P0131 on Subaru
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
P0131 on Subaru vehicles indicates o2 sensor circuit low voltage (bank 1, sensor 1). Au7o has documented this code across 1 Subaru model — most commonly on Forester. This code means the upstream oxygen (O2) sensor on Bank 1, Sensor 1 — located before the catalytic converter — is reporting an abnormally low voltage, which the computer reads as a persistently lean exhaust (too much oxygen). The computer uses this sensor to fine-tune the air-fuel mixture, and a stuck-low signal disrupts that feedback. The low reading can come from an actual lean condition in the engine or from a faulty sensor or its wiring. Symptoms may include rough running, hesitation, and reduced fuel economy. Typical repair costs on Subaru range from $150 to $450, depending on the specific model and root cause.
Common Causes of P0131
- •Vacuum or intake air leak (true lean condition)
- •Failed or contaminated upstream O2 sensor
- •Exhaust leak ahead of the sensor
- •Low fuel pressure or weak/clogged fuel injectors
- •Corroded or loose O2 sensor connector
- •Damaged or shorted-to-ground sensor wiring
- •Dirty mass airflow sensor skewing fuel trims
- •Faulty PCM/ECM (rare)
P0131 on Subaru by Model
Subaru Forester(1 issue)
- Front Air/Fuel Ratio (Wideband O2) Sensor Heater Circuit Failure — P0031/P00372009-2018
Foresters commonly throw front oxygen/air-fuel-ratio sensor heater-circuit codes (P0031 low, P0037 downstream) as the sensors age. The sensors live on the hot exhaust, so the wiring harness near them turns brittle and chafes, and the internal heater element degrades. The result is an illuminated check-engine light, slightly richer/leaner running while the heater is cold, and a guaranteed emissions-test failure. Many owners replace the sensor only to find the real fault is damaged harness wiring at the connector.
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What does P0131 mean on Subaru?▼
P0131 stands for "O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)." This code means the upstream oxygen (O2) sensor on Bank 1, Sensor 1 — located before the catalytic converter — is reporting an abnormally low voltage, which the computer reads as a persistently lean exhaust (too much oxygen). The computer uses this sensor to fine-tune the air-fuel mixture, and a stuck-low signal disrupts that feedback. The low reading can come from an actual lean condition in the engine or from a faulty sensor or its wiring. Symptoms may include rough running, hesitation, and reduced fuel economy. On Subaru specifically, this code is documented across 1 model.
What causes P0131 on Subaru vehicles?▼
Common causes on Subaru: Vacuum or intake air leak (true lean condition), Failed or contaminated upstream O2 sensor, Exhaust leak ahead of the sensor, Low fuel pressure or weak/clogged fuel injectors, Corroded or loose O2 sensor connector. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0131 on a Subaru?▼
Repair costs on Subaru range from $150 to $450, depending on the specific model and root cause.
Which Subaru models have P0131 documented?▼
Au7o has documented P0131 on 1 Subaru model: Forester.