A diode is a electronic device that passes current in only one direction.  It can be thought of as being a bit like a non-return valve for electricity.

Diode test results analysis:

• A good forward-based diode displays a voltage drop ranging from 0.5 to 0.8 volts for the most commonly used silicon diodes.


• The multimeter displays OL when a good diode is reverse-biased. The OL reading indicates the diode is functioning as an open switch.


• A bad (opened) diode does not allow current to flow in either direction. A multimeter will display OL in both directions when the diode is opened.


• A shorted diode has the same voltage drop reading (approximately 0.4 V) in both directions.

Diodes are used for a huge amount of things within a gas boiler, one of the main uses in to form a bridge rectifier either on the PCB or in the gas valve plug. A bridge rectifier uses four diodes to take the AC voltage from the mains or the output of a transformer and “rectify” it to the DC voltage required by the circuit, in this case the solenoid.

When testing a diode you are measuring the voltage drop across the component, a healthy diode will show a drop in one direction (forward biased) but not the other (reverse biased), therefore to fully check a diode it should be tested in both directions. The diode here is shown out of circuit for clarity, however there is no need to remove the diode to test it. Click here for help setting your meter to the Diode test mode.