P0036 P0056 P0137 P0157 2020 Ford Mustang GT

[AmkoA]

Hello all,

2020 Mustang GT (Gen 3 Coyote, manual transmission).

In May 2024 I installed Stainless Works catless long tube headers. Shortly afterward, I hit a bump and the Bank 1 rear O2 sensor wiring was pulled into the driveshaft and destroyed. This initially caused P0036 and P0137.

Since then I have replaced both rear O2 sensors multiple times using OEM Motorcraft sensors. I also replaced the rear O2 sensor harness (LR3Z-14300-C). No O2 extension harnesses are installed.

The issue has gradually progressed to:

• P0036 (HO2S Heater Control Circuit B1S2)
• P0056 (HO2S Heater Control Circuit B2S2)
• P0137 (HO2S Circuit Low Voltage B1S2)
• P0157 (HO2S Circuit Low Voltage B2S2)

The codes return after every DTC reset and typically return together on both banks.

Diagnostic work already performed:

• Multiple new OEM rear O2 sensors
• New rear O2 harness
• Connector inspections
• Continuity checks from sensors to PCM
• Voltage checks at sensors and PCM
• Fuse checks
• PCM relay checks
• Wiggle tests
• Visual inspection of wiring
• No catalytic converter efficiency codes present
• Rear sensors use 90-degree spacers

One thing I found interesting is that on startup the front O2 sensors behave normally, but both rear O2 sensors remain around battery voltage (~14V) and do not appear to behave like a normal heated sensor circuit. The same behavior was observed from the PCM side of the circuit.

At this point I am starting to suspect PCM damage from when the original Bank 1 rear O2 wiring was pulled into the driveshaft. I have found a few reports of similar Mustang PCM failures causing rear O2 heater and low-voltage codes on both banks.

Before I spend approximately $960 on a new PCM and programming, does anyone have any other ideas or experience with these specific codes on a Gen 3 Mustang?

Thank you.