Check Engine Light Anomoly!!

Hi,

I am hoping someone can solve this issue that I have with my check engine light on my 1994 cavalier. Pretty much; I am getting a code 45 and code 25. The code 45 is oxygen sensor reading rich and the code 25, is the MAT (or IAT) sensor being shorted.

After replacing both sensors and checking for any loose wiring or grounds and finding nothing, I have a pretty good idea what is causing the probelm, but before I mention it and discuss with you all, I am hoping anyone can share some information or suggestions as to what is causing the check engine light to come on. Thanks again.

- Greg

A little more info would help i.e. what size engine ?

the car is a 1994 cavalier 2.2 L

Also, I should have mentioned that the check engine light comes on just after start up.

Usually the car will run for about 2 minutes and then the light comes on along with the fan and the temperature is still at 40 degrees. After about 2-5 minutes, the light goes off and the fan goes off as well.

I have checked all the wiring and checked for any grounds, replaced both sensors.......

So basically, i figure the MAT (IAT) sensor is sensing extreme heat when the engine is completely cold. I roughly know how to fix this problem, but I would like to hear your input first...........

Usually the car will run for about 2 minutes and then the light comes on along with the fan and the temperature is still at 40 degrees.

Are we talking about coolant temp at 40 degrees or the IAT (Intake Air Temp) which is the sensor in the snorkel that measures the air temp going into the engine. Two very different things.


I have checked all the wiring and checked for any grounds, replaced both sensors.......
You replaced the O2 sensor and the intake air temp sensor? Are you sure you shouldn't be looking at the coolant temp sensor? I would bet that the O2 did not need to be replaced and was reading rich as a result of the engine running rich because the PCM thought it was at 40 degrees instead of what it was actually at (much warmer) which would require a leaner mixture.

Are we talking about coolant temp at 40 degrees or the IAT (Intake Air Temp) which is the sensor in the snorkel that measures the air temp going into the engine. Two very different things.


You replaced the O2 sensor and the intake air temp sensor? Are you sure you shouldn't be looking at the coolant temp sensor? I would bet that the O2 did not need to be replaced and was reading rich as a result of the engine running rich because the PCM thought it was at 40 degrees instead of what it was actually at (much warmer) which would require a leaner mixture.


What happens is, when the car is initially started, the temperature when a car starts is 40 degrees after I think about 30 seconds or so, maybe even a minute. The IAT sensor takes a temperature reading in the air duct where the air filter is.....now for some strange reason the cooling fan is turning on as soon as the check engine light comes on.

Well, this is what I have come to conclude:

I think the EGR valve needs to be replaced.

Since the oxygen sensor is reading rich, the valve must be stuck open or just shot. This will cause higher amounts of fuel to pour into the exhaust manifold and thus setting off the O2 sensor and reading is RICH.

The reason for the MAT (IAT) sensor getting such a high temperature is because of the EGR valve. The valve must be shot becuase high temperatures are present due to the EGR valve not being able to have the correct A/F mixture thus causing high temperatures to exceed causing a high enough temperatures to cuase the MAT sensor to put a code out. And the EGR valve also lessens high temperature inputs and allows the engine to be a lot cooler.

Anyways, this is what I think is causing the check engine light to come on, and that all 3 mechanics I took the car too couldnt figure out the problem or tell me what was causing the check engine light to come on.

- Greg

Does it run rough? Does it idle? Does it run fine under moderate acceleration? is the brake pedal harder than normal to push? Since you have not mentioned any of these symptoms I would doubt the EGR is stuck open.

Could you at least try to answer some of the questions I had? Why do you think the IAT is reading high? How would it be possible fo hot exhaust gases to reach the IAT sensor? I don't think your Diag holds water but at least your trying to make sense of what is happening. Give us some answers to the questions we have asked and lets see if we can't nip this in the backside!!!

Does it run rough? Does it idle? Does it run fine under moderate acceleration? is the brake pedal harder than normal to push? Since you have not mentioned any of these symptoms I would doubt the EGR is stuck open.

Could you at least try to answer some of the questions I had? Why do you think the IAT is reading high? How would it be possible fo hot exhaust gases to reach the IAT sensor? I don't think your Diag holds water but at least your trying to make sense of what is happening. Give us some answers to the questions we have asked and lets see if we can't nip this in the backside!!!

