DTC P1643: Wait To Start Indicator Control Circuit: Description
WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
A Wait to Start indicator is illuminated by the Engine Control Module (ECM) when the glow plugs are commanded ON. When the ECM is commanding the Wait To Start indicator ON, the voltage potential of the circuit will be low, near 0 volts. When the ECM is commanding the Wait To Start indicator OFF, the voltage potential of the circuit will be high, near battery voltage. The primary function of the ECM is to supply the ground for the Wait To Start indicator control circuit.