Driving along, temp gauge in the middle, all seems fine.(4.2 S/C)

Suddenly gauge drops to just above blue.
Fan roaring on full.
Plug IID tool in, coolant temp steady between 80 and 90. I drive off again and Fan stops running on full speed and temp gauge goes back to normal. IID coolant temps stays the same between 90 and 90.

What the hell is going on?

Thanks!

Scan for fault codes when the fan is roaring

Had this issue once or twice

P0116

Changed the temp sensor, but still happens after clearing codes , key off restart and its cleared itself

Thanks. I am getting P0151 occasionally, but I dont think thats related.

No other codes.

When you say sensor, is that the coolant temp sensor? It weird because on the return journey everything was fine, and the coolant temp via the IID is always where it should be, even when the temp gauge drops!

Yes coolant temp sensor, changing it out didn't sort it out for me, still happens once in a blue moon.

Happens when clearing either rich or lean codes with iid (can't remember which) but clears immediately with ignition off and restart

How strange!
I am going to change the coolant temp sensor and may do the oil temp sensor too-there is a post on here somewhere saying can cause the temp gague to drop No idea how but they are cheap.

Anyone else had anything similar?