how does check engine light turn on

on a ten year old troope with a little over 64,000 miles, just noticed the check engine light on but then it turned off
the timing belt and tensioner (pusher) were replaced over a year ago
am I correct in assuming that at some point close to 65k miles, the light will turn on or is this perhaps some other unrelated situation (ie. oxygen sensor etc) ?
in case it matters, I've had to switch to premium unleaded several months ago to stop pinging
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Is this a '95 Trooper? Then I imagine it has and OBD-2 system (On Board Diagnostics ver 2). The engine control computer turns on the check enginer light when it detects something is not right with the engine.
Probably it is related to your having to switch to premium gas... I'm assuming your manual doesn't indicate premium is required. The engine should have an anti-knock sensor which the computer monitors. When it detects knocking the computer retard the timing to make it stop. If it can't do it then something's wrong.
Take it to an Autozone, they will connect to the diagnostics port and read the trouble codes for you.
BuddyWh

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BuddyWh wrote:

I think it's either 94 or 93

so there is no such thing as the "service light due to it's time to replace timer belt" ? I remember reading somewhere that it happens at 65k miles?

only problem is that the light does not stay on but if it does, will do, PepBoys is nearer
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Not sure PepBoys will do it... it's an AutoZone thing.
If it is an OBD-2 system the computer will store trouble codes even if the light isn't on. The computer uses a fairly complicated algorithm to determine when to set the light: based on things like type of problem detected, number of consecutive starts, driving pattern (miles driven at given MPH), and how many times the error is detected.
In your case, you know you have a problem since the engine pings when it really should not.
I don't think the lamp lights when it's time to replace the timing belt based on mileage... at least, I've never heard of it on Isuzu's. My Rodeo certainly didn't... I replaced the timing belt at 90K. It's really not so critical on these V6 engines since they are not an interference design. Check your owner's manual, it should have some information on it if that is the case.
BuddyWh
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There are a number of things that can cause the check engine light to come on. One thing to keep in mind though, the check engine light is really an emissions control system diagnostic warning. It can come on for reasons as simple as a gas cap not being on tight. Also, I would try to find an Autozone for the code check, Pep boys will charge you $75 to read it and reset the codes.
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