Check Engine Light Function
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Joined: Aug 2003
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From: Canada
Check Engine Light Function
Does the check engine light only come on when you have a sensor failure or sensor out of range? I would think that any sensor out of range should cause the check engine light to come on. However, in reading this forum and talking to others it seems that many of you have had sensor failures but no check engine light. As a part of maintenance should a person periodically check for trouble codes even when the light is not on? Or, if the light is off does that mean all sensors are functioning within their normal range??
Thanks for your input.
Thanks for your input.
Posting Guru
Joined: Apr 2000
Posts: 1,746
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From: PNW
Check Engine Light Function
My understanding is that the computer only knows a sensor is bad if it fails outside of the expected range for that sensor. So, even though the value is considered valid by the computer, it still could be wrong for the given condition. Also, I don't think the computer has the ability to check the response rate of a sensor. For example, O2 sensors tend to get "lazy" (sluggish) rather than just fail. Changing a sluggish O2 sensor results in better performance, even though the computer didn't flag it. For older computers (OBD I), if you don't get a light, then you won't get a trouble code. I think OBD II systems have the ability to store non-latching fault codes for a certain number of start cycles. The only sure way that I know of to detect a bad sensor that still reports a valid value is to monitor it real time. Some of the fancier readers will let you do this. The only problem is that you would need to know what value the sensor should give for a certain condition (i.e., for an O2 sensor reading of X volts, and air MAF reading of Y volts, the injector pulse width should be Z msec). Or, I suppose you could look to see if the value changes when conditions change.
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