Abstract: A pellet resistor gas sensor includes a housing and sensor, the sensor being located in said housing and being adapted to provide a signal indicative of the presence of a gas in said housing. A test gas generator supplies a test gas into the housing on demand. This enables the operation of the sensor to be verified. The test gas generator preferably includes electrodes having a catalyst such as carbon, platinum, ruthenium or rhodium for promoting the generation of the test gas, and electrical contacts, in the form of pins, which connect electrodes within the housing to external wires, which lead to a remote current source. The pins may be in seating engagement with apertures in the housing through which they protrude.