101 sends a pulse signal to an injector and expects to receive an output if no output is detected via the output signal line a fault has been detected for example the pcm 101 would perform a stuck open test on the gasoline relay 103 where only gasoline injection is required because gasoline injection is desired the gasoline relay 103 should have connected the gasoline injector 105 to the power source to perform the stuck open test the pcm 101 sends a pulse signal to the gasoline injectors 105 and expects to receive a corresponding output via the output signal line if the pcm 101 does not receive a corresponding output from the gasoline injectors 105 then the pcm 101 infers that the gasoline relay 103 has not connected the gasoline injectors 105 to the power source in the same scenario the pcm 101 may also perform a stuck closed test on the compressed natural gas relay 102 because compressed natural gas injection is not desired the compressed natural gas relay 102 should have disconnected the compressed natural gas injectors 104 from the power source in order to perform the stuck closed test on the compressed natural gas relay 102 the pcm 101 sends a pulse signal to the compressed natural gas injectors 104 and expects to receive no corresponding output via the output signal line if the pcm 101 receives an output from the compressed natural gas injectors 104 then the compressed.