Diagnostics: Scan Tool
Scan tools are great for monitoring sensor input from the different onboard computers. Many scanners can also actuate items in mode 8, bypassing sensor input. If the vehicle is overheating, and the ECT, or engine coolant temperature sensor is suspected of being at fault, bypass the sensor and activate the fan circuit. If the fan works, inspect the ECT and associated circuitry.
The same is true for the fuel sending unit. If the gauge is continuously reading full or empty, connect the appropriate scan tool to the DLC and monitor the fuel level sensors input to the PCM as the tank is filled. This will indicate if the sending unit is functioning correctly, or if there is a problem with the instrument panel gauge or circuitry.