If I start my engine and let it idle for 15 or 20 minutes the radiator fan on my 140K miles 2004 TL never turns on. However, if I turn on the A/C it start spinning. Obviously, there is a switch/relay problem. I checked all of the relays, but they seem OK and I can't tell where the problem is. I'm worried that this can have the potential to cause a serious damage because the in dash temperature indicator never goes above halfway no matter how long I sit and idle. I'm afraid that the engine may overheat one day and I'll never know until it's too late.

I suspect a bad coolant temperature sensor, but before I start replacing parts, I was wondering if somebody knew of some tests I could run to confirm that the temperature switch is the problem. Also, does anybody have any schematics so I can trace the circuit? Thanks.