1992 Buick Lesabre Limited - 207,000 miles

A little back story:
Car'* been out of commission for about 2 months due to an intake manifold gaskets spilling tons of coolant on the throttle body side of the engine. [Still not sure if it was the actually lower intake gasket, or the small gasket on the side of the lower plenum that holds... I'm not sure what that piece is, looks like it just holds coolant...] Also replaced PCV valve and thermostat while we were at it.

Anyway - that was a big project for us, took us a while, but we got her going, all seemed well, car ran great for the first go (probably about 20 miles).

Filled her up with gas, turned on engine, now got a couple of codes - Code 42 and 58.

Code 58 is just the "immobilizer control module" which went away after a couple of drives. We figured that maybe it was just because we disconnected the battery for a couple of weeks while we replaced the gaskets.

Code 42 is "The EST signal did not change when the ECM applied bypass voltage to the ignition module."
When I looked up this code originally, it didn't make sense to me because the car was running FINE. (Other people with this code had stalling problems...) It probably made 10 trips over a week (maybe 100 miles worth) and nothing seemed wrong.

We checked all the wiring, made sure we got everything plugged back in, no obvious problems there and car was running fine so didn't know what to do about it. Plus we didn't really do anything near the ICM when replacing the gaskets.

Today, couldn't get the car to start at work. Ran fine all week and to work this morning with this code 42. Now, the engine tries to start, but won't take off. Very much assuming it must be with this code, and probably will need a new ICM or ECM.

Feels odd that this problem came up right after the gasket ordeal, and didn't seem to matter for 1 1/2 weeks before the car would stall.

Any ideas of where to start? Could just take it to a mechanic, but I feel like diagnosing will cost way more than we want to put into the car, obviously she'* not worth much, except to us. Electrical is not my forte, so I'm little worried to tackle it myself. What would be better to replace first, ICM or ECM?

I would start by checking wiring on the passenger side. There is a lot of wiring right in the area you would be messing with when you pull the LIM.

If you still have the Magnavox ignition system (one single large coil pack with 3 posts on each side) you may wish to convert it to the later Delco style unit (3 individual coils with 2 posts each, all on the same side). It should be a direct swap (ICM and coils), and is usually recommended because the Magnavox systems tend to be problematic. It also seems like a PROM on the PCM that needs to be reseated could also cause issues, so it may be beneficial to check and reseat the major wiring connections at the ICM, firewall, and PCM before throwing more parts at it.

Sorry for no updates, got very busy with the Christmas season, and then was out for the count for 3 weeks with an illness I just couldn't get over.

So we haven't done very much except just kind of look things over, double check that all electrical was plugged in and nothing looked fishy. The day I posted last was the day the car first died at work, and couldn't get it started. Waited a day, planned on towing it, but miraculously, got it started! (It took a couple tries, and it barely, barely took off, but when it did, it ran and drove home just fine.) Tried two weeks ago to get it started in the garage, nothing again, it just can't take off. It starts and then stalls immediately. Then, tried two nights ago, and it started fine and drove fine.

Still getting the code 42. Having a hard time diagnosing this myself, looking at all the electrical is very overwhelming and I don't know where to start. Especially since we have this sometimes start/sometimes not, but run fine when it does-symptoms, which is confusing.

Magnavox coil pack was replaced about 6 years ago, its actually the one with three packs, 3 posts of each side. Is this style problematic as well? It looks like this.

That is the Magnavox system referenced. Both the coil pack and ICM are known to be weak points. It does have 3 separate coils, but they are all integrated in to one single assembly.