P0031 Code: O2 Sensor Heater Control Circuit Low (Bank 1 Sensor 1)
The upstream sensor heater on bank 1 draws little or no current – usually an open heater or a dead fuse.
At a glance
Severity: Low
- Meaning
- O2 Sensor Heater Control Circuit Low (Bank 1 Sensor 1)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$1,200
- DIY difficulty
- Easy
P0031 is stored when the engine computer switches on the heater of the bank 1 upstream sensor but sees almost no current flow, or sees the voltage on its control wire sit where it would if nothing were connected. The circuit is behaving as if it were open. The heater gets battery voltage from a fused ignition or relay feed and the computer completes the path to ground, so a break anywhere in that loop – the element itself, the fuse, the feed wire, the connector or the ground-side wire back to the computer – produces this code. On most vehicles that log P0031 the sensor is a wideband air-fuel sensor whose heater measures only a few ohms, so an open reading is easy to recognize. Until the circuit is restored, the engine runs without mixture feedback for several minutes after every cold start.
Can I keep driving? Fine to drive for now. Expect slightly higher fuel use on short trips and a failed emissions test, but there is no risk of engine damage.
How to read P0031
- P Powertrain
- 0 Generic (SAE)
- 0 Fuel/air & emissions (aux.)
- 31 Specific fault
Symptoms
- Check engine light returns within a minute or two of every cold start
- Air-fuel sensor value in live data stays flat and unchanging during warm-up
- Short- and long-term fuel trims stuck at 0 % for longer than normal after start-up
- Oxygen sensor and heater monitors both show not ready
- Other heater codes such as P0037 or P0051 at the same time when a shared fuse has blown
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Heater element inside the upstream air-fuel sensor burned open
- Blown heater fuse – it blew for a reason, so look for a short before replacing it
- Connector not latched or feed wire pulled loose after engine or exhaust work
- Break in the ground-side control wire between sensor and engine computer
- Heater relay or main engine relay not delivering power
- Heater driver inside the engine computer failed open
Diagnosis step by step
Look at what else is stored
Read freeze frame data and all codes. If P0037, P0051 or P0057 are logged alongside P0031, four heaters failing at once is unlikely; the common fuse or relay that feeds them is the real suspect.
Tools: OBD-II scanner
Check for power at the harness plug
Unplug the sensor connector (on most engines it's clipped to a bracket near the valve cover or on the transmission bell housing). With the key on and engine off, the feed pin on the harness side should show about 12.4 to 12.6 volts to ground. Zero volts sends you to the fuse, relay and feed wire, not the sensor.
Tools: Multimeter
Measure the heater cold
Measure across the two heater pins on the sensor side with the engine cold, since heater resistance rises as it warms. Wideband air-fuel sensors usually read roughly 2 to 5 ohms, narrowband sensors more. OL or several thousand ohms means the element is open and the sensor must be replaced.
Tools: Multimeter
Do a voltage-drop test on the feed under load
A feed that reads 12 volts with nothing connected can still collapse under a 2 to 4 amp heater load. Back-probe the feed pin with the sensor connected and the engine idling cold, and measure between it and the battery positive post. More than about 0.5 volt of drop points to a corroded fuse terminal or relay contact.
Tools: Multimeter
Check the control wire back to the computer
Disconnect the battery, locate the heater control pin at the engine computer connector from a wiring diagram and measure continuity to the sensor connector. You want well under 1 ohm, and the reading must stay steady while you flex the harness at clips and bends.
Tools: Multimeter
Fit the sensor and confirm the fix
Once the harness checks out, install a direct-fit sensor with an O2 sensor socket and torque it to spec. Clear codes, let the car sit until fully cold, then start it and watch the air-fuel reading come alive within about a minute.
Tools: O2 sensor socket, Torque wrench, OBD-II scanner
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace blown heater fuse after tracing why it blew | Yes | $1–$15 |
| Re-latch connector or repair broken feed wire | Yes | $0–$180 |
| Upstream air-fuel sensor, part only | Yes | $90–$270 |
| Upstream air-fuel sensor replaced at a shop | No | $190–$460 |
| Engine computer heater driver repair | No | $300–$1,200 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Toyota
On many Toyota models the bank 1 upstream sensor is an air-fuel ratio sensor whose heater has its own fuse in the underhood box, often labeled A/F. Check that fuse before buying a sensor, and if it's blown, look along the harness for chafing before fitting a new one.
Field data
| Chevrolet | 4 | Volt | |
|---|---|---|---|
| Toyota | 2 | Avalon, Corolla |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
An OBD-II emissions test fails a car with the check engine light on for P0031. Many states also reject cars with too many incomplete monitors, so don't clear codes just before the test; drive a few cold starts first.
Frequently asked questions
Is P0031 always a bad sensor?
An open heater element is the most common cause, but a fuse, an unlatched connector or a broken feed wire give the identical code. Five minutes with a multimeter at the connector tells you which side of the plug the fault is on.
P0031 appeared right after other engine work. Coincidence?
Probably not. The upstream sensor connector sits close to the exhaust manifold, valve cover and intake, and it's easy to leave it half-latched or strain the pigtail during plug, gasket or manifold work. Check the connector before buying parts.
The heater fuse was blown. Will a new fuse fix P0031?
It may for a while, but a fuse only blows when something draws too much current. If the new fuse blows again, look for a shorted heater (often logged as P0032 or P0038) or a harness rubbing through on a bracket.
Where exactly is bank 1 sensor 1?
It's the first sensor in the exhaust stream on the cylinder 1 side, threaded into the exhaust manifold, turbo outlet or the front of the close-coupled converter. On transverse four-cylinder engines it often faces the firewall and is easiest to reach from underneath.