P0031 is a very common check engine light code that points to a problem with the heater circuit of your front oxygen sensor (also called the air-fuel ratio sensor on many newer cars). When this circuit isn’t working correctly, the sensor can’t heat up fast enough, so your engine computer (ECU) may run a richer mixture, hurt fuel economy, and increase emissions. The good news: P0031 is usually straightforward to diagnose with basic tools and a bit of patience. Let’s walk through what it means, common causes, and how you can fix it.
What Does P0031 Mean?
P0031 stands for “HO2S Heater Control Circuit Low (Bank 1 Sensor 1).” In plain terms, your ECU has detected a low voltage or low current condition in the heater circuit of the upstream oxygen sensor on Bank 1. Bank 1 is the side of the engine with cylinder #1, and Sensor 1 is the sensor before the catalytic converter.
The heater inside this sensor helps it reach operating temperature quickly so the ECU can go into closed loop and fine-tune the air-fuel mixture. When the heater circuit isn’t working correctly, the ECU sets P0031 and turns on the check engine light.
Quick Reference
- Code: P0031
- Meaning: HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
- Main area: Upstream O2 / A/F sensor heater wiring and sensor
- Typical causes: Failed sensor, blown fuse, damaged wiring, poor connection
- Risk level: Low immediate risk, but bad for fuel economy and emissions
Real-World Example / Field Notes
In the shop, I see P0031 a lot on daily drivers that are 8–15 years old. A common scenario: you start the car on a cold morning, the engine runs fine, but the check engine light pops on. Scan shows P0031, maybe with no other codes. On inspection, the upstream O2 sensor wiring is brittle from heat, or the connector has green corrosion. Sometimes the heater circuit fuse is blown from a shorted sensor. Replacing the sensor and repairing the wiring usually clears the code and restores normal operation.
Symptoms of P0031
- Check engine light on: The MIL (Malfunction Indicator Lamp) will almost always be illuminated.
- Poor fuel economy: You may notice you’re using more fuel, especially on short trips and cold starts.
- Rough cold start: The engine might feel a bit rough or “lazy” until it warms up.
- Rich exhaust smell: You may smell extra fuel from the tailpipe, particularly when the engine is cold.
- Failed emissions test: Higher emissions or readiness monitors not setting can cause an inspection failure.
- Slight hesitation: Mild hesitation or sluggish throttle response during warm-up is possible.
- No obvious drivability issues: In many cases, the vehicle still drives “fine” with only the light on.
Common Causes of P0031
Most Common Causes
- Failed upstream O2 / A/F sensor heater: The heater element inside the Bank 1 Sensor 1 oxygen sensor burns out or shorts internally, causing low current or a blown fuse.
- Damaged sensor wiring near exhaust: Heat from the exhaust or contact with moving parts melts, chafes, or breaks the heater wires, causing low voltage or an open circuit.
- Corroded or loose connector: Moisture, road salt, or previous repairs can cause poor contact in the O2 sensor connector, dropping voltage to the heater.
- Blown O2 heater fuse: Many vehicles have a dedicated fuse or shared fuse for sensor heaters; if it blows, the ECU sees low heater circuit voltage and sets P0031.
- Poor ground for heater circuit: A loose or rusty ground connection can prevent proper current flow through the heater.
Less Common Causes
- Faulty ECU driver: The internal driver in the engine control unit that powers the heater can fail, though this is rare compared with sensor or wiring issues.
- Incorrect aftermarket sensor: A universal or wrong-spec sensor with different resistance can confuse the ECU and trigger P0031.
- Previous wiring repairs: Poorly done splices, wrong gauge wire, or cheap crimp connectors can cause high resistance and low voltage to the heater.
- Relays or power distribution issues: On some models, a relay feeds multiple heaters; a failing relay can cause low power to Bank 1 Sensor 1.
