New to the forum and I couldn’t find any post with a similar problem. I have a 1997 626 2.0L with 128K miles. The automatic transmission is failing (natch), but recently I was having an issue with low/rough idle and the occasional stall at a stoplight. I cleaned the MAF sensor, the IAC valve, and the transmission range sensor. While I had the airbox off I noticed the hose from the air intake to the airbox had a huge crack in it, so I sealed it with duct tape. While putting the airbox back on I broke the ECT sensor (cheap plastic).

The ECT sensor that NAPA had for my car wouldn’t fit; after some trial and error, I used the sensor to a 1996 manual transmission 626. It fits, but isn’t exactly the same length.

Now the A/C compressor won’t come on – no matter what. The car is extremely difficult to start when cold, but when I disconnect the harness to the ECT sensor, it starts right up. Also the secondary radiator fan does not come on, and the engine stumbles on acceleration from a dead stop. I’m wondering if the wrong ECT sensor could cause all of this? Thanks in advance for any ideas.

 

I'm basing this on my '99 Protege, 1.8L FP engine, and manual transmission, but it's generically valid for your machine.
This sensor is a thermistor and tells the PCM what is the coolant temperature. The symptoms you describe match a situation where a mismatched sensor is telling the PCM you are running a hot engine.
The A/C compressor and fan impose a heavy accessories load on the engine, and the hot air extracted from the car imposes a heavy thermal load on the radiator too. To protect the engine from thermal overload damage, the PCM will prohibit operation of the A/C in its entirety in such an event. It will also throw a CEL, and may (don't know for sure), put your car in limp mode, in which case if an automatic, may prevent the transmission from shifting up from 2nd gear (not good for the transmission if prolonged).
Needless to say, the engine fuel/ air ratio will be completely out of whack and your car will neither run nor idle worth a damn.
The correct voltage range for the sensor output is not less than 0.2V and not greater than 4.9 V.
Looks like you'd better match that sensor with an exact or equivalent, based on the Mazda part No.

 

Thanks, oldeng. I'm going to run up to the wrecker yard and see if I can't get lucky today. Otherwise, I'll order a sensor tonight.

 

So I got the correct sensor from Advance Auto Parts and I have air conditioning again! Just in time for rainy season in Florida. The Mazda dealer quoted me $60.25 and said it would take 3 days (Mazda part # F7B918840), while Advance cost $16.29 (Advance Auto Parts #WT5053). Looks like NAPA is a bunch of morons, and I'll be shopping at Advance Auto Parts from now on.