Greetings, this started happening after I had an LED lightbar installed on my 2008 Mazda5.
We had to take out the battery and the ECU to have access to the firewall where the wiring went in. That night, it was the first time I noticed the radiator fan on when normally it's not (A/C was off). The next day on the way to work, the gauge went all the way up to H but the engine showed no signs of overheating. I plugged in my OBD sensor, the temperature read 262 degrees but no fault codes and no engine warning light. When I arrived, I even let the engine idle for about 15 minutes to observe but even the overflow tank and radiator cap were cool to the touch. What do you reckon the problem is? It's like the computer is sending the wrong signals or the cooling system is.
Thanks!

 

What temp is your OBD reader showing when the engine is dead cold? After 5 minutes run time? Decent drive?

 

Thanks for your response. Upon start up, 86 degrees.
At 159.8 degrees, normal operating temperature has been reached according to the app.
After about 17 minutes of normal highway driving @ 55mph, the needle starts to depart from the normal middle part of the gauge and starts climbing fast towards H. This has been happening consistently for three days now

 

Thermostat. a malfunctioning thermostat on that engine will more often than not, open early and you will run a 156 to 159 degree temp. PCM should pick up on it, if your in the States that is, and set a P0126 code. After driving for a distance, it sounds like either the thermostat is closing or at least restricting the flow enough to overheat. Either way, stop driving until you get the problem rectified. 262 temps will set you up for a head gasket failure real quick. Thermostat is located within the black plastic outlet, located on the block on the passenger side of the motor. The upper radiator hose and a heater hose both connect to it.

 

My engine light finally lit up and I got a P0117 fault code.
This time it actually overheated so I had to let it cool down.

 

Well? What did you end up doing?

 

Get the cooling system pressure tested, including the radiator cap. Also get a TK test done, ( checks for combustion gases in the cooling system).

 

Replaced the Thermostat, problem solved!
Thanks all!

 

Thanks for sharing. At least you had a warning that the thermostat was failing. When mine failed, I had no warning but luckily, it failed as I was pulling into my house and replaced it.