1996 Ford F350 460 Engine P1131 and P1151 Codes: O2 Sensor Signal Failure Diagnosis

Can you monitor the upstream B1S1 and B2S1 oxygen sensors? The downstream sensors were replaced—could they be contributing to the issue or is it a deeper sensor signal problem?

My Innova scanner doesn't support real-time O2 monitoring, but I'm currently checking. I previously received questionable advice when my CEL first appeared—I replaced the upstream sensors based on that recommendation. It was a straightforward job with no repair history, and after clearing codes, they returned after driving.

Freeze frame data confirms both P1131 and P1151 errors. P1131 means 'Lack of HO2S11 Switches - Sensor Indicates Lean,' and P1151 indicates 'Lack of HO2S21 Switches - Sensor Indicates Lean.'

If the upstream O2 sensors have no history and are not responding, consider replacing them. Use a scan tool to perform quick snap throttle tests—ideally, the voltage should rise to around 0.8V. Also check whether the system is in open or closed loop; if sensors aren't active, it will remain in open loop.

Thanks! I'll try that right away.

I visited a friend's shop and used his advanced scanner. The O2 sensors show no signal—only heating element voltage (around 8V). This matches the description of 'Lack of HO2S11 Switches' in P1131 and P1151. I'm going to re-install the original upstream sensors today and see if the error codes return. Could this be an ECU failure? Or a wiring issue?

Just to clarify—were the replaced sensors upstream or downstream? The P1131/P1151 errors refer to upstream O2 sensors. If new sensors show no response and the ECU doesn't detect them, it could point to a wiring issue or ECU malfunction.

Yes, I replaced both upstream O2 sensors and have now reinstalled the original ones. The same P1131 and P1151 codes appear with the CEL on. After performing a Ford KOER test, four codes are present: P1131, P1151, P1000 (OBD Systems Readiness Test Not Complete), and P1127 (Exhaust Temperature Out of Range; O2 Sensor Tests Not Completed). I've rechecked all connectors—no signs of damage. The wiring harness looks intact. I'm now completely stuck on this issue.

I cleared the codes and drove nearly 10 miles before they returned. Then I cleared them again and performed a full KOER test to confirm the diagnosis.

P1127 error says 'Heated Oxygen Sensor (H02S) Heater Circuit Malfunction.' Which O2 sensors does this refer to? Is it upstream or downstream?

Only the upstream O2 sensors. Same codes appear on both left and right banks.

P1131, P1151, P1000, and P1127—all detailed above.

After testing with a more advanced scanner, I saw voltage on the heating elements (estimated ~8V) but no signal from the O2 sensors. Without a wiring diagram, I assume the upstream O2 sensors connect directly to the ECU. I've traced the harness as far as possible and found no visible damage. Could there be a blown fuse or relay affecting the heater circuits for the O2 sensors?

See bbbind.com for official wiring diagrams. To verify sensor signal integrity: - Use an incandescent test light and scan tool to monitor ECU input voltage. - Apply positive voltage from the test light to the O2 sensor signal wire at the connector. If the scan tool reads 1V, the signal wire and ECU are functional. If it still shows 0V, the issue is in the wiring or ECU. - Apply test light to the ECU input side. If voltage appears, the problem lies in the wiring; if not, the ECU may be faulty. To check heater circuit continuity: - Verify 12V at the sensor connector's heater positive terminal during KOER (key-on engine-off). - If present, connect a test light between heater positive and negative. If it lights up, the heater circuit is good; if not, trace back to source. - The heater resistance on an O2 sensor should be around 2–12 ohms. An open or shorted heater indicates a defective sensor.

Thanks for the detailed test procedure, CJ1! This is very helpful and will guide my next steps in diagnosing the issue.