Error P0031 – how to fix?

Error code definition P0031

P0031 is a code for the HO2S Low heating control circuit (Bank 1 Sensor).

What does the code P0031 mean

The code P0031 indicates that the engine exhaust must have an air-fuel ratio of 14.7 to 1 for the engine to function properly, as indicated by Heated Oxygen Sensors (HO2S) which detect the oxygen content in the vehicle exhaust. This information is sent to the engine control module (ECM) to adjust the proper amount of fuel to the engine.

The HO2S system is heated to provide a very fast data signal to the ECM’s closed-loop system to help control exhaust emissions at startup and when the engine is cold or warm.

What causes the code P0031?

In general, if the code has been cleared and reset and then the engine warning lamp illuminates and the OBD-II code returns, the HO2S sensor for engine bank 1 and sensor 1 in front of the catalytic converter is not sending the correct data to the ECM and may be damaged or defective.

The sensor may have a damaged heating element inside, or it may have the wrong ground, a 12-volt battery input, or a damaged connector. It is quite rare for the ECM to be bad.

What are the symptoms of the P0031 code?

• The Check Engine light will come on.
• The ECM detects a low O2 sensor heating circuit and will go into emergency mode until the ignition is turned OFF.
• The failsafe mode causes various discomforts depending on the vehicle and the manufacturer of the vehicle’s ECM program.
• This safe mode will continue until the problem is fixed.
• Some of the most common riding problems are jitter, erratic operation and low power.

How can a mechanic diagnose the code P0031?

P0031 is only diagnosed with the OBD-II scanner. The mechanic can reset the OBD-II code and test the vehicle to see if the code returns. If the code and engine warning light illuminate again, the mechanic should check both power and ground to the sensor, including the wiring and the sensor.

Many of the problems with this code are wiring related, due to heat from the exhaust system and catalytic converter.

The most common errors when diagnosing the code P0031

• The HO2S should not be simply replaced without thoroughly checking the wiring and connector.
• The mechanic must make sure that the sensor has a voltage of 12 volts and that the ground is good.
• Both sides of the connector wiring should be checked to make sure the connector is good.

How serious is the P0031 code?

Code P0031 is usually preceded by illuminating the dashboard engine warning light during operation. The vehicle is still drivable but must be fully diagnosed as soon as possible to avoid problems such as a broken sensor loop, excessive fuel consumption, malfunction or possible damage to other components.

In many cases, if the engine warning light illuminates immediately after startup, the OBD-II system can be reset and the vehicle will operate normally.

What repairs can remove the code P0031?

The most common potential repairs to fix the P0031 code are as follows:

1. Have an authorized technician check the code with a scanner. Reset fault codes and perform road test.
2. If P0031 returns, perform the test procedure. You may have a few problems, but damaged wiring due to excessive exhaust heat is the most common. Before replacing a sensor, make sure that the wiring is good and has the correct voltage and ground for the sensor.
3. With the key on and the engine off, check with a voltmeter that the voltage from the battery fuse to the heater element is 12+ V. If there is no voltage, repair the open or shorted 12-volt power circuit by first determining if a short circuit needs replacing any blown fuses.
4. If battery power is intact, disconnect the ground (control) circuit from the ECM wiring connector and check the resistance on the circuit.
5. If resistance is infinite, repair open circuit.
6. If the control circuit works, we suspect a faulty O2 sensor. Replace sensor and check again.

An example from my experience:

After some frustration of dealing with the P0031 code and not having the procedure to fully diagnose the problem due to a very tight tight on the Mercedes CLK, I replaced the very expensive HO2 # 1 sensor, only found that after a test drive that the engine warning light came back and I had this same code again. I did a wiring inspection to make sure it wasn’t damaged or fused and then found I had less than 10 volts on the sensor.

Originally I only used a 12 volt test light and it came on which means I have a full circuit, or so I thought. Upon further investigation, I found that the HO2 sensor ground wire was heavily corroded and the nut was loose where it attaches to the chassis. I cleaned it and added another washer to increase the contact area, checked it again and it was 12.5V. I reset the code with an OBD-II scanner, tested the car on the road and it was fine.

Additional considerations for P0031 code.

Many vehicles over 100,000 km have temporary problems with the sensors, which often occur during starting or prolonged stress on the driveline.

If the engine warning light comes on and the vehicle appears to be operating normally, the OBD-II system can be reset using a scan tool and the problem may not recur. Therefore, it is important to check the fault and reset it before making any repairs.