion valve will fix the AC".

(After replacing the condensor and wiring harness, turning on the AC button does not have any affect on the temperature of the air coming out.)


I don't want to spend hundreds of dollars to replace a part that may or may not fix the problem. If it were a small amount ... I might go for it. But for a job that will cause $600-1200... I would like to know with greater certainty if replacing the expansion part will fix the problem.

Is there some test I can have a mechanic do to verify that the AC expansion valve is broken and needs to be replaced? Or do I have to just spend the money and hope the AC works again?

(If there is a test to check the AC expansion valve, how do I ask to have such a test done?)

  • 2
    Wait, your mechanic replaced your condensor (evacuated the system, removed old condensor, installed new condensor, and added freon and oil), but didn't replace the $20 expansion valve while he was in there?! I am 99% sure that it is standard practice to replace the expansion valve or (orifice tube) and the drier whenever any A/C work is done. Aug 27, 2015 at 17:48
  • So what was wrong with the AC to begin with?
    – Zaid
    Aug 27, 2015 at 17:54
  • @zaid After taking the car back I spent some time researching a better mechanic and I went to the new mechanic. I shared what the dealer's mechanic had said about the expansion valve being clogged. The new mechanic did his own analysis and he found the expansion valve to be the issue. His estimate was $850 and he did the job for about. Having a mechanic that you can trust and is a good communicator and very competent and meets his estimates makes a world difference!
    – syn1kk
    Sep 19, 2015 at 11:31

1 Answer 1


An air conditioning system is separated into two sides, high pressure side and low pressure side. There are service fittings on each side. If you think of the AC system as a hula hoop, the system is separated on one side by the compressor and on the other by the expansion valve. The high side has the condenser and the low side has the evaporator (the cold thing inside the dash). To check the system a mechanic would use a manifold gauge set, which has one gauge to show the low side pressure and one gauge to show the high side pressure.

The test for a clog in the system is to install the gauge, the pressures should be equal on both sides. Run the car with the AC, compressor running and the air in the car blasting full force. The low side should go down to around 30 psi or so. The high side to 250 psi or so. If there is a clog the low side will draw down into a vacuum and stay there. The high side will go way past 250 psi. This test only shows the existence of a clog and can't say where the clog is. Also it is a concern regarding where the clog came from.

The given numbers aren't a hard rule, more like a general guide line. Every car is slightly different.

  • re - "This test only shows the existence of a clog and can't say where the clog is": it sounds like you are saying, the expansion valve could be the problem but there there is more than one thing that could cause the test to fail?
    – syn1kk
    Aug 28, 2015 at 12:01
  • Correct, the test shows whether refrigerant is moving form the high side to the low side or not. When it's not then more investigation is required. If i was diagnosing the car and i found a clog i would use the process of elimination. Evacuate the system and blow through all the hoses and parts either with compressed air (lightly) or use a rubber hose and use my mouth. If every component is unrestricted then replace the expansion valve.
    – vini_i
    Aug 28, 2015 at 12:36

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