Code Definition

P1131 — HO2S Bank 1 Sensor 1 Signal Below 0.45v (A/F Ratio Too Lean)

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Mazda

About P1131

P1131 is a manufacturer-specific powertrain diagnostic trouble code recorded for Mazda vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1131 is stored, the module has detected a signal or supply voltage lower than the expected operating range; a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1131 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The upstream heated oxygen sensor (HO2S) on Bank 1 Sensor 1 reports exhaust oxygen content to the Powertrain Control Module (PCM) as a switching voltage, high for rich and low for lean. DTC P1131 sets when this sensor signal stays below about 0.45V and does not switch, telling the PCM the air fuel ratio on Bank 1 is too lean or the sensor is biased low. This can be a genuine lean condition from unmetered air or a fuel delivery shortfall, or a sensor, wiring, or exhaust leak fault that makes the mixture appear lean. The driver may notice hesitation, surging, rough idle, poor power, and an illum...

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