Doug Rose is the author for this bit of wisdom:

Here is my procedure for troubleshooting gauges and sensors:

Every Marinette toolbox should have a 5 watt 33 ohm resistor and a 1 watt 240 ohm resistor, each with clipleads, along with a multimeter with an extra set of very long leads with clips.

To test the meter:

=> Turn the ignition off.

=> Remove the wire from the "S" terminal of the meter.

=> Turn the ignition on.

=> Verify that the needle is all the way left below the zero mark.

=> Turn the ignition off.

=> Clip the 240 ohm resistor from ground to the "S" terminal of the meter.

=> Turn the ignition on.

=> Verify that the needle is on minimum: "E" for fuel, zero for oil pressure and 120 degrees or something for temperature.

=> Turn the ignition off.

=> Remove the 240 ohm resistor and clip the 33 ohm resistor in its place.

=> Turn the ignition on.

=> Verify that the needle reads maximum: "F" for fuel, 80 psi or something for oil pressure, and 240 degrees or something for temperature.

=> Turn the ignition off.

If the three verifications are made, meter head is good.

To test the sensor:
=> Turn the ignition off.

=> Connect an ohmmeter between ground and the wire that was on the "S" terminal.

=> Verify 240 ohms +/- 30 ohms, or less if appropriate for a temperature sensor.

=> Turn the ignition on and start the engine.

=> Verify resistance between 240 and 33 ohms, scaled to the expected reading for the sensor.

=> Turn the ignition off.

If two verifications are good then sensor is good.

If the sensor does not read good, then remove the sensor wire at the sensor on the engine block, and repeat the sensor test to see if the wiring is bad.

To test the wiring:

=> Turn the ignition off.

=> Connect one lead of ohmmeter to the engine block and the other lead to the "ground" in the instrument panel.

=> Verify resistance of less than one ohm.

=> Disconnect wire from sensor and connect to one lead of ohmmeter and the other lead to the sensor wire that was removed from the "S" terminal of the meter.

=> Verify resistance of less than one ohm.

=> Remove lead from sensor wire and attach to "ground".

=> Verify resistance is high, over 1 Megohm.

Test Closeout:

=> Turn the ignition off.

=> At engine block, connect sensor wire to sensor.

=> At instrument panel, connect sensor wire to "S" terminal of meter.

=> Record test results in logbook.

=> Remove Prepositioned Refreshment Asset from cooler. Insert index finger into aluminum tab, and pull.