Metal Oxide Varistor (MOV) are voltage  dependent symmetrical resistors intended to protect equipment from transient voltages resulting from lightning strikes. This component is located near the ac supply input and before the bridge rectifier.  
The use of MOV not only reduces the chance of electrical damage buut also improves equipment reliability. Metal oxide varistor have many types of colors and to test it you can use an analog meter set to X 10 K ohm and it should show no reading on both ways. If you got any readings then it had developed a short circuit.

Some important parameters of MOV are a varistor’s energy rating (in joules), response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown (clamping) voltage. The energy rating is often defined using ‘industry standard’ transients such as 8/20 microseconds or 10/1000 microseconds. Metal Oxide Varistor are designed for shunting short duration pulses. For example, 8 microseconds is a transient’s rise time; 20 microseconds is the fall time. If it break down get back the same spec or some closer value.