[gstsaver]

Have these codes come up every once in a while. Delete them and they go away for some time (anywhere from a day to a month).

What could be causing these?

It's a 2002 Blazer 4x4 LS low km

 

[MEP1]

Did you try Google? The codes themselves tell you what you need to know. I already know from the first one that you've either put a 165° thermostat in it or the temp sensor is bad or unplugged.

 

[gstsaver]

Did you try Google? The codes themselves tell you what you need to know. I already know from the first one that you've either put a 165° thermostat in it or the temp sensor is bad or unplugged.

Thanks. I know what each one individually is but there has to be a reason they come and go altogether

 

[oldeerslayer]

Your alternator is a 3-wire. 1 for battery hookup (charging post), one to turn the alt on (L terminal), and one for the PCM to sense alt output/load (F terminal). P01638 means the alternator is not putting out what it should. Or at least it's not telling the PCM. Which could trigger the others because they are voltage dependent. Although the temp sensor is only 5V. Could be a bad alternator, bad connection between the alt and the PCM or bad PCM. Or it could be 3 separate issues.
Look at the connector to the alternator carefully for corrosion or damage or looseness. Check the O2 fuse. Check the connector to the temp sensor. Look at the O2 sensor wiring for heat damage.

 

[MEP1]

Thanks. I know what each one individually is but there has to be a reason they come and go altogether

That's the sort of thing that it helps to add in the initial post. I'd be looking for a used PCM.