Contour Enthusiasts Group Archives Forums General Troubleshooting lessons learned plus a question

A while ago I posted some questions regarding things a mechanic told me I had to get fixed and the prices I was quoted. I was told by some ceg members to get second opinions. This is simple advice (when you have a lot of time) but is very wise. Thank you.
First guy told me there's a leak in the muffler had to replace the whole thing. Need new rotors all around and needed front brakes. (wanted $600 total)
I went to another place where they showed me the rotors and brake pads with the tires off. Turns out the front brakes were near new, but the rear ones needed replacing. The rotors were a bit rusty, but didn't need replacing. The muffler hole was just a little dent. (cost me $100)
Also, I had a vibration problem when going 60mph. They checked the balancing, alignment etc. Nothing wrong. Finally, the 3rd person I took it too told me there's a defect with one of the tires I had recently gotten. It had a bump that made it not quite round. Happy to hear it.. the original place I bought it at will swap it out for me. Just wished I didn't have to go through 3 shops to hear it.
Here comes the question: I have a very pesky CEL that comes on quite frequently. In the past, on 3 separate occasions, I've had the O2 censor replaced, new spark plugs, and um... something else replaced with a complicated name I can't remember. I have a 1997 V6 with 63K miles. Over the last 6 months, I've had the light come on and off. Usually, it stays on for a couple weeks and goes off before I've had a chance to take it in. I'm a procrastinator..
Anyway, can you get it checked when the light is off? I've heard people tell me both ways.. yes, they can, or no they can't, the computer won't pick anything up.
Also, any idea what it could be? The ONLY difference I've noticed is that occasionally, when idle, it almost stalls, then the engine kicks in and it revs up just a tiny bit to prevent stalling. Possibly lower gas mileage too.

Thanks for any help.
tnt

You can look for a PTC (Pending Trouble Code) when the light is off, it's like a code that is on the verge of throwing a Check Engine Light. When it grows up to be a DTC (Diagnostic Trouble Code), your CEL will illuminate. If the CEL goes out, I'm pretty sure that the DTC that set it will be gone, too. Some codes will go away if the computer sees a certain amount of drive cycles or key on events without seeing the same problem again. A CEL doesn't necessarily mean that something is wrong right now, but may mean something happened that worries the computer and it wants you to give it some attention.

Depending on the type of code reader and the skill of its operator, you may be able to look up what is called a "Freeze Frame". When a DTC is set, the computer stores the current sensor conditions at the time the code is set (like RPM, Vehicle Speed, Coolant Temp, O2 readings, etc.). So you can see what was going on when the code went on. But this goes away after some set period as well.

Best bet: next time the Check Engine light comes on, get over to an Autozone and you can get the code read for free. Without the code, there is no point in trying to diagnose or fix the problem because it could be hundreds of things.