What does code P1139 mean?

P1139 · SubaruFront Oxygen AF Sensor #1 Heater Circuit Performance Range Problem

P1139 on Subaru vehicles indicates that the engine control module (ECM) has detected an abnormal performance range in the heater circuit of the front air-fuel ratio sensor, also called the AF sensor, for bank 1. The ECM monitors the heater current and voltage to ensure the sensor reaches operating temperature quickly; this code sets when the heater circuit's electrical behavior falls outside the expected range, which can delay closed-loop fuel control and increase emissions.

SystemPowertrain

Code typeSubaru specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Guidance is general and written for Subaru vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.