CTS (Coolant temperature sensor) at back of head under plenum, is tricky to get at it's a thermister that provides the ECU with info on coolant temperature from 0 degrees to boiling these do go out of calibration after a while and hence give false readings leading to rich mixture.

Forgot to say in last post but a common one, check the MAP sensor hose, under header tank beneath a fender this connects to the plenum. These can simply come off (It's a good idea to cable tie for this reason) when they do they run very rich, if not this they also split!

Same applied to fuel pressure regulator if this comes off though you go to other extreme of leaniness and could lose engine so again good to make sure it's in good condition/connected.

Martin