There are all sorts of error tolerances programmed into the computer for monitoring systems. If the computer set a code and turned on the light every time it saw something out of spec it would turn into the proverbial “boy cries wolf” situation. The dumb light would be going off all of the time and people would ignore them even more than they already do.
So most errors have to occur something like a certain number of times over a certain amount of time and/or over a certain number of drive cycles. The specific conditions for setting any particular code on any particular vehicle can normally be pulled from a factory service manual. The computer will also store “pending codes.” These can be read by many code readers, but haven’t yet turned on the light. It’s sort of a 3 strikes policy.
These days I often find and fix problems that should set codes before a code even gets set. My most recent included an EGR system problem and fuel injector problem, both of which caused me noticable issues but the computer wasn’t ready to call them “errors” yet. My minivan has an issue right now that should be setting a code. I’ve started trying to diagnose, but am sort of waiting (and waiting…and waiting…) for a code to help me out a little.
Anyway, you should have the codes read again, though I wouldn’t be surprised if it’s P0128 yet again. You do know that many big-box auto parts stores will read these for free right?
Then if you have the P0128 or anything else with cooling system reference, your best bet, if your BF does have an IR thermometer, is to just use it. It really is as simple as point and click and you’ll know in seconds.