Diagnosis: Step-by-Step Guide
To properly diagnose P0031, you’ll want a basic scan tool, a digital multimeter (DMM), and ideally a wiring diagram for your vehicle. A test light, back-probe pins, and access to live data or Mode $06 information make the job easier, but you can still do a lot with simple tools and careful inspection.
- Confirm the code and check for companions. Use a scan tool to read codes. Note any related O2 heater codes (P0032, P0051, P0052) or fuel trim codes. Clear codes, then see if P0031 returns quickly after a cold start.
- Visual inspection of sensor and wiring. Locate Bank 1 Sensor 1 (upstream, before the catalytic converter). Inspect the harness for melted, cut, or rubbed-through insulation. Check that the connector is fully seated and not oil-soaked or corroded.
- Check the O2 heater fuse. Find the fuse box and locate the O2 sensor / EFI / heater fuse as per your owner’s manual or wiring diagram. If the fuse is blown, replace it once and see if it blows again. A repeatedly blown fuse usually means a shorted heater or wiring.
- Test for heater power supply. With the key ON (engine off), back-probe the heater power wire at the sensor connector. You should see battery voltage (around 12V). If there’s no power, trace back to the fuse, relay, or power supply circuit.
- Check heater ground or control side. Depending on the design, the ECU may control the ground or the power side. Use your wiring diagram to identify the control wire. With the engine running from a cold start, you should see the ECU switching the circuit (using a DMM or test light). No switching may indicate a driver or wiring issue.
- Measure heater resistance. With the sensor unplugged and the engine off, measure resistance across the heater pins on the sensor itself. Typical values are often in the 3–20 ohm range, but check your service data. Infinite resistance (open circuit) or near-zero (short) means the heater is bad.
- Wiggle test the harness. While monitoring heater voltage or resistance, gently wiggle the wiring harness and connector. If readings jump around or the engine stumbles, you likely have an intermittent connection or broken wire inside the insulation.
- Check live data and readiness. Look at O2 sensor data on your scan tool. If the sensor is slow to respond or stays “cold” for a long time, the heater may not be working correctly even if resistance looks okay.
- Verify ECU output (advanced). If power and ground to the sensor are good, and the sensor tests fine, you may need to check continuity between the sensor connector and the ECU, and verify the ECU driver is commanding the heater. This step is usually best left to a professional if you’re not comfortable with pin-testing modules.
Pro tip: On high-mileage vehicles, if the upstream sensor is original and you have P0031, it’s often smart to replace the sensor proactively after confirming power and ground are good. A tired sensor can cause subtle fuel trim issues even before it totally fails.
Possible Fixes & Repair Costs
Most repairs for P0031 are straightforward once you’ve pinpointed the cause. Typical fixes include:
- Replacing the Bank 1 Sensor 1 O2 or air-fuel ratio sensor
- Repairing or replacing damaged wiring and connectors in the heater circuit
- Replacing a blown O2 heater fuse and correcting the underlying short
- Repairing grounds or power supply issues in the heater circuit
- In rare cases, repairing or replacing the ECU
In terms of cost, an upstream O2 or A/F sensor replacement at a shop usually runs around $200–$450 parts and labor, depending on vehicle make, sensor type (standard vs. wideband), and rust or access issues. Wiring repairs can range from $100–$300 if the harness is damaged. An ECU replacement, while rare, can exceed $800–$1,500. DIY repairs using quality parts can significantly reduce these costs if you’re comfortable working around the exhaust and wiring.
Can I Still Drive With P0031?
In most cases, you can still drive with P0031 without immediate danger to the engine. The vehicle will usually run, start, and drive reasonably well, especially once warm. However, the ECU may run richer during warm-up, hurting fuel economy and increasing emissions. Over time, extra fuel can stress the catalytic converter and may cause other codes to appear. If the check engine light is flashing or you notice severe drivability issues, that’s a different problem and you should stop driving and diagnose immediately.
What Happens If You Ignore P0031?
