P0031 on Toyota: How to Diagnose & Fix
HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
1 Toyota model · 1 documented issue
P0031 on Toyota by model: symptoms, diagnosis and fix
Owners of fourth-generation 4Runners report code P0031, a heater circuit fault for the bank 1 upstream sensor, often together with P0051 for bank 2, with the check engine and VSC lights on. In one 2003 V6 case, replacing both sensors and the engine computer did not clear the codes; the cause was a broken wire through the firewall to the A/F heater relay. Applies to: 2003-2009 Toyota 4Runner with the 4.0L 1GR-FE V6 or the 4.7L 2UZ-FE V8. The owner reports linked here are from a 2003 V6 and 2004, 2005 and 2007 V8 trucks. The V6 uses air-fuel ratio sensors in this position; owners say early (pre-2005) V8s use conventional oxygen sensors there. Mileage when it happened, as stated in the linked owner reports: 36,000 miles on a 2007.
Common Symptoms
- Check engine light on with code P0031
- VSC TRAC and VSC OFF lights come on with the engine light
- Check engine light comes straight back after codes are cleared
How to Fix
Have the codes diagnosed before buying parts: a technician should check the sensor heaters, the A/F heater relay, and the wiring and connectors. One owner who fitted the wrong type of replacement sensor got both codes right after installation.
Sources for this issue (5)
- Toyota 4Runner Forum: O2 Sensor Replace - p0031 Code?forum
- Toyota 4Runner Forum: P0031 oxygen sensor locationforum
- Toyota 4Runner Forum: Simultaneous P0031 and P0051 Codesforum
- The Track Ahead: P0031 and P0051 Codes - Check Engine Light, VSC TRAC, VSC OFF Light (owner project blog)forum
- Toyota 4Runner Forum: Swapped out O2 sensors and getting P0031 and P0051forum
P0031 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0031 mean on a Toyota?
P0031 stands for "HO2S Heater Control Circuit Low (Bank 1 Sensor 1)." The PCM detects low voltage (short to ground or open) in the heater control circuit of the upstream O2 sensor on Bank 1, Sensor 1. On Toyota it is documented across 1 model: 4Runner.
What are the most common causes of P0031?
The most common causes of P0031 are: Open/short in heater wiring, Failed O2 sensor heater, Corroded connector, Blown fuse. The specific cause varies by vehicle.
How much does it cost to fix P0031 on a Toyota?
Repair costs vary widely with the root cause and Toyota model; each issue on this page carries its own estimate where one is published.
Looking for P0031 on a different make?
View P0031 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.