My check engine light comes on after the car is completely warm and at idle, with a code 23 (MAT sensor). IF i am rolling, at about 15mph or more, it doesn't come on, or goes off. I replaced the sensor about 3 months ago, and the problem went away until now. I see no decrease in power or anything, but does anyone have any ideas on what to do besides buy a new sensor? I dont want to keep buying 22$ sensors that go out this frequently, especially when it doesnt seem to harm performance at all. But the bright orange light also gets annoying. What exactly is ithe MAT's function anyway, besides telling the ecm the temp of the air cleaner (does it affect fuel mixture?).

 

If I were you, I'd check on the harness connector (corrosion on terminals, broken wire inside the connector), and check for any missing insulation/wire breaks on the harness wires themselves.

 

All wires look clean, connector is in great condition... i just checked the wiring back to the harness... it all looks like it is in decent shape. Any other ideas?

 

It really is only used in open loop mode, before the O2 sensor is warmed up and working properly.

According to the error code list in the tech section of this site, code 23 could also mean that you have a TPS error, so you might want to chack that. Also are you sure that you are getting 23, not 24?

I used to get a code 24 when I sat at idle, didn't see a difference in power, but did have a slightly high idle. It turned out that the return spring on the TB wasn't closing the plate all the way, but far enough to make the TPS think that it was closed fully. When the ECM saw that the idle speed was up, but there was not throttle opening or speed, it thought that I should have been slowing down (decel) which would account for the idle speed with no throttle. With the VSS not showing a speed, it thought that the VSS was dead.

As soon as I started moving, got up to around 15 mph, the light went off. I went down to AutoZone, picked up a carb spring kit, used one of them to make my throttle plate close all the way at idle, and the problem went away.

Long story short, the MAT is used mainly for cold start and open-loop, not so much in closed loop.

 

Ill have to check the return spring, but my idle seems normal. I had a slight problem with it the other day when my TPS stuck on a little for some reason, but i pushed it back and hasnt caused any problem since. The car gave my this code once before, i repleced the sensor, and it went away for a couple months. I'll check the return spring idea, might be a viable solution.