P0031

DTC P0031 Oxygen Sensor Heater Control Circuit Low (Bank 1 Sensor 1)

CIRCUIT DESCRIPTION

DTC Detecting Condition:

The front Heated Oxygen (HO2) sensor is used to monitor oxygen concentration in the exhaust gas. For optimum operation of the TWC, the air-fuel mixture must be maintained near the stoichiometric level. The HO2 sensor output voltage changes suddenly in the vicinity of the stoichiometric ratio. The ECM adjusts the fuel injection time so that the air-fuel ratio is nearly stoichiometric level. If the oxygen concentration in the exhaust gas increases, the air-fuel ratio is LEAN. The HO2 sensor voltage drops below 0.45 V and the sensor informs the ECM of the LEAN condition. If oxygen is not in the exhaust gas, the air-fuel ratio is RICH. The sensor voltage increases above 0.45 V and the sensor informs the ECM of the RICH condition.









HINT: The ECM provides a pulse width modulated control circuit to adjust the current through the heater. The heated oxygen 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 sensor to regulate the air-fuel ratio close to the stoichiometric level. This maximizes the ability of the TWC to purify the exhaust gas. The 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 outside 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 sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio.

When heated, the sensor becomes more efficient. When the temperature of the exhaust gas 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 sensor will be inaccurate and the ECM cannot regulate the air-fuel ratio properly. When the heater current is outside the normal operating range, the ECM interprets this as a malfunction in the sensor and sets a DTC.

Example:
The ECM sets DTC P0032 or P00038 if the current in the sensor heater is more than 2 A when the heater is OFF. Conversely, the ECM sets DTC P0031 or P0037 if the current is less than 0.25 A when the heater is ON.

Wiring Diagram:

INSPECTION PROCEDURE

Step 1:

Step 2 - 3:

Step 4:

Step 4 (Continued):

HINT: Read freeze frame data using the hand-held tester or the 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 occurred.