Well, i went to go check to see if the EGR needs replacement and it does. You can tell when the EGR is done when you need a fair amount of pressure to push up on the diaphram, which I tested and it was very difficult to do. it should move with ease. The car does run a bit rough but not exremely rough, on a scale I would say 1/10.

It idles fine, and it runs fine when the car is accelerating. the brake pedal is not rough or hard to push.

....And I think you are wrong about the MAT sensor not having to do with manifold temperatures. I am pretty sure that the senor senses the temp. within the manifold. Hence the name Manifold Air Temperature sensor.

This sensor relays information back to the ECU and this is why it is reading at such high temperatures. I believe it is the EGR valve being stuck open allowing high temps. to cirrculate and thus causing an error in the sensor.................

That sensor is in the air snorkel near the air filter. If the EGR is stuck open it would have a ruff idle if it even idled at all. Since there would be a lack of manifold vacuum the brake pedal would also be harder to move. If the EGR is stuck down (as you have described in your poor definition of when an EGR is bad) than it is possible that the EGR would not even open. If that is the case than there would be no drivability symptoms other than a possible "ping" under hard acceleration. The proper way to check an EGR valve is to apply vacuum (if it is vacuum operated) and in your case see if the diaphram moves up. Or do the same thing with the engine running and see if the engine stalls. Maybe you should try this so you can see what an open EGR valve really does to the idle of an engine!!!!.

Once again I credit you for trying but your logic does not make sense!

Surprise you haven't given any answers! You still haven't said whether the 40 degrees was the IAT or the coolant temp sensor. If the sensor is reading cold than the engine is going to run rich!!!!

That sensor is in the air snorkel near the air filter. If the EGR is stuck open it would have a ruff idle if it even idled at all. Since there would be a lack of manifold vacuum the brake pedal would also be harder to move. If the EGR is stuck down (as you have described in your poor definition of when an EGR is bad) than it is possible that the EGR would not even open. If that is the case than there would be no drivability symptoms other than a possible "ping" under hard acceleration. The proper way to check an EGR valve is to apply vacuum (if it is vacuum operated) and in your case see if the diaphram moves up. Or do the same thing with the engine running and see if the engine stalls. Maybe you should try this so you can see what an open EGR valve really does to the idle of an engine!!!!.

Once again I credit you for trying but your logic does not make sense!

Surprise you haven't given any answers! You still haven't said whether the 40 degrees was the IAT or the coolant temp sensor. If the sensor is reading cold than the engine is going to run rich!!!!

Ok. 40 degrees is the temp. guage on the dashboard (so the coolant temp.). As soon as the car fires up, atleast 30 seconds to a minute, the check engine light comes on.

Temp of the engine, thats a good start! Fahrenheit or Celsius? I have a feeling it is celsius. If this is the case and it never get above that than I think It would be well worth looking at scan tool data to see what the temp sensor is reporting. Since the fans come on it would be worth monitoring the sensor with a scan tool from a cold start to see what it is doing through the warm up cycle. If the engine sees an anamoly with engine tempurature it will fire up the fans as a safe mode to protect the engine. Since the anamoly could very well be within operating perameters than it is very possible it will never flag a code. All the PCM knows is what the temp is that the sensor is reporting. It has no idea what the actual conditions are that the vehicle is in.

If the engine is actually running at full operating temp and the PCM sees a cooler temp reading it will think the engine is still cold and add extra fuel based on its programing for those conditions. Solve the temp issue and the others will probably take care of themselves.

Ok, thats is great to know! thanks for your insite, greatly appreciated. So basically what you are telling me is to replace the coolant temp. sensor. This could very well be the problem?? I will scan my car again and look at the start up temp when the engine is cold.......

What I am saying is you need to monitor what the coolant temp sensor is reporting to the PCM. It could be the temp sensor but it could also be the connector/harness/PCM. Any additional resistance in the connector or wiring will cause a cooler temp reading. A temp sensor is a thermistor which is opposite of a resistor. A thermistors resistance is higher when cold and lower when hot. More resistance = cooler temp. A resistor has lower resistance when cold and higher when hot. Based on that higher resistance could be the issue not the temp sensor.

Just something else to think about.

Ok! that is great to know! thanks again and I really appreciate your insite!! thanks again!!


- Greg

Ok! that is great to know! thanks again and I really appreciate your insite!! thanks again!!!!


- Greg