Fault code P0031-Heated oxygen sensor (HO2S) 1, bank 1, heater control-circuit low means that the heater in your oxygen sensor is not working. The heater helps the oxygen sensor get to operating temperature faster, improving fuel economy.
P0031 quick overview
| Meaning | P0031: Heated Oxygen Sensor (HO2S) Heater Control Circuit Low (Bank 1, Sensor 1) |
| Is it serious? | This code is not immediately critical but should be addressed promptly to maintain fuel efficiency and avoid potential long-term damage to the engine or catalytic converter. |
| Possible causes | -Faulty heated oxygen sensor (HO2S) on Bank 1, Sensor 1 -Blown fuse for the oxygen sensor’s heater circuit -Damaged or corroded wiring or connectors for the oxygen sensor -Rarely, a faulty Powertrain Control Module (PCM) |
| How to diagnose? | Check Fuses: Start by checking the fuse for the oxygen sensor’s heater circuit for signs of damage or a break. Inspect Wiring: Visually inspect the wiring and connectors for the oxygen sensor for any signs of damage, wear, or corrosion. Test the Sensor: Use a multimeter to test the resistance and voltage of the oxygen sensor’s heater element. Replace Faulty Parts: If the fuse, wiring, or sensor is faulty, replace them and clear the code using an OBD-II scanner. |
Test oxygen sensor PDF
- How to locate a specific sensor (bank1, 2)
- How to test with a multimeter
- How to test using an OBD-II reader
- Verify O2 sensor/CAT just with live data
Case study: How to fix P0031
I looked into 10 different cars with P0031 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what a real mechanic would do to solve it.
Common fix
| Fix | Frequency (%) |
|---|---|
| Replace O2 sensor with OEM part | 50% |
| PCM/ECM replacement | 30% |
| Repair wiring/connection issues | 10% |
| Fuse replacement | 5% |
| Throttle body inspection/reconnection | 5% |
Key takeaway: Replacing the O2 sensor with an OEM sensor is the most frequent fix, accounting for 50% of cases. PCM/ECM replacement is also common, especially when O2 sensors and wiring check out but the issue persists.
Common mistakes
- Using Aftermarket Sensors: Several mechanics installed aftermarket O2 or air/fuel ratio sensors that caused persistent P0031 codes. In many cases, switching to an OEM sensor resolved the issue.
- Misidentifying Bank and Sensor: Some technicians confused Bank 1 and Bank 2 sensors, particularly in cases where the sensor was installed on the wrong side of the vehicle (e.g., Bank 1 vs. Bank 2 confusion).
- Overlooking ECM Compatibility: Mechanics sometimes installed incorrect ECMs, leading to unresolved P0031 codes. This was most apparent in cases where the wrong year’s PCM/ECM was provided.
- Neglecting Grounding Issues: In some cases, mechanics did not properly check the grounding circuits of the O2 sensor heater, leading to incomplete diagnoses.
- Failing to Perform Thorough Continuity or Load Testing: Some mechanics hesitated to perform voltage drop tests or full continuity tests on the O2 heater circuits, relying on part replacement instead.
Key insights learned!
- Use OEM Sensors: The most effective solution across all cases was replacing the O2 sensor with an OEM part. Aftermarket sensors frequently caused improper function and persistent codes.
- Check Heater Circuit Voltage and Grounding: A key diagnostic step is verifying both power (12V) and ground connections in the O2 heater circuit. Load testing with a light bulb is effective in detecting weak or unstable connections.
- PCM/ECM Testing: If replacing the O2 sensor and checking wiring/grounds does not resolve the issue, the PCM/ECM may be at fault. Cross-check ECM part numbers, as incorrect ECMs may be provided based on VIN mismatches.
- Inspect for Short Circuits or Damaged Wiring: Thoroughly check the wiring harness, especially in areas prone to rubbing or exposure (e.g., under the intake manifold). Intermittent shorts can trigger P0031 codes.
- Consider Fuse Issues and TSBs: Blown fuses (e.g., SBF7) can cause the O2 sensor heater circuit to fail. Additionally, check for applicable technical service bulletins (TSBs) related to ECM or heater circuit issues for your vehicle.
