WhiskeyHotel

Yes, the actual equation is R=Rref(1+a(T-Tref)) and the temp coef (a) of Cu is +0.000393 per degree centigrade. That means the effect is pretty small at the lengths and temps you would experience under your car's hood. (That Bunsen burner temp in your example link is ~1700 degrees C!)

Example: 100 feet of 20 gauge wire with resistance of 1.015 ohms at 20° C (room temp). If the temperature of the wire goes up 10°C, the resistance will change by 0.0399 ohms (10 degrees * 0.00393 per degree * 1.015 ohms = 0.0399 ohms).

The wire resistance will now be 1.015 ohms + 0.0399 ohms = 1.0549 ohms.

In my opinion, that wouldn't be enough change to account for the large increase in resistance your mechanic is seeing. (I think - I don't know what the increase was. I also don't know how the resistance was measured nor do I know which of the conductors he measured and there are grounds, power, and sensor signals going to the throttle body. You can see that I am shooting from the hip here.) My electrical engineering spider sense is tingling at this point. There could be something else going on - connector pins heating up and losing continuity - something. I hope the proposed solution works - but I am a bit skeptical at this point. Let us know how it goes.