my 850 starts runs it idles at 2000rpms after u drive it and the temp gauge goes down to cold even when the temp is hot. i shut the car off and try to restart it but it doesnt start fuel pump is running dont no where to start. if you let it cool down it starts up idles fine i drove it and it did it again no fault codes but idle air control vibrates with key on

 

It's normal for the ICV to run with the key on. Not sure why it won't start unless you have a temp sensor bad.

 

+1 possible bad temp sensor. Maybe?!?

 

the temp sensor will make it not start

 

If you are asking if it can cause a no start, yes it can.
If the sensor "tells" the computer the engine is hotter than it really is the computer will be giving the engine a leaner mixture and it might not be enough fuel to fire. If it "tells" the computer it's colder than it is the mixture would be richer than necessary and if started run rough until the extra gas was burned off and it warmed up a bit. Or it could flood the engine.

When you start it up and the engine is hot does the temp gauge actually go down? If so you need a new temp sensor no matter what else. This would be first thing to try.

 

thank you for all the help guys to use to working on american cars that arent as advanced but i love my volvo so its cool to learn new stuff

thanks