P0031 trouble code definition
P0031 is the code for HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
What the P0031 code means
P0031 code means for the engine to run correctly, the engine exhaust must have a specific air-fuel ratio of 14.7 to 1 as signaled by the heated oxygen sensors (HO2S) which detect the oxygen content of the vehicle exhaust. This information is then sent to the Engine Control Module (ECM) to adjust the correct volume of fuel delivered to the engine.
An HO2S is heated to insure very rapid signal data to the ECM closed loop system which helps control emissions during start-up and when the engine is cold or warming up.
What causes the P0031 code?
Generally, if the code is cleared and reset, then the engine warning light comes on and the OBD-II code comes back, the HO2S sensor for the engine bank 1 and sensor 1 which is upstream of the catalytic converter, is not sending the proper data to the ECM and may be damaged or defective.
The sensor may have a bad heater element internally or it could have a bad ground, a failed 12 volt battery input or connector. It is quite rare if the ECM was bad.
What are the symptoms of the P0031 code?
The Check Engine Light will come on.
The ECM detects the O2 sensor heater circuit low and will enter the failsafe mode until the ignition is turned off.
The failsafe mode will cause various drive complaints depending on the vehicle and manufacturer of the vehicles ECM program.
This failsafe mode will continue until the problem is repaired.
Some of the common drive problems are hesitation, runs rough and low power.
How does a mechanic diagnose the P0031 code?
P0031 is only diagnosed with an OBD-II scanner. The mechanic can reset the OBD-II code and do a test drive the vehicle to see if the code returns. If the code and the engine warning light come back on, the mechanic should check both power and ground to the sensor including inspecting the wiring and sensor.
Many of the problems for this code are wiring related, due to the heat of the exhaust and catalytic converter.
Common mistakes when diagnosing the P0031 code
Do not simply replace the HO2S before checking the all related wiring and the connector closely.
The mechanic should insure there is 12 volts to the sensor and the ground is good.
Both sides of the connector wiring need to be checked to insure that the connector is good.
How serious is the P0031 code?
The P0031 code will usually be preceded by the engine warning light coming on the dash while it is in operation. The vehicle can still be driven but it should be completely diagnosed as soon as possible to prevent issues like the sensor loop failing, excessive fuel consumption, poor operation, or possible damage to other components.
Often times, if the engine warning light came on immediately at start up, the OBD-II system can be reset and the vehicle will operate normally.
What repairs can fix the P0031 code?
The most common potential repairs to address the P0031 code are as follows:
Have a certified technician verify the code with a scanner. Reset the fault codes and perform a road test.
If the P0031 code returns, then follow the test procedure. It can have several problems, but wiring being damaged by excessive heat from the exhaust is most common. Make certain the wiring is good and has proper voltage and ground to the sensor before replacing the sensor.
With the key on and engine off, use a voltmeter to check for 12+ volts fused battery feed to the heater element. If there is no voltage present, repair the open or short in the 12 volt feed circuit by first determining if it is necessary to replace any fuse blown from the short.
If the battery feed is intact, remove the ground (control) circuit from the ECM wiring connector and check for resistance on the circuit.
If there is infinite resistance, repair open in the circuit.
If the control circuit checks out, suspect a bad O2 sensor. Replace the sensor and re-check.
One example from my experience:
After some frustration dealing with a P0031 code and not following procedure to diagnose the problem completely due to very tight confines on a Mercedes CLK, I changed out the very expensive HO2 #1 sensor, only to find that after a test drive that the engine warning light came back on and I had the same code again. I had done a visual inspection of the wiring to make sure it was not damaged or melted, then found that I had less than 10 volts at the sensor.
Originally, I just used a 12 volt test light and it came on, meaning it had a complete circuit, or so I thought. After further investigation I found the ground wire on the HO2 sensor circuit was heavily corroded and the nut was loose where it attached to the chassis. I cleaned it up and added another washer to increase the contact area, rechecked it, and I had 12.5 volts. Reset the code with the OBD-II scanner, road tested the car, and it was fine.
Additional comments for consideration regarding the P0031 code.
Many vehicles with mileage over 100,000 have momentary sensor problems that usually occur during start up or prolonged stress situations on the drive train.
If the engine warning light comes on and the vehicle seems to be operating normally, the OBD-II system can be reset using the scanner and the problem may not reoccur. This is why it is important to verify the fault and reset it before doing any repairs.
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