P0031 Code- HO2S Heater Control Circuit Low Bank 1 Sensor 1

What Does Code P0031 Mean

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.

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Possible symptoms

• Engine Light ON (or Service Engine Soon Warning Light)

Possible causes

• Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1
• Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 harness is open shorted to ground
• Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit poor electrical connection
• Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit fuse
• Faulty Engine Control Module (ECM)

Troubleshooting P0031

NOTE #1: Since each oxygen sensor has its own dedicated heater control circuit, you will need to have a repair manual for the application being worked on in order to be able to correctly determine the location, function, color-coding, and routing of all wiring associated with the oxygen sensor being investigated.

NOTE #2: Diagnosing code P0031 requires that the engine be in good running order, with no unmetered air entering the engine. If other codes relating lean running, or misfiring conditions are present long with code P0031, repair these issues first before attempting an electrical diagnosis of this code.

NOTE #3: It is important to retest the system after each step in the repair procedure to ensure a reliable repair.

Step 1

Record all fault codes and freeze frame data present for future reference. This information can be valuable in identifying intermittent faults.

Step 2

Since burnt wiring is a common cause of code P0031, thoroughly inspect all associated wiring and connectors for signs of damage, short circuiting, or corrosion. Pay particular attention to the routing of wiring, since unsecured wiring can burn against hot exhaust components. Repair all wiring issues as required.

Step 3

If the code persists, perform resistance, continuity, ground, and reference voltage checks on the wiring in the circuit, but be sure to consult the manual to identify the wiring associated with the sensor in question. Compare obtained readings against values stated in the manual. Reference voltages should always closely match battery voltage; about 12.6-, to 13.8 volts. Be sure to disconnect all control modules before performing continuity checks to prevent damaging a control module.

The oxygen sensor itself forms part of the control circuit, and must therefore be checked as well, but be sure to disconnect the sensor from its wiring before performing resistance and continuity checks on the sensor. Compare resistance readings to values stated in the manual, and replace all sensors that do not comply with the manufacturer’s specification.

NOTE: On some applications, the reference voltage and ground is supplied by the PCM. In these designs, there is no fuse incorporated into the circuit. Consult the manual on which design is used on the vehicle being worked on.

Step 4

If the fault persists and scanner can monitor live data streams, start the engine and monitor the live data. Typically, the signal voltage from pre-catalytic converter oxygen sensors should constantly vary from around 0.1V, to about 0.9V once the engine has reached working temperature and the PCM has entered into closed loop mode.

If obtained readings do not fall within this broad range, or if there are long periods (typically 8 seconds and more) during which the readings remain constant, remove the oxygen sensor(s) and inspect it/them for signs of discoloration, or the presence of undesirable deposits. Both conditions can cause code P0031, meaning that the cause/origin of the discoloration or deposits must be found and corrected.

However, oxygen sensors that are damaged by additives cannot be repaired, and replacement is the only reliable remedy.

NOTE: At this point it is important to determine if the vehicle had been running with any type of additive in either the fuel or the engine oil, or both. Some aftermarket oil and fuel additives contain silicone-based substances that can permanently damage oxygen sensors.

If it is suspected that oil or fuel additives are the cause of discoloration or deposits, check the condition of the spark plugs, since the plugs will often also be discolored or covered in harmful deposits due to long term exposure to some, mostly poor quality additives.

Step 5

If sensors are replaced, reconnect (and secure) all wiring, and operate the vehicle to see if the code returns. It is a good idea to leave the scanner connected during this drive cycle so that live data from the sensor control circuit can be monitored effectively.

If all electrical values correspond to the values stated in the manual, and new oxygen sensors had been fitted, it is unlikely that the fault code will return. However, if it does return, there is likely to be an intermittent fault present. Intermittent faults can sometimes be extremely difficult to find and repair, meaning that the fault may have to be allowed to worsen before an accurate diagnosis and repair can be made.

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