P0131 Toyota 4Runner
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1) · Powertrain Code · Medium · Repair: Moderate
What is P0131?
Low voltage from the upstream O2 sensor on bank 1, indicating a possible lean condition or sensor failure.
When the Toyota 4Runner triggers code P0131 (o2 sensor circuit low voltage (bank 1, sensor 1)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Toyota 4Runner.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Toyota 4Runner.
Common Causes on Toyota 4Runner
- Bank 1 upstream O2 sensor stuck lean — contaminated or aged sensor element
- Vacuum leak causing actual lean condition that sensor is correctly reporting
- Cracked exhaust manifold allowing air to reach the O2 sensor
- Failed Denso O2 sensor heater circuit causing slow warmup
Symptoms
- Check Engine Light on
- Engine running rich as ECM compensates for false lean reading
- Black exhaust smoke at idle
- Reduced fuel economy
- Sulfur or rotten egg smell from exhaust
Notes for Toyota 4Runner
On Toyota V6 models, bank 1 is the firewall side — ensure you are replacing the correct sensor before purchasing the part.