When the key is in turned ON, the PCM energizes the fuel pump relay which sends a voltage to the lift pump. The lift pump remains during the glow plug cycle. If the engine is not started immediately after the glow plug cycle, the PCM shuts the fuel lift pump OFF and waits until engine RPM is greater than zero and then turns the relay back ON. As a backup system to the fuel pump relay, the fuel lift pump can also be turned ON by the fuel pump/oil pressure switch. When the engine oil pressure reaches 28 kPa (4 psi), and the fuel pump relay does not complete the circuit, the fuel pump/oil pressure switch will close and complete the circuit to run the fuel lift pump.