GMC service engine soon light
 

I recently purchased a 1997 GMC 1500 series pickup with the 5.7 litre engine. Truck has 98,000 miles and starts and runs excellent no issues. Today out of the blue the service engine soon light came on. This truck has OBDII and I know I can have the code scanned but I was wondering what triggers the light? I removed the gas cap and put a little teflon grease on the rubber seal and tightened the cap. Being new to this vehicle I don't know if this light is triggered by needing an oil change? I also have to figure out how to reset the light? Any help is appreciated.

It could be anything. Go to an auto parts store, they will read and reset it for free. There is no way to guess, it really could be anything.

Oil change won't do it. Gas cap could. Get it scanned, then you'll know for sure.

Soon? Think it knows about the bk?

Bet its the oz sensor or sensors. It has four I think. Get someone to scan it and that should tell you whats going on. Universal ones are cheaper if you can do it yourself. Hint, don't try and remove one when the pipe is hot. Let it cool for hours first. Threads will strip.

heh - now we see where all that money went in "engineering." All GM products have a doomsday circuit that is automatically activated either by bankruptcy or attempts to break the UAW.

Most times when that light is lit in any GM you're also not getting any mileage or driveability problems. IOW, you can ignore it in most cases.

If the light comes on and you detect a noticable performance/mileage drop, then get it checked out.

'97 Vortec 350: my bet is the EGR valve/solenoid

CEL and a no inspection state=good. Blow it off, disconnect the terminals, or have it scanned.

That's right about time for one of the cats to go, probably a "catalytic converter under efficiency" light. At any rate, if its not flashing its a tertiary emissions component and still needs to be dealt with.

Often if one component goes out, the computer puts that device or subsystem into a loop mode and you loose fuel efficiency. And don't pull the light whatever you do, that puts the system into limp mode.

I have the same engine in my Tahoe. My service engine light comes on from time to time. I recently had it scanned and the fault code was the heater in the O2 sensor. The light goes out in a couple of days and won't come back on for months.
My truck has over 240,000 km's.

Today I stopped by the local FLAPS and had the code read. Said it was " bank # 1 O2 sensor ". He cleared the code and I drove home. Light did not come back on but it was only a 10 minute drive. Now the question is: does this mean the O2 sensor is bad/defective or is it reading a malfunction? Meaning is there a problem upstream?

With the same scanner they can see if the O2 sensor is showing voltage with the engine running. Dead O2 sensors show, well, nothing.

You should have paid attention to the code as in P0XX. There are about 20 generic OBD oxygen sensor faults.

An o2 sensor can read voltage, and still be bad, The computer is looking at fluctuations in voltage. If the computer pulls back, or opens up the injector pulse width, and the O2 doesn't react, or reacts too slow, it will throw a code.

Just because it has an O2 sensor code, doesn't mean it's an O2 sensor. Unless it was a failed Heater code.

1st O2 sensor actually does feedback to CPU to adjust air fuel. 2nd o2 is there as differental to show the 1st one is working. Most don't change 2nd one till outright failure.

the 1st O2 has to cyclevoltage readings a certain amount of times and if it doesn't then it stores the code tenatively as pending, then eventually a failure. You reset the code it'll come back eventually.

rjp

The 1st O2 sensor is used to adjust fuel trim at cruise and idle. The 2nd O2 sensor is used to test the catalytic converters ability to store oxygen or how efficient it is.

Excellent feedback and I'm learning. With my Porsches I don't have computer engine control and my daily drivers I've been fortunate to not had a problem yet. I started looking on Ebay for code scanners as it looks like a valuable tool to own. In the mean time I'll drive the truck and see if the light comes on. Thanks guys.