CHT and EGT
Low or intermittent readings. Make sure all connections are clean tight and secure. Try unplugging connectors and re-attaching.
Check thermocouple resistance. Use an ohmmeter. With the thermocouple disconnected from the gauge and at ambient temperature. The CHT thermocouple will have around 1.5 ohms. The EGT thermocouple will have around 2.5 ohms. This will vary if the ambient temp varies between the connector end and the probe end of the thermocouple. This is not an absolute test but it is a good quick field test. We are looking for an open circuit here if the sender is dead.
If you suddenly have no readings you can test the sender as above and also check the gauge. With the thermocouple disconnected use an ohmmeter across the gauge connections. CHT will be about 8.5 ohms and EGT will be around 10-18 ohms depending on scale range and calibration. Here we are looking for an open circuit if the gauge itself is in need of repair or replacement.
Temperature Gauges / senders
If your gauge jumps when you turn on lights, fans etc. The most common problem is that the gauge ground and the sender ground are at different locations. Make sure that these grounds are at one single grounding point. It can be under the dash or in the engine compartment. Do not jumper from one ground point to another or daisy chain them.
To test the temperature sender ( oil, water, air 399-series ) use an ohmmeter and a known source of heat. Boiling water at sea level is 212 deg f. = 212 ohms on the sender with the sender disconnected from the gauge. Other values are 32 deg f = 9800 ohms 70 deg f = 3570 ohms 100 deg f. There are other ranges in the misc info page.
To test the gauge , substitute the sender with these known resistance values to simulate the temperature and corresponding reading on the gauge.
"KV" pressure senders
These operate on 5 vdc from the gauge. The output is .5 to 4.5 vdc. 0 to max rating of the sender. 387-15KV 0-15 psi 387-100KV 0-100 psi. You can use a voltmeter to test this. Also you can use a regulated air supply so you can adjust the pressure applied to the sender and monitor the output. This can be tested with the gauge connected.