Circuit/System Description
WARNING: This page is about a different car, the 2010 Pontiac G5 and 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM) has 2 internal 5 V reference busses called 5 V reference 1 and 5 V reference 2. Each reference buss provides 5 V reference circuits for more than one sensor. A fault condition on one 5 V reference circuit will affect the other 5 V reference circuits connected to that reference buss. The ECM monitors the voltage on the 5 V reference buss.