I am very mad right now

Why are you angry? This is very common with today's vehicles. GM especially has a a lot of tricks built in to get you back to the dealer, such as the need to program computers to the vehicle. There is no need to design them that way but that means they can charge you more money.

First of all, there is a huge list of "cells" that make up the fuel trim numbers. Those tell he computer how much fuel to inject for every possible combination of load, coolant temperature, air temperature, speed, rate of speed change, etc, and all of those numbers are wiped out when you unplug the computer or disconnect the battery. You start out with factory-programmed values, which are close but never perfect, then those "look-up tables" start to be rebuilt as you drive. You didn't give it time to do that. "Short-term fuel trims", (STFT), are recorded right away and you will rarely notice that unless you have some gauge that claims something is wrong. Unless you were immediately trying to race someone on the highway, you should have noticed no running problem, yet now the dealer knows you're angry when they did nothing wrong. "Long-term fuel trims", (LTFT), take longer to update and you will never know that is occurring. Until those numbers are added to the look-up tables, there's no way you can expect the best possible emissions, performance, or power. Most drivers would not even notice less than full horsepower unless they stomped it to the floor as they left the dealer's parking lot. That's not normal driving behavior so it's not something the dealer or the manufacturer planned for. You have to give the new computer time to relearn the parameters of the engine's sensors and its performance characteristics.

The second problem is believing the dealer when they said the computer can cause slow cranking. I think that's what you're trying to describe.

"I went to take the truck back and it won't start at all the engine try's to turn over."

I assume there's supposed to be a period in there so that's how I read it. Slow cranking is a product of the battery, charging system, or less likely, the starter. GM has had a huge problem with their generators since they redesigned them for the '87 model year. They develop high voltage spikes that can destroy the internal diodes or voltage regulator and they can interfere with computer sensor signals by radiating across adjacent wires. Running problems can often be attributed to the generator. It is common to go through four to six of them in the life of the vehicle, but experienced mechanics know that to reduce the number of repeat failures you must replace the battery at the same time. As they age, they lose their ability to dampen and absorb those spikes. Typically the battery does not need to be replaced if it is less than about two years old. This cause and fix is not common knowledge among owners of other car brands because it only affects GM products, and few GM owners are aware of it or the reason. They just unfairly assume the mechanic is trying to sell them more stuff than necessary when all they think they need is a new generator.

The first thing you need to do is measure the battery voltage with a digital voltmeter. It must be 12.6 volts. If it's closer to 12.2 volts it is discharged. Charge it at a slow rate for an hour, then try to start the engine. If it cranks normally, you know the starter is good. If you find around 11 volts, it has a shorted cell and must be replaced. Next, measure the battery voltage again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator, and if it's bad, replace the battery too, especially if it's still original.

The third problem is GM cleverly designed in a number of "got'chas" from simply disconnecting the battery or letting it run dead. Those are handled by towing it to the dealer to have them unlock or reprogram some computers. THAT is not the issue now because it was running for you when you picked it up, but it could become a problem if you disconnect it to replace it or the generator. To avoid that, use some type of "memory saver" while the battery is disconnected. Some plug into the cigarette lighter but those only work if the lighter works with the ignition switch turned off. If you have to leave the switch on, that little battery can't possibly meet the demand so it won't maintain the needed voltage. I use a small battery charger connected to the cables but then you have to be careful the positive one doesn't touch anything metal on the truck, and that a charger's cable clamp doesn't pop off.

I don't know how your mechanic diagnosed a bad computer as causing a slow crank condition. He might have reached a valid conclusion, and apparently it cranked okay for him after the service was completed, (after all, it also cranked okay for you at the dealership when you picked it up), so he had no reason to doubt his repair. But keep in mind GM dealers are the most high-pressure when it comes to new-car sales, and they are in the top three world-wide at squeezing money from unsuspecting owners after the sale. I don't want to imply that all GM dealerships are shady, (you can find them in any car brand, and in any other profession), but in my city we have over 15 new cars dealers that are ethical and easy to work with, including two GM dealers, but we have three dealerships well-known for counties around to be crooks, and all three are owned by the same fellow who owns the Chevy dealership. Anything with his name on it is hurting for repeat customers. Too many current owners of his products have said "never again". I have never heard of that many complaints from our Cadillac or GMC dealers or those of any other brands.

The mechanic may have misdiagnosed the cause of the problem, or he might have been going off past experience based on an inaccurate description you provided of the problem. That happens all the time, but the mechanic's first task always is to verify the complaint. Had he witnessed the slow cranking, he might have had a different diagnosis. If all you said was "it starts hard", that could be a computer problem but it is not as common as everyone thinks. Also, this may not have happened in your case, but WAY too many people can't describe what they mean by "doesn't start". They stare with a blank face if you ask if the starter cranks the engine, if there's no sound at all when they turn the ignition switch to "crank", if it cranks fine but won't run. They are lost on the sarcasm of "I'm in pain, ... Why". They don't understand that a dozen different symptoms can be lumped into "doesn't start".

