I have an 89 mustang with a 5.0 HO. When engine warms up, the check engine light comes on. When I shut it off and start it up again, it stays off for about five minutes but then comes right back on. After the light comes on, it feels like it idles leaner. I have not scanned it yet for lack of a scanner. My first guess would be the coolant temp sensor but i'm just not sure. Any possibilities of what could be causing this would be greatly appreciated.

 

o2 sensors

 

go to autozone for a free check....

 

How funny. The other day I noticed my water temp guage was acting funny - when the car is off it reads about 20% warm, but turn the ignition on and it pegs out on the cold side no matter how hot the engine is. I've got a new sensor to put in but anybody know what causes it?