[ChevyTech]

I was hoping you would chime in ChevyTech, I've read some of your replies(in other threads with tons of info) and you're the kind of people that make this site awesome.

Thanks

Do I clear the codes without replacing any of the parts?

Yes

I don't have an ohm reader, but I can replace the thermo sensor. I know it's been there for a while, 15+ years.

I would not replace any parts yet. This group of codes does not make sense. Wait until we see what else we can figure out.

I forgot to mention, the service engine doesn't come on until i'm at 45mph and faster. If I'm around town it doesn't light up.

Clear the codes and then as soon as the light come on, check the codes and see which one gets set first.

When the truck is at idle, I can feel a slight miss.

You need to find out why it misfires. This will make the system lose control of the mixture.
These systems work great when every thing is right but they have a big flaw. Once the engine misfires for any reason it throws the system in the wrong direction. This system watches the O2 sensor, but the O2 only will see how much oxygen is in the exhaust stream NOT burnt gas. When every thing is right the O2 knows just how much of the oxygen should have been burned by combustion. Once it misfires, there is way more oxygen in the exhaust, because none got burned in a cylinder. The system thinks it is to lean so it gives the engine a richer mixture. The richer mixture often makes it misfire more (especially if it is cause by weak ignition, bad plugs…) so it dumps in more fuel. Now it is so rich the plugs and O2 sensor get soot on them. It misses more. The system goes richer and richer.

First check for a vacuum leak such as the base gasket under the TBI unit, and the ignition parts would be the next place to look for a problem.

I'll check out the MAP sensor's negative wire.

The sensor wires connect to ground at one of the eyelets that are at the front intake bolts or the Thermostat bolts. Make sure they are clean and tight.