Once you test the charging system and battery, I don't know what to tell you to expect from the dealer, but I hope you'll post the results so we'll know how you made out. I CAN share a story that has parallels to this. I helped out at a tv repair shop for a few years owned by one of my state's biggest crooks. Some of his tactics would make the Mafia green with envy. Once something was fixed, he regularly charged for additional parts that weren't needed or put in. How are you going to know? Eventually the word-of-mouth advertising got so bad he ran out of customers in my extended community of well over 100,000 people and went out of business. I worked at my cousin's little 2 1/2 man tv repair shop for over 30 years in a tiny farming community of about 2,000 people. He was so ethical and honest it was embarrassing, but he survived only on word-of-mouth advertising and had a lot of loyal repeat customers. Those are the two extremes of the profession, but even my cousin ran into a dilemma. A lot of parts in a tv can't be tested, they can only be replaced to see if that solves the problem, and many of those parts have to be mangled to get them out. Also, a lot of those parts do not fit in any other model, even of the same brand, so you're very unlikely to ever need them again, but they can't be sent back, so what do you do with them? We have to charge for those parts and hope our training and experience keeps the number of those wrong guesses to a minimum.

That never used to be the case with Engine Computers in cars. Plug one in and if the problem isn't corrected, take it out and put the old one back. What do you do now with a computer that has to be programmed to the car, then won't work in a different car? Just ask the guys at any salvage yard how they love GM for making all those computers worthless because they won't work in another car. Even if they can be reprogrammed by the dealer to work, how happy do you think they're going to be to do that programming for you when they didn't get to sell you the part? So what should your dealer do with that new computer that didn't fix the problem? Ethically, the old computer should be reinstalled. I suppose the case could be made they made the attempt at fixing the problem so you can be expected to pay the labor, but the question remains about what to do with the new computer. My understanding is the dealer only can reprogram it to put it on the next car, and the ethical thing to do would be to credit you the cost of it, but if I'm wrong, how many parts like that can they afford to give away before they have to charge even more to cover those costs?

My recommendation is check the battery and charging system first, just with a cheap digital voltmeter so you have an idea of what might be going on, then, after you cool down, give the dealer a chance to check their work and explain what happened. Listen to their new recommendations. If that does involve the battery or generator, ask them what will be done about the computer that wasn't needed. If they told me it was too late to take it out or give a refund, I would try to calmly say, "I see, ... Interesting". Believe me, they will know you will be telling your friends and coworkers this story. The owner of the dealership I worked for told us very often that it cost him more in tv, radio, and print advertising to get one new customer than it took to keep ten current customers happy and returning. (Remember that Chevy dealer with no repeat customers)? What he meant was the service department didn't need new customers if we just kept our happy ones coming back. GM as a whole, and many of their dealers concentrate on making the biggest possible sale now rather than numerous smaller sales over time. That was the quote from the unethical tv shop owner I worked for. "Get as much money as possible from them the first time because they aren't coming back a second time". He was pretty proud of that saying, but guess who is out of business now?

From what I've read from other people, it sounds like you got some good deals, especially on the computer but from the dealer too. Like I said earlier, they aren't all crooks. Buying parts somewhere else, then taking them to the dealer is like taking your own food to the restaurant and asking them to cook it. That would typically not bother me but some shops won't do that. Some want the profit on the parts, but most don't want to get in the middle of a warranty argument if the part is defective or fails in a few days. The parts profit helps cover the cost of the mechanic replacing it again under warranty at no charge to you.

I'm still disgruntled that the manufacturers even put in computers that have to be programmed to the vehicle. I can understand the advantage of updating the software if necessary but to just make it so a salvage yard computer can't be used is not in the customers' best interest.

Keep us informed of your progress.

Given the recent history, the lights would appear to be related to the work under the hood and not an engine problem. Regardless, don't run the engine any more until you check the wires on the oil pressure sending unit. If those look fine, use a mechanical gauge to check the actual oil pressure. My best guess is it will be fine, and safe to drive. A wire could have gotten pinched and finally rubbed through and grounded out to turn the light on.

The Check Engine light means a diagnostic fault code is stored in the Engine Computer. Having that code read is the place to start. It will indicate the circuit or system that needs further diagnosis. Years ago oil pressure was not monitored by the Engine Computer so logic would dictate the two things are not related, but every manufacturer makes so many changes now that maybe it IS monitored on your vehicle.

Check Engine lights are always yellow. You can tell the relative severity of an intermittent code by how the light acts. It must turn on when the fault detected could have an adverse effect on emissions. If it goes back off while you're driving, the intermittent problem went away and it's relatively minor, but there will be a code stored in the computer. If the problem goes away but the light stays on, then resets to off when you stop and restart the engine, that's a little more severe. When the light is always on, even after restarting the engine AND after the problem is no longer occurring, that's still more severe. When the light starts flashing, that's the time to bail out and duck for cover. That means too much raw fuel is going into the exhaust system and the catalytic converter will overheat shortly. That's the time you must stop the engine right away.

The plug-in PROM is the personality for your specific car. That's how GM made any needed changes or updates when necessary without having to replace the whole computer. All computers of the same part number will work the same way. You just transfer your old PROM into it. No programming is needed. Those computers from that era did cause a lot of trouble and it's why to this day so many mechanics still jump on the computer as the cause of every conceivable problem they don't know how to diagnose.