DTC P0037: Ho2S Heater (Rear) Circuit Low: Condition

WARNING: This page is about a different car, the 2002 Mazda Protege5 and 2002 Mazda Protege. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when PCM detects low input voltage from ground side of rear HO2S heater circuit after PCM turns HO2S heater circuit off. This is an intermittent monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

Loose connection at connectors.
Open or short circuit between rear HO2S and PCM.
Open circuit between rear HO2S and ignition switch.
PCM malfunction.
Rear HO2S malfunction.