Code Definition

P1143 — HO2S Bank 1 Sensor 3 Signal Below 0.45v (A/F Ratio Too Lean)

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

About P1143

P1143 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 P1143 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.

P1143 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

Some Mazda exhaust systems use a third oxygen sensor on Bank 1, Sensor 3, located furthest downstream after the rear catalytic converter, to refine fuel trim and monitor catalyst performance. DTC P1143 sets when this sensor signal stays below about 0.45V, telling the Powertrain Control Module (PCM) the exhaust at that point is lean or the sensor is biased low. This can reflect a genuine lean condition, an exhaust leak drawing in air near the sensor, or a sensor or wiring fault. Driveability is often unaffected, but the MIL will illuminate and related fuel trim or catalyst monitor codes may be ...

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