Definition of error code P0031
P0031 is the code for the HO2S Low (Bank 1 Sensor) heating control circuit.
What the P0031 code means.
The P0031 code means that for the engine to operate properly, the engine exhaust must have an air to fuel ratio of 14.7 to 1, as indicated by the heated oxygen sensors (HO2S), which detect the oxygen content of the vehicle exhaust. This information is sent to the engine control module (ECM) to adjust the proper amount of fuel supplied to the engine.
The HO2S system is heated to provide a very fast data signal to the ECM’s closed loop system, which helps control exhaust emissions during starting and when the engine is cold or warming up.
What causes the P0031 code?
In general, if the code has been cleared and reset, and then the engine warning light comes on and the OBD-II code returns, the HO2S sensor for engine bank 1 and sensor 1 before the catalytic converter are not sending the correct data to the ECM and may be damaged or defective.
The sensor may have a faulty heating element inside, or it may have a bad ground, a 12 volt battery input, or a faulty connector. It is quite rare that the ECM is bad.
What are the symptoms of the P0031 code?
- Engine control light comes 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 different complaints depending on the vehicle and the manufacturer of the vehicle’s ECM program.
- This fail-safe mode will continue to operate until the problem is corrected.
- Some of the most common driving problems are hesitation, erratic operation, and low power.
How can a mechanic diagnose the P0031 code?
P0031 is diagnosed only with an 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 the engine warning light come back on, the mechanic should check both the power and ground of the sensor, including checking the wiring and sensor.
Many problems with this code are wiring-related, due to heat from the exhaust system and catalytic converter.
The most common errors when diagnosing the P0031 code
- Do not simply replace the HO2S before carefully checking the wiring and connector.
- The mechanic needs to make sure that there are 12 volts at the sensor 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?
The P0031 code is usually preceded by the engine warning light on the instrument panel that comes on during operation. The vehicle can still be driven, but should be fully diagnosed as soon as possible to avoid problems such as a damaged sensor loop, excessive fuel consumption, malfunctions, or possible damage to other components.
In many cases, if the engine warning light comes on immediately after starting, the OBD-II system can be reset and the vehicle will operate normally.
What fixes can clear the P0031 code?
The most common potential fixes to clear the P0031 code are as follows:
- Have an authorized technician check the code with a scanner. Reset the trouble codes and perform a road test.
- If the P0031 code returns, perform the test procedure. You may have several problems, but damaged wiring due to excessive exhaust heat is the most common. Before replacing the sensor, make sure the wiring is good and has the correct voltage and ground for the sensor.
- With the key on and the engine off, check with a voltmeter that the voltage from the battery fuse to the heating element is 12+ volts. If no voltage is present, repair the open or shorted 12-volt power circuit, first determining if the blown fuses need to be replaced due to the short.
- If the battery power is intact, disconnect the ground (control) circuit from the ECM wiring harness connector and check the resistance in the circuit.
- If the resistance is infinite, repair the open circuit.
- If the control circuit is checked, suspect a defective O2 sensor. Replace the sensor and check again.
Example from my experience:
After some frustration dealing with the P0031 code and not following the procedure to fully diagnose the problem due to very tight cramps in the Mercedes CLK, I replaced the very expensive HO2 sensor #1, only to find that after a test drive, the engine warning light came on again and I had the same code again. I did a visual inspection of the wiring to make sure it wasn’t damaged or melted, and then discovered that I had less than 10 volts on the sensor.
Originally I just used a 12 volt test light and it lit up, which meant I had a complete circuit, or so I thought. Upon further investigation, I found that the ground wire in the HO2 sensor circuit was badly corroded, and that the nut was loose where it was attached to the chassis. I cleaned it up and added another washer to increase the contact area, checked it again and it had 12.5 volts. I reset the code with an OBD-II scanner, road tested the car and everything was fine.
Additional notes for consideration of code P0031.
Many vehicles with more than 100,000 miles experience temporary sensor problems, which often occur during starting or prolonged charging of the drivetrain.
If the engine warning light comes on and the vehicle appears to be operating normally, the OBD-II system can be reset with a scanner and the problem may not recur. Therefore, it is important to check the malfunction and reset it before making any repairs.
