the switch can also become intermittant or oil fouled, once that happens its not far off leaking.

to check that you really need a decent multimeter OR a range of bulbs.

if you test it with one thats of higher wattage than the one in the dash you may get a false OK as theres not enough current passing through the switch to light up a bigger bulb.

theres also a varying range of pressure sensors that will fit look about the same and work pretty close to right and will give the same symptoms if you hook up a light instead of a gauge.