If you ignore P0031 long-term, you risk poor fuel economy, higher emissions, and potential damage to the catalytic converter from running rich during warm-up. You’re also likely to fail an emissions or inspection test, and you may miss other new problems because the check engine light is already on. Fixing P0031 sooner rather than later is usually cheaper and better for the vehicle.
Related Codes
- P0027 – Exhaust Valve Control Solenoid Circuit Range/Performance Bank 1
- P0026 – Intake Valve Control Solenoid Circuit Range/Performance Bank 1
- P0019 – Crankshaft Position Camshaft Position Correlation Bank 2 Sensor B
- P0018 – Crankshaft Position Camshaft Position Correlation Bank 2 Sensor A
- P0017 – Crankshaft Position Camshaft Position Correlation Bank 1 Sensor B
- P0016 – Crankshaft Position Camshaft Position Correlation Bank 1 Sensor A
- P0009 – Engine Position System Performance Bank 2
- P0008 – Engine Position System Performance Bank 1
- P0007 – Fuel Shutoff Valve “A” Control Circuit High
- P0006 – Fuel Shutoff Valve “A” Control Circuit Low
Key Takeaways
- P0031 points to a low-voltage problem in the heater circuit of the Bank 1 Sensor 1 oxygen (or air-fuel ratio) sensor.
- The code is usually caused by a failed sensor heater, damaged wiring, a blown fuse, or a bad connection.
- Symptoms are often mild: check engine light, worse fuel economy, and possible roughness on cold starts.
- Diagnosis involves checking power, ground, heater resistance, and wiring integrity before condemning the ECU.
- Fixes are typically affordable, with sensor replacement being the most common repair.
Vehicles Commonly Affected by P0031
P0031 shows up across many brands because all modern gasoline engines use heated oxygen or air-fuel ratio sensors. It’s especially common on:
- Toyota / Lexus: Camry, Corolla, RAV4, Tacoma, Highlander, ES, RX models with A/F sensors.
- Subaru: Outback, Forester, Impreza, Legacy, especially in colder climates where heaters work harder.
- Hyundai / Kia: Elantra, Sonata, Santa Fe, Optima, Sportage with high mileage.
- Nissan / Infiniti: Altima, Maxima, Rogue, G35/G37, often due to harness wear near the exhaust.
- Honda / Acura: Civic, Accord, CR-V, Pilot, MDX with aging upstream A/F sensors.
- Domestic brands: Ford, Chevrolet, Dodge trucks and SUVs also see P0031, especially on older high-mileage vehicles.
FAQ
Can P0031 go away on its own?
It’s unlikely. You might clear the code and it may stay off for a short time, but if the underlying heater or wiring issue is still there, P0031 will usually return, especially after a cold start. The root cause needs to be repaired for a permanent fix.
Is P0031 serious enough to stop driving?
P0031 is not usually an emergency code like a severe misfire, but you shouldn’t ignore it forever. You can typically drive short-term without major risk, but expect worse fuel economy and a higher chance of catalytic converter stress over time. Plan to diagnose and repair it soon.
Can a bad O2 sensor cause P0031?
Yes. A failed heater element inside the upstream O2 or air-fuel ratio sensor is one of the most common reasons for P0031. The sensor may still read exhaust gases somewhat, but if the heater is open or shorted, the ECU will detect a low circuit condition and set the code.
Can I replace the O2 sensor myself for P0031?
Many DIYers can replace the upstream sensor with basic tools and some patience. You’ll need an O2 sensor socket or wrench, penetrating oil for rusty threads, and safe access under the vehicle. Always disconnect the battery if you’re working near the starter or main power cables, and avoid twisting the harness when removing the sensor.
How do I know if it’s wiring or the sensor causing P0031?
Testing is key. If you have proper power and ground at the connector and the heater resistance on the sensor is out of spec, the sensor is likely bad. If there’s no power, poor ground, or voltage changes when you move the harness, you probably have a wiring or fuse issue. A multimeter and a wiring diagram make this distinction much easier.