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 in front of the catalytic converter. “Bank 1” indicates that the sensor is on the engine side with cylinder #1.

The HO2 sensor is a component that measures the level of oxygen in the exhaust gas. 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 oxygen sensor has a heating element that allows the circuit to reach its optimum operating temperature, so it can send information as quickly as possible.

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

Common symptoms

If you encounter the p0031 DTC 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
• Vehicle has low power
• Excessive fuel consumption

These problems are the result of 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?

Yes, you’ll still be able to 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 or poor operation as well as 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. 

P0031 causes

Whether you own a Honda or a Toyota Motor Corporation vehicle, several common factors could cause 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
• Improper battery input to the O2 sensor
• A faulty or outdated PCM/ECM (this is rare, and all other causes should be investigated first)

Diagnosis

Diagnosing and repairing a P0031 fault 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 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 that the mechanic will identify by:

• Inspecting the O2 sensor, the heater element circuit wiring and connector: Check 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.
• Checking 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.
• Checking power flow to the O2 heater 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.
• Measuring 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.
• Replacing the heated oxygen sensor: If the mechanic has verified that the oxygen sensor circuit, voltage, and resistance is sound, it may be necessary to replace the AF sensor.
• Checking 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.
• Replacing the ECM: As a last resort, the mechanic may need to replace a faulty Engine Control Module that is wrongfully activating the P0031 code, though this is very rare.

The mechanic shouldn’t attempt repairs to the O2 sensor or heater circuit without resetting the codes, as the P0031 fault code and 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. 

Possible repairs for P0031 & Costs

To address the P0031 DTC (heater control circuit low), the mechanic may need to make replacements and repairs to one or more components of the circuit, wiring harness, or sensor element. The cost of repair will include all the parts and labor involved in the repair.

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

• To replace the heated oxygen sensor: $200-$300
• To repair or replace the AF sensor heater or wiring in the heater element circuit: $100-$1,000