SL2's computer monitors read "not ready," can't pass inspection!

Hello, I am hoping someone here can solve my annoying problem!

Last Friday I took my 1996 SL2 to be inspected. Everything would have passed fine BUT, the guy said three of the sensors could simply not be read by his computer. He gave me a paper, it says that "catalyst," "02 sens," and "EGR sys" are all reading as not ready. Everything else was read and passed. If one more sensor could have been read, I'd have passed, since you can get away with 2 "not readys" in TX evidently.

So the guy says that I probably need to reset the computer because the battery had been disconnected (has not been), and told me to go drive about 40 miles to do this. He told me to come back and not to shut the car off and he'd read it again. I did all this, he hooked it back up, and guess what, now it wasn't reading ANY codes! He was baffled and couldn't tell me what I should do but take it to the dealership.

Instead I took it to a local mechanic I've known first. They hooked it up to their machine, now, once again it's reading all codes but those 3. They said there's nothing they can do with it, so I gave in and took it to the dealer. Guy at the dealership says it could be faulty wires or the computer could be shot (wants 400 bucks for just that, $69 to diagnose). I didn't leave the car because of the 2 hour wait, so now it's sitting here with me, expired inspection and all. I've got 10 days to fix this.

Something maybe important, and confusing: for a while now, my battery light and coolant light will randomly come on and start flashing together. Sometimes for a couple seconds, maybe minutes. Then it goes off for equally random amounts of time. My temp gauge never fluctuates, always says everything's cool. So I know the flashing lights mean something, but what? My car runs absolutely fine, but because of some computer issue, I'm being forced to drive it with no inspection. Is the dealer right, am I going to have to bite the bullet and pay up, or is there another way to fix this?

Thanks for any advice you have!

PCM Rarely goes bad. The '96 year is the only year where a PCM flash is needed however thats to remove a misfire...not your issue.

SES Light on? Does the SES light turn on when you put the key in RUN? Last tune up? (aka plugs, wires, oil change..etc etc)

What to do:
If the SES light is on, go to autozone/advanced auto and check the codes. WHile your there get a new Engine Coolant Temperature Sensor. Post the codes here if there are any. Replace the ECTS and clean the EGR and throttle body (if the ECTS connector has corrosion on it, replace it also). From there drive on the highway from one exit and then back home. Note the operating temperature on the highway. Should be 3/8 as per the gauge. If it is less...dont go back home, but go to the dealership and get a new thermostat and coolant from autozone. Once home and cooled replace the thermostat. I have posted a drive cycle in another thread recently. You can do that to force the PCM to run its monitoring tests or just drive untill they are eventually completed.

Explanation: The ECTS is VITAL to many subsystems of the car including air/fuel ratio. This includes almost every on board diagnostic test. However the stock one is made of resin and cracks...sending false data to the PCM. New ones are brass.

A cold running engine (one that is below the correct operating temperature) that is from a faulty thermostat is then read by the ECTS again failing the monitoring tests.

A dirty EGR and throttle body will again cause failures in the onboard monitoring tests. The EGR is an emission device, and when clogged with dirt and carbon, it cannot open the pintle. Carb cleaner and a rag does well.

Instructions and prices:
ECTS-$10 at autozone and $2 for the teflon tape. $20 at the dealer that comes with sealant on it. I have posted a video for this.
Thermostat- $22 at the dealer. (DEALER ONLY ITEM) I again have posted a video on this
Coolant-$20 bucks

There ya go. Now you know what to do on saturday and sunday :)

Thank you very much for this advice. These are things I'd never have thought of. As to your questions, the light does not come on just by simply having the key in RUN. It usually will come on when I have been driving for anywhere between 30 seconds to a few minutes. Completely random.

Last tune up, I believe was when I got the car. Probably should have been more often, I know. I got the car in 2003 I think, and the car lot paid for a tune up at the dealership for me. But ever since then, I've been replacing oil regularly and have zero mechanical issues. I don't think the battery has died in years. Only problem was my brake pads wearing down which is normal anyway after so long. Car's got about 109,000 miles on it, still running fine.

I'll try to get all the things you suggested done today and tomorrow; hopefully it'll save me hundreds of dollars in tests. If I run into any issues or changes, I'll post back.

Once again, thanks!