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Q: A/C quits cooling at idle on 2004 Lincoln LS

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While going down the road the a/c cools fine. If you stop at a red light, it keeps blowing, but not cool air. Local mechanic checked and said the high pressure side was reading 140 -should be 225 or so. Suggested replacing compressor. Does this sound correct?

To properly diagnose, it is important to have both readings, LOW pressure side and HIGH pressure side. You said the HIGH side indicates a pressure of 140 psi which is low, but what does the LOW side indicate? If the LOW side pressure is high (above about 50 psi) then I'd say, yes, the compressor may be bad. However, if both the LOW side and HIGH side are indicating low readings, then the problem might be simply that the system needs to be charged (add refrigerant). Or, it could be that the EXPANSION VALVE is stuck closed.

So, as you can see, the LOW and HIGH side readings need to be checked at the same time. Just looking at one or the other is not ever sufficient enough for someone to diagnose an AC problem.
The Air Conditioner compressor has lost the ability to operate properly if unable to produce more than140 psi with system fully charged. A low Refrigerant condition could also result in low readings in high pressure system. When was the last time Air Conditioner was serviced and inspected for leaks? If Compressor is replaced, Air Conditioner system filter should be replaced also.
the same thing happens to my Lexus and was told I need a new throttle! A Lexus dealer told me this. I don't go there anymore but still have the problem.
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