Oxygen sensor circuit high input

P0138 - downstream of catalyst (rear) / rear (RH) bank

P0158 - downstream of catalyst (rear) / front (LH) bank

Emissions Equipment Information

Three o² sensor system (KL engine with one catalytic converter) - A front sensor is positioned on the LH bank and a middle sensor is positioned on the RH bank upstream of the catalyst for closed loop fuel control.  A rear sensor is mounted downstream of the catalyst to monitor its efficiency.

Four o² sensor system (KJ / KL engine with three catalytic converters) - Front sensors are mounted upstream of the RH and LH warm-up three-way catalyst (WU-TWC) for closed loop fuel control.  Rear sensors are mounted downstream of the WU-TWCs to monitor their efficiency.

Detection Conditions

• If input voltage from the oxygen sensor remains above 0.45V for 12 seconds during deceleration fuel-cut, the ECU determines that the circuit input is too high.

Diagnostic Support Notes

• This is a continuous monitor (CCM)
• Check engine indicator illuminates if the ECU detects either of the above malfunction conditions in two consecutive drive cycles
• PENDING CODE is available
• FREEZE FRAME DATA is available
• DTC is stored in ECU memory

Possible Causes

• Oxygen sensor malfunction
• Short to power circuit in wiring between oxygen sensor terminal A and ECU

[P0138 / P0158 - From 2002 Mazda Millenia factory service manual]