P0125 coolant code will set if the engine coolant temperature cannot be reached at a specified time. Most of the time, it detects if the engine coolant thermostat is defective or not. Other causes of this faulty code are bad coolant sensor, inoperative radiator cooling fan, defective thermostat. To make sure this code is triggered, reset the code by disconnecting your battery for 1 minute and reconnect. Take the vehicle for a road test and if the code returns, it shows the coolant code is serious.
Basic testing of P0125 coolant code requires verifying the coolant fluid level and touching by hand the thermostat housing for any signs of temperature change (be careful when doing this). The coolant sensor wirings can be checked visually and sometimes disconnected to see if it is getting electrical contact from the engine computer. A wiring diagram can be used to check more complicated wiring issues but getting the assistance of an online mechanic can give you a faster result. Using a voltmeter, you can check the resistance for continuity. Also, you can also probe the coolant sensor housing if it is grounded.
Fixing this P0125 code depends on your findings shown above after check ups. Here are other common fixes suggested by ATS. Verify the voltage readings of the coolant sensor as suggested by the engine specifications. Do the same on the air temperature sensor whenever the sensor is used. On some throttle sensor (tps) circuit, the coolant sensor has a common ground shared with the tps sensor. If the tps sensor wirings are shorted, it will also affect the coolant sensor. Again make sure to use a wiring diagram because not all engines have the same set up.
Quick P0125 Code Tip:
On some imported and domestic vehicles, if this code shows up, check the voltage fluctuations of the upstream oxygen sensors located in the exhaust manifolds. If you are lucky, you will find one of them is defective. When this code is triggered, it will prevent the engine computer to close loop.
Finally, getting a P0125 coolant code fixed can be done easily if you follow these basic tests.