P0031

DTC P0031 OXYGEN SENSOR HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 1)

HINT:
- Sensor 1 refers to the sensor mounted in front of the Three-Way Catalytic Converter (TWC), and located near the engine assembly.
- Sensor 2 refers to the sensor mounted behind the TWC, and located far from the engine assembly.

CIRCUIT DESCRIPTION

DTC Detection Conditions:

The front Heated Oxygen (HO2) sensor monitors the oxygen concentration in the exhaust gas. For optimum TWC operation, the air fuel mixture must be maintained as close as possible to the stoichiometric ratio. The HO2 sensor output voltage changes dramatically in the vicinity of the stoichiometric ratio. By making adjustments in accordance to these signal voltages, the ECM adjusts the fuel injection time so that the air fuel ratio remains close to stoichiometric levels.

If the oxygen concentration in the exhaust gas increases, the air fuel ratio is lean and the HO2 sensor output voltage to the ECM drops below 0.45 V. If the oxygen concentration in the exhaust gas decreases, the air fuel ratio is rich and the HO2 sensor output voltage to the ECM increases to above 0.45 V.

HINT:
- When any of these DTCs are set, the ECM enters fail-safe mode. During fail-safe mode, the ECM turns off the Heated Oxygen (HO2) sensor heater. Fail-safe mode continues until the ignition switch is turned to OFF.
- The ECM provides a pulse width modulated control circuit to adjust current through the heater. The sensor heater circuit uses a relay on the B+ side of the circuit.

MONITOR DESCRIPTION

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunction Thresholds:

Component Operating Range:

Monitor Result:

The ECM uses information from the Heated Oxygen (HO2) sensor to regulate the air-fuel ratio and keep it close to the stoichiometric level. This maximizes the ability of the Three-Way Catalytic Converter (TWC) to purify the exhaust gases.

The HO2 sensor detects oxygen levels in the exhaust gas and transmits the information to the ECM. The inner surface of the sensor element is exposed to outside air. The outer surface of the sensor element is exposed to the exhaust gas. The sensor element is made of platinum coated zirconia and includes an integrated heating element.

The zirconia element generates a small voltage when there is a large difference in the oxygen concentrations between the exhaust gas and outside air. The platinum coating amplifies the voltage generation.

The HO2 sensor is more efficient when heated. When the exhaust gas temperature is low, the sensor cannot generate useful voltage signals without supplementary heating. The ECM regulates the supplementary heating using a duty-cycle approach to adjust the average current in the sensor heater element. If the heater current is outside the normal range, the signal transmitted by the HO2 sensor will be inaccurate, as a result, the ECM will be unable to regulate air-fuel ratio properly.

When the current in the HO2 sensor heater is outside the normal operating range, the ECM interprets this as a malfunction in the sensor heater and sets a DTC.

Example:
The ECM sets either DTC P0032 or P0038 when the current in the HO2 sensor heater is more than 2 A despite the heater being OFF. Conversely, when the heater current is less than 0.25 A despite the heater being ON, DTC P0031 or P0037 is set.

Wiring Diagram:

Step 1 - 2:

Step 3:

Step 4:

INSPECTION PROCEDURE

HINT: Read freeze frame data using a hand-held tester or OBD II scan tool. Freeze frame data record the engine condition when malfunctions are detected. When troubleshooting, freeze frame data can help determine if the vehicle was moving or stationary, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data, from the time the malfunction.