You can also read all individual cases of solving P0031 which I used to create this study result and diagnostic guide!
Case #1: Dodge
Vehicle information: 2001 Dodge Ram 1500 Laramie 5.9L
Issue + repair history:
The vehicle had the MIL (Malfunction Indicator Lamp) on, with code P0031 indicating a B1 S1 (Bank 1 Sensor 1) heater circuit low. The O2 sensor was replaced, but the code returned. Further diagnostics with a DRB3 scanner showed inconsistent voltages for the O2 sensors. The technician followed diagnostic procedures, tested the heater circuit, and replaced the PCM, but the problem persisted.
Other codes + test results:
None reported.
Other mechanics suggestions:
- Check the O2 sensor grounds, as Chrysler vehicles often have issues with ground wires breaking off.
- Perform a voltage drop test on the system, especially under load, to identify weak connections.
- Inspect the battery and cables for potential issues affecting sensor readings.
- Use an OEM (factory) O2 sensor for better reliability.
Final fix:
The issue was traced to a bad connection at the O2 sensor connector on the PCM side. After repairing the connector, the O2 sensor heater circuit functioned properly, and the problem was resolved.
Case #2: Nissan
Vehicle information: 2003 Nissan Sentra GXE 1.8L
Issue + repair history:
The vehicle came in with the MIL (Malfunction Indicator Lamp) on, displaying code P0031, which indicates an HO2S1 (Heated Oxygen Sensor 1) heater control circuit low. A pin-point test revealed high resistance (10.2 ohms) in the O2 sensor, above the specification of 2.3 to 4.3 ohms. Despite replacing the sensor, the same resistance issue persisted. After further investigation, it was found that the ECM was not grounding the heater circuit.
Other codes + test results:
None reported.
Other mechanics suggestions:
- Verify the 12 volts required to operate the heater and check the ground.
- Check if there are multiple O2 sensors.
- The resistance of 10–17 ohms might be acceptable, as aftermarket sensors can sometimes cause issues.
- Inspect the ECM for moisture damage, particularly if there’s foam inside the ECM box, as this model had a recall related to moisture problems affecting the ECM.
Final fix:
The ECM was replaced with a used one, and the heater circuit worked correctly, resolving the issue.
Case #3: Subaru
Vehicle information: 2008 Subaru Outback 2.5i 2.5L
Issue + repair history:
The vehicle came in with the MIL (Malfunction Indicator Lamp) on, displaying codes for both O2 sensor heater circuits. Upon inspection, no power was found to the front or rear O2 sensors. The fuse for the heater relay and throttle module relay (SBF-7) was blown, and replacing the fuse did not solve the issue as it kept blowing. After disconnecting the throttle body, the fuse stopped blowing, and power was restored to the O2 sensors. However, after reconnecting the throttle body and performing a test drive, the issue did not recur.
Other codes + test results:
- P0031 – Bank 1 Sensor 1 heater circuit low
- P0037 – Bank 1 Sensor 2 heater circuit low
- Ohm check of heater circuit wiring showed 0.4 ohms
- Voltage test showed 0V
Other mechanics suggestions:
- The throttle body might have an intermittent short to ground, and checking continuity between the throttle body and O2 sensors could help isolate the issue.
- A dirty or binding throttle body may cause the issue.
- Check the wiring harness under the intake manifold for possible rubbing on bolts, which could cause a short.
- Inspect the underside of the loom near the throttle body and O2 sensors for worn wiring.
Final fix:
Although the exact cause was not conclusively identified, the throttle module was suspected to be the issue. The vehicle was driven for two weeks with no recurrence of the problem after the throttle body was reconnected. Wiring under the intake manifold was also flagged as a potential future concern.
Case #4: Mazda
Vehicle information: 2000 Mazda MPV DX 2.5L
Issue + repair history: The van came in with a MIL lamp on and code P0031. The O2 sensor had previously been replaced along with the intake manifold runner control unit (IMRC), which fixed code P1512. However, the P0031 code returned. Wiring was checked but no issues were found. Voltage from the PCM to the O2 sensor was missing. The vehicle was sent to a Mazda dealership where the PCM and immobilizer were replaced and reprogrammed, but the P0031 code persisted. The O2 sensor was replaced again with no improvement.
