Service Engine light is on....

[mkalcev1]

My service engine light came on today. I've been told that I need to replace the coolant temp sensor, but in the past, I don't remember that sensor turning on the engine light. Could this be the reason for the light this time, or is it most likely something else? If so, any input would be greatly appreciated!!!

[sierrap615]

ask them what the code was - or goto autozone or your local parts store, some of them will pull codes for free. if you post the code i can confirm the problem. saturn had two designs of the coolant temp sensor(ECT), the first was made with a nylon case, the second with a brass case. the sensor itself is exactly the same, just the case was different, they are 100% reverse compatible. the nylon case ECT failed massively after a few years, so around 2001 they came out with the brass ECT, it should last forever. when the nylon ECT fails, it will do so slowly over time. it can throw a SES light, but 90% of the time it be noticed by drivabilty problems first (high idle, hard start, poor MPG). it is a $15 peice and a easy repair for the DIYer.

[mkalcev1]

Thanks a lot for the input! The ECT sensor is the most likely culpret. My question is though, would this be something that, once it starts going bad, can not trigger the alert to begin with, and eventually degrade to the point that it does switch on the SES light? Also, I was told that it's possible for a faulty ect sensor can allow fuel into the exaust system and degrade the O2 sensor. If a new ECT sensor doesn't turn the light off, do you think the O2 sensor would be next on the list?

Originally Posted by sierrap615

ask them what the code was - or goto autozone or your local parts store, some of them will pull codes for free. if you post the code i can confirm the problem. saturn had two designs of the coolant temp sensor(ECT), the first was made with a nylon case, the second with a brass case. the sensor itself is exactly the same, just the case was different, they are 100% reverse compatible. the nylon case ECT failed massively after a few years, so around 2001 they came out with the brass ECT, it should last forever. when the nylon ECT fails, it will do so slowly over time. it can throw a SES light, but 90% of the time it be noticed by drivabilty problems first (high idle, hard start, poor MPG). it is a $15 peice and a easy repair for the DIYer.

[sickcallawayc12]

My coolant light has been constantly flashing since the dealer replaced what was thought to be a faulty cooling temp sensor (Back in april). It never made my SES light go off.

[sierrap615]

Originally Posted by mkalcev1

My question is though, would this be something that, once it starts going bad, can not trigger the alert to begin with, and eventually degrade to the point that it does switch on the SES light??

yes, it can be bad for a long time before the SES light comes on, if ever. the computer expects to see a certain range of numbers from the ECT, (IE: -40F to 250F). when the ECT fails, the reading will slightly differ from the accual number, then a little more, then a little. is if the engine is 180F, the sensor may read 160F, for example. the reading is wrong, but well within the range allowed by the computer.

Originally Posted by mkalcev1

also, I was told that it's possible for a faulty ect sensor can allow fuel into the exaust system and degrade the O2 sensor.

sorta, when the computer thinks the engine is cold, it will dump extra fuel in to warm up the engine. if the sensor is always reading cold, it will always be throwing fuel in, leading to the problems i listed earlier. although it COULD shorten the life of the O2 sensor or the catalytc converter, it rarely is the sole problem leading to their failures.

Originally Posted by mkalcev1

If a new ECT sensor doesn't turn the light off, do you think the O2 sensor would be next on the list?

i really don't like just throwing parts at a car hoping something will fix the problem, i highly beleave in proper diagionois. but the ECT design inprovement is so great i would even replace a good old sensor with the new type. geting the code will tell us exactly what system is at failt. just like it would have cured sickcallawayc12's SES light.

what year is your car? if it is 91-95, you can pull the codes yourself with a paperclip...