P0031
2TR-FE ENGINE CONTROL SYSTEM: SFI SYSTEM: P0031: Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1)
DTC P0031 - Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1)
DTC P0032 - Oxygen (A/F) Sensor Heater Control Circuit High (Bank 1 Sensor 1)
CAUTION / NOTICE / HINT
HINT:
- Although the DTC titles say the oxygen sensor, these DTCs relate to the Air-Fuel Ratio (A/F) sensor.
- Sensor 1 refers to the sensor mounted in front of the Three-Way Catalytic Converter (TWC) and located near the engine assembly.
DESCRIPTION
The A/F sensor generates a voltage* that corresponds to the actual air-fuel ratio. This sensor voltage is used to provide the ECM with feedback so that it can control the air-fuel ratio. The ECM determines the deviation from the stoichiometric air-fuel ratio level, and regulates the fuel injection time. If the A/F sensor malfunctions, the ECM is unable to control the air-fuel ratio accurately.
The A/F sensor is the planar type and is integrated with the heater, which heats the solid electrolyte (zirconia element). This heater is controlled by the ECM. When the intake air volume is low (the exhaust gas temperature is low), current flows into the heater to heat the sensor, in order to facilitate accurate air-fuel ratio detection. The heat generated by the heater is conducted to the solid electrolyte through the alumina, therefore the sensor activation is accelerated.
In order to obtain a high purification rate of the carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) components in the exhaust gas, a TWC is used. For the most efficient use of the TWC, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric level.
*: Value changes inside the ECM. Since the A/F sensor is the current output element, a current is converted to a voltage inside the ECM. Any measurements taken at the A/F sensor or ECM connectors will show a constant voltage.
HINT:
- When either of these DTCs is set, the ECM enters fail-safe mode. The ECM turns off the A/F sensor heater in fail-safe mode. Fail-safe mode continues until the ignition switch is turned to OFF.
- The ECM provides a pulse width modulated control circuit to adjust the current through the heater. The A/F sensor heater circuit uses a relay on the +B side of the circuit.
MONITOR DESCRIPTION
The ECM uses information from the Air-Fuel Ratio (A/F) 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 A/F 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 the 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 this voltage generation.
The A/F 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 A/F sensor will be inaccurate, as a result, the ECM will be unable to regulate air-fuel ratio properly.
When the current in the A/F 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 DTC P0032 when the current in the A/F sensor heater is more than 10 A. Conversely, when the heater current is less than 0.8 A, DTC P0031 is set.
MONITOR STRATEGY
TYPICAL ENABLING CONDITIONS
P0031:
P0032:
TYPICAL MALFUNCTION THRESHOLDS
P0031:
P0032:
COMPONENT OPERATING RANGE
WIRING DIAGRAM
INSPECTION PROCEDURE
HINT: Read freeze frame data using the Techstream. Freeze frame data records 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.
PROCEDURE
1. INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE)
(a) Disconnect the A4 Air-Fuel Ratio (A/F) sensor connector.
(b) Measure the resistance between the terminals of the A/F sensor connector.
Standard Resistance:
(c) Reconnect the A/F sensor connector.
NG -- REPLACE AIR FUEL RATIO SENSOR
OK -- Continue to next step.
2. INSPECT AIR FUEL RATIO SENSOR HEATER RELAY (A/F HEATER RELAY)
(a) Remove the A/F HEATER relay from the engine room R/B.
(b) Check the A/F HEATER relay resistance.
Standard Resistance:
(c) Reinstall the A/F HEATER relay.
NG -- REPLACE AIR FUEL RATIO SENSOR HEATER RELAY
OK -- Continue to next step.
3. INSPECT ECM (HA1A VOLTAGE)
(a) Turn the ignition switch to ON.
(b) Measure the voltage between the terminals of the E4 and E5 ECM connectors.
Standard Voltage:
OK -- REPLACE ECM
NG -- Continue to next step.
4. CHECK HARNESS AND CONNECTOR (A/F SENSOR - ECM, A/F SENSOR - A/F HEATER RELAY)
(a) Check the harness and connectors between the ECM and A/F sensor.
(1) Disconnect the A4 A/F sensor connector.
(2) Disconnect the E4 ECM connector.
(3) Check the resistance.
Standard Resistance (Check for open):
Standard Resistance (Check for short):
(4) Reconnect the A/F sensor connector.
(5) Reconnect the ECM connector.
(b) Check the harness and connector between the A/F sensor and A/F HEATER relay.
(1) Disconnect the A4 A/F sensor connector.
(2) Remove the A/F HEATER relay from the engine room R/B.
(3) Check the resistance.
Standard Resistance (Check for open):
Standard Resistance (Check for short):
(4) Reconnect the A/F sensor connector.
(5) Reinstall the A/F HEATER relay.
OK -- REPLACE ECM
NG -- REPAIR OR REPLACE HARNESS OR CONNECTOR