Other codes + test results:
- P1512 – Intake manifold runner control stuck closed (previously fixed)
- Fuel pressure – within specifications
- Voltage test – no reading from PCM to O2 sensor
- Wiring – no broken or pinched wires found
Other mechanics suggestions:
- The PCM grounds the heater circuit, and it may not be receiving power from another source.
- Check if the wiring can power a test bulb to verify circuit functionality.
Final fix: The aftermarket O2 sensor was wired incorrectly. Installing an OEM sensor resolved the issue.
Case #5: Toyota
Vehicle information: 2003 Toyota Camry SE 2.4L
Issue + repair history: The vehicle originally had a P0420 code and the front flex catalytic converter assembly was replaced. Two weeks later, the MIL light came back on with code P0031 (B1S1 sensor circuit low). The front oxygen sensor was replaced with an aftermarket part, but the code returned. The sensor was replaced again with a dealer part, and the ECU was flashed, but the light came back. The technician suspected a possible ECU issue.
Other codes + test results:
- P0420 – Catalyst system efficiency below threshold (fixed)
- Wires at EFI relay – 3.3V on one wire, 3.0V on the other
- Grounds for the relay and fuse – checked out fine
- Wires from computer to sensor – no broken wires found
Other mechanics suggestions:
- Ensure the correct O2 sensor is used based on emissions type (Federal or California).
- Verify proper grounding of the O2 heater circuit and inspect the overall wiring.
- Check for an Air/Fuel ratio sensor instead of an O2 sensor, referencing the VIN to match parts.
- Some mechanics believed ECU failure was unlikely, though one reported having replaced an ECU for a similar issue.
Final fix: The first dealership had sold the wrong sensor. After replacing it with the correct part, the issue was resolved.
Case #6: Subaru
Vehicle information: 2005 Subaru Outback R L.L. Bean Edition 3.0L
Issue + repair history: The vehicle arrived with the MIL lamp on and the cruise control light flashing. Scanning revealed code P0031 for the Bank 1, Sensor 1 O2 sensor (front left). The SBF5 and SBF7 fuses were checked and found to be good. Power and grounds at the O2 sensor were also verified as good. Initially, a non-OEM air/fuel ratio sensor was installed in haste, which was incorrectly identified as an oxygen sensor.
Other codes + test results:
- P0031 – Bank 1, Sensor 1 low
- 12V at O2 sensor – confirmed
- Fuses SBF7 and SBF5 – confirmed OK
- Replaced B1S1 O2 sensor – no change
Other mechanics suggestions:
- Check the heater control signal and the duty cycle on the scan tool, as the heater signal should be pulsed, not steady.
- Verify if the sensor is an air/fuel ratio sensor rather than an O2 sensor and ensure it is OEM.
- Clarify the sensor location; Bank 1, Sensor 1 is on the right (passenger) side, not the left (driver’s) side.
- The heater circuit is controlled by the ECU, which pulses the ground for the sensor.
Final fix: The incorrect (aftermarket) air/fuel ratio sensor was initially installed. Replacing it with an OEM Bosch sensor resolved the issue.
Case #7: Toyota
Vehicle information: 2011 Toyota 4Runner SR5 4.0L
Issue + repair history: The customer brought the vehicle in with the check engine light on. A friend had installed a new O2 sensor for Bank 1, Sensor 1 (B1S1), but the P0031 code returned. Upon inspection, it was found that the new sensor had been incorrectly installed on the driver’s side (Bank 2). The technician removed the B1S1 sensor, checked the resistance on pins 1 and 2, and found it open. The sensor was replaced, but the code reappeared before the engine could be started.
Other codes + test results:
- P0031 – Air/Fuel Sensor (Bank 1, Sensor 1) Heater Control Circuit Low
- Resistance check on B1S1 pins 1 and 2 – open
Other mechanics suggestions:
- Ensure the correct installation of connectors, as O2 and Air/Fuel sensors have different connectors and should not be interchanged.
- Test the sensor’s main signals to check for weak or missing power and ground.
