The other day I decided to check my CEL because it had been on for a while. I wanted to check if anything else popped up. I found P0483 control fan rationality check malfunction. I did a search and learned some things.

 

I checked my coolant and I added maybe a few cups to bring the overflow tank to "full". The car was running with the AC on and the fans would cycle on and off for 15 seconds. At this point the gauge was reading normal. I took the family on a nice long drive (80 degree heat) and the gauge read normal the entire time.

 

When we returned I let the car idle in front of the house with the AC off as we unloaded. I noticed the fans were running for more than 15 seconds now. I got curious and looked inside only to find the gauge pegged in the red. The coolant level was normal and the car did not seem that hot. I again pulled the diagnostic out and there was no code for P0483.

 

What gives?

You may have air trapped in the system. With the engine cooled off, open the radiator cap and check the coolant level. Add coolant if need, leave the cap off, and start the car. Let it run and warm up with the cap off. Put the cap back on and see how things run. You may have other issues if that doesn't help.

So what happens if you let the vehicle get up to normal operating temp (no AC on)? do the fans kick on and cycle off as normal? or do they just say running or dont come on at all? Its pretty common that a stuck thermostat can throw this code...however as mentioned make sure all air is properly bled out of the system first...what year and model is the vehicle? how many miles? any maintenance history?