Hi, I hope I am in the right place to post this. I am having problems with my 2000 bonnevile 3800 non supercharged with 200,000 miles on it. I bought it in 2002 and it'* always been a reliable car. usually it runs great until last week and need some input as to what I should do.
here is what I have done to the car recently. new plenum gasket, valve cover gaskets, new axle bearings front and rear, new cv half shafts right and left, new rotors, brakes, calipers, new tps sensor, new coils.
now the problems that have started since I did all the work. getting codes p0418, p0201,
p0300. at first I was showing the 418 code only and ended up taking it to firestone after I changed the relay and they couldn't figure it out and said it was in the computer. while it was there I had them do a brake fluid change for me since everything else was new with the brakes. then a couple days later the headlights would turn off and on at random. then the abs light and the brake light would come on at random (usually after hitting a bump) so I checked all connections again they were all tight.
2 days ago I started it and the engine had a miss. hooked up the scan tool and the p0201 and p0300 codes came up. I checked the injectors with a ohm meter they all read 12.5 ohms so I'm thinking that is fine.
so I guess now I am wondering if I really do need a computer or maybe it'* just some kind of a short. I have about tapped out on the money I was going to spend on it and didn't want to get a computer without another opinion. at this point I may be overthinking the problem.
thank you for reading this far and for any comments I recieve......

not looking at a diagram but it seems more likely that all those circuit'* relays are powered by the same feed and ground, i would be looking for something like that

thank you! that may be correct. I am starting to check all the grounds I can find now. also I was just now looking up how to test the icm. just trying everything I can think of, I will search for a wiring diagram to pin it down better. thanks again

Roger...

also forgot to mention it has all new plugs and wires.

those arent going to cause your headlights and abs to go out

ok here is what happened. I cleaned all the grounds I could find and a couple of them really needed it, especially the one on top of the transmission. I started the car and it was running perfectly, no misses and the only code was the p0418 for the air pump relay. so it ran fine for about 10 minutes put the fuse box cover back on under the hood so I could test drive it and it started to miss again. so I kinda shook the fuse box and the motor died altogether. turned the key and not even a light would come on! so I disconnected the negetive battery cable then the battery cable at the fuse box and lifted the fuse box up and pushed all the connectors all the way on. a couple of them were pretty loose.
and that did the trick. I drove it around for about 50 miles and had no codes at all and the abs brakes worked perfectly!
I wanted to thank you for pointing me it the right direction, it led me to the actual problem. been messing with this thing for a week now and was ready to pull my hair out. LOL

glad to hear you got it, and thanks for posting the fix im sure it will help others

I had almost same problem, codes P0201 and P0301 after I changed oil and a fuse. Turned out when I went to remove the cover to the fuse box I thought I had to loosen the three bolts. I never tightened them back up. I did that and changed the two fuses for good measure (eventhough they were fine) and car is running fine.