- Unplug both front sensors and see if open circuit codes set, to rule out defective replacement sensors.
Final fix: The issue was resolved by correctly installing the proper sensor after recognizing the initial mistake of overlooking the sensor installation.
Case #8: Dodge
Vehicle information: 2001 Dodge Ram 1500 Laramie 5.9L
Issue + repair history: The truck originally came in with severe engine misfire and flashing check engine light, displaying only misfire codes. The injector wires were found chewed by rodents, and a Mopar service kit was used to replace the damaged wiring. Even after fixing the wiring, the engine still had a dead cylinder, and the driver for injector #6 was determined to be bad. After replacing the ECM and flashing the VIN, the engine ran perfectly, but a new P0031 code appeared for Bank 1, Sensor 1 O2 heater circuit low. The O2 sensors were replaced, but the P0031 code persisted.
Other codes + test results:
- P0031 – O2 sensor heater circuit low (Bank 1, Sensor 1)
- Circuit load tested using a headlight
- Front and rear O2 sensors replaced – no change
Other mechanics suggestions:
- Load test all ECM powers and grounds using a headlight to ensure proper connections.
- The wrong ECM may have been provided by the dealer, especially if drivers for the O2 sensors were missing.
- Consider trying the old ECM to see if the new code still appears.
Final fix: The truck required a PCM from a 2000 model year instead of the 2001 model year due to a manufacturing discrepancy. Although the truck was labeled as a 2001 by the VIN, it used a 2000 PCM, which ultimately resolved the issue when the correct PCM was installed.
Case #9: Jeep
Vehicle information: 2004 Jeep Grand Cherokee Laredo 4.0L
Issue + repair history: The vehicle came in with the MIL lamp on and code P0031 (HO2S11 heater circuit low). Testing showed resistance in the upstream O2 sensors: 5.2 ohms for HO2S11 and 4.3 ohms for HO2S21. The heater circuits were tested with a load light, showing 3 amps and proper duty cycling. HO2S11 was replaced with a Denso sensor (4.2 ohms), but the P0031 code persisted.
Other codes + test results:
- P0031 – HO2S11 heater circuit low
- Resistance of 5.2 ohms for HO2S11 and 4.3 ohms for HO2S21
- The voltage drop test showed instability in the ground from the PCM
Other mechanics suggestions:
- Check for software updates or TSBs related to the heater circuit for vehicles built before 6/2/03 (TSB 18-032-03).
- Inspect Fuse #16 in the PDC for shorts to ground.
- Use only factory (OEM) sensors, as aftermarket sensors may not be compatible with the vehicle’s heater circuit calibration.
- The ground circuit is VPW modulated, and incorrect O2 sensors can damage these circuits.
Final fix: The PCM was not maintaining a stable ground, so it was replaced. The dealer also replaced the O2 sensor, stating that the aftermarket sensor was incompatible. The final fix involved installing both a new PCM and a factory O2 sensor.
Case #10: Nissan
Vehicle information: 2002 Nissan Sentra SE-R 2.5L
Issue + repair history: The vehicle arrived with a P0031 code for the O2 sensor heater circuit (HO2S1). Initial tests confirmed battery voltage at the O2 sensor plug. However, the technician encountered difficulties with continuity tests due to issues with the Nissan technical site. The technician considered whether to continue testing or simply replace the O2 sensor.
Other codes + test results:
- P0031 – HO2S1 Heater Circuit Low
Other mechanics suggestions:
- Many suggested replacing the O2 sensor, as defective O2 sensors and clogged catalytic converters are common issues with this vehicle.
- Inspect the catalytic converter for clogging, as it could cause further engine damage.
- Check for any ECU-related recalls, especially regarding the ECU foam liner, which could damage the ECU.
- Use only OEM O2 sensors, as aftermarket parts may not work properly in OBD-II systems.
Final fix: The O2 sensor was replaced, which resolved the issue. No further problems were encountered, and the vehicle did not require additional repairs related to the ECU or catalytic converter.
Test oxygen sensor PDF
- How to locate a specific sensor (bank1, 2)
- How to test with a multimeter
- How to test using an OBD-II reader
- Verify O2 sensor/CAT just with live data
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko