P0031: HO2S Heater Control Circuit Low (Bank 1, Sensor 1)

What Is P0031?

P0031 is an OBD-II Diagnostic Trouble Code (DTC) defined as “HO2S Heater Control Circuit Low (Bank 1 Sensor 1)”. It’s a generic Powertrain Control Module (PCM) code common with vehicles made after 1996.

DTC P0031 means there’s a fault with the heated oxygen sensor (HO2S) control circuit 1, located on the exhaust manifold before the catalytic converter. “Bank 1” indicates that the sensor is on the engine side with cylinder #1.

The O2 sensor (also called the upstream sensor) is a component that measures the oxygen level of the gas in the exhaust system. This oxygen sensor, also called the AF sensor, reports this data to the Powertrain Control Module (PCM) or Engine Control Module (ECM), which then adjusts the air-fuel ratio (AF ratio) as required.

In other words, the engine increases or decreases fuel flow to bring your vehicle to a fuel-efficient and emission-friendly state.

The upstream O2 sensor has a heating element that allows the circuit to reach its optimum operating temperature to send information as quickly as possible.

But when the Engine Control Module detects low voltage from the upstream sensor circuit or perceives that the O2 heater wire is failing, it sets off trouble code P0031 and turns the Check Engine Light on.

Likewise, the code P0037 indicates a problem with the rear sensor (rear O2 sensor or downstream sensor), located after the catalytic converter.

Common Symptoms

If you encounter code P0031 due to a fault with your heated oxygen sensor control circuit, you may experience some of the following symptoms:

• Check Engine Light turns on
• Vehicle rides rough
• Engine hesitation or low power
• Drivability issues as the vehicle enters failsafe mode (in some cases)
• Excessive fuel consumption

These problems result from the vehicle’s inability to maintain the correct air-fuel ratio (or oxygen-to-fuel ratio) due to the faulty O2 sensor or circuit.

Can I Still Drive With a P0031 O2 Sensor Code?

Yes, you can drive your vehicle with the P0031 code and the engine light turned on. However, it’s recommended that you have this code fixed as soon as possible.

If your heated oxygen sensor is faulty, the engine won’t be able to maintain the correct air-fuel ratio. This can lead to low mileage, poor operation, and high exhaust gas emissions.

Additionally, leaving code P0031 unresolved for an extended period could cause damage to other engine parts. Repairing some of these parts, like the catalytic converter, is quite expensive.

DTC P0031 Causes

Whether you own a Honda or a Toyota Motor Corporation vehicle, several common factors could trigger the P0031 code and turn your engine light on.

These include:

• A faulty bank 1 sensor or AF sensor heater
• A damaged heated oxygen sensor connector
• A damaged connector at the ECM
• Frayed wires or a torn wire harness in the heater circuit
• A damaged heated oxygen sensor fuse or relay
• A short or open circuit 
• Bad engine ground
• Improper battery input to the O2 sensor, leading to low voltage
• A faulty or outdated PCM/ECM (this is rare, and all other causes should be investigated first)

Diagnosis and Possible Repairs

Diagnosing and repairing the P0031 trouble code requires specialized knowledge and tools. It’s not recommended to attempt DIY repairs. Instead, leave this to a qualified mechanic.

Here’s what you can expect the technician to do to find the fault that’s causing the p0031 code:

• Use an OBD II scanner to check for other trouble codes and address them first

• Reset the O2 sensor code and test drive the car

If the P0031 trouble code and check engine light return, there’s likely a fault in the oxygen sensor circuit. The mechanic will:

• Inspect the O2 sensor, the heater element circuit wiring, and the connector: Check the wiring harness for frayed wiring and blown fuses that could disrupt the oxygen sensor circuit. Ensure the oxygen sensor element connector is not damaged or overheating.

• Check the engine ground to the heater circuit: If the control circuit seems sound, your mechanic will use a digital multimeter to measure the resistance and voltage of the engine ground and ensure continuity to the heating element and AF sensor.

• Check power flow to the bank 1 sensor: Use the digital multimeter to check the battery voltage flowing into the heater element. It should measure around 12V. If no voltage is detected at the oxygen sensor heater, this likely indicates an open circuit of a faulty heated oxygen sensor fuse.

• Measure the circuit resistance: If the battery voltage to the heater element circuit is lower than 12V (but not zero), your mechanic may need to check for a high resistance problem.

• Replace the heated oxygen sensor: If the mechanic has verified that the oxygen sensor circuit, voltage, and resistance are sound, it may be necessary to replace the AF sensor.

• Check the ECM connector and AF connector: If replacing the sensor doesn’t solve the P0031 code, it may indicate a problem with the ECM connector or AF sensor connector.

• Replace the ECM: As a last resort, the mechanic may need to replace a faulty Engine Control Module that is incorrectly activating the P0031 code, though this is rare.

Note: The mechanic shouldn’t attempt repairs to the O2 sensor or heater circuit without resetting the codes, as the P0031 fault code and check engine light sometimes go away when reset.

Likewise, they shouldn’t replace the oxygen sensor heater or diagnose a faulty Engine Control Module as a first step without thoroughly checking the circuit. Failing to check these components could lead to unnecessary repairs and costs.

Replacement Costs

To address the P0031 DTC (heater control circuit low), the mechanic may need to replace or repair one or more components of the circuit, wiring harness, or O2 heater element. 

The cost of repair will include all the parts and labor involved.

Here’s an estimate of the P0031 fault code repair costs:

• To replace the heated oxygen sensor: $200-$500
• To repair or replace the wiring harness in the heater element circuit: $100-$1,000
• To replace the ECM: $800-$1500