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Oxygen sensor failures in Mitsubishi Lancer, Galant, and Eclipse models commonly trigger diagnostic trouble codes such as P0135, P0141, P0155, and P0157. P0135 and P0155 indicate heater circuit malfunctions in the upstream sensors, while P0141 and P0161 indicate downstream sensor heater issues. Replacement should be done with the correct OEM or equivalent sensor for the engine family, including 4G63 and 4G64 variants. Upstream sensors are located before the catalytic converter and monitor air-fuel ratio; downstream sensors are after the converter and monitor catalyst efficiency. Always disconnect the battery, allow the exhaust to cool, and apply anti-seize compound only if specified by the sensor manufacturer. Modern OBD-II systems, including 2026 updates to SAE J1979-2, support enhanced diagnostic data and standardized sensor readiness monitoring, making accurate sensor selection and correct installation critical for emissions compliance and driveability.