PZEV Engine
DTC P0037 OXYGEN SENSOR HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 2)HINT:
- These DTCs are recorded when a heated oxygen sensor heater circuit is not working properly.
- DTCs P0043 and P0044 have been added to meet the requirement for Partial Zero Emission Vehicle (PZEV).
- Sensor 2 is the sensor positioned in the 2nd location from the engine assembly, located after the front catalyst.
- Sensor 3 is the sensor positioned in the 3rd location from the engine assembly, located after the rear catalyst.
CIRCUIT DESCRIPTION
HINT:
- These DTCs indicate abnormal voltage output from the heated oxygen sensor 2.
- Sensor 2 is the sensor positioned in the 2nd location from the engine assembly, located after the front catalyst.
DTC Detection Condition:
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way catalytic converter is used. For the most efficient use of the three-way catalytic converter, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio.
The heated oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio. This is used to detect the oxygen concentration in the exhaust gas and provide the ECM with feedback control the air-fuel ratio.
When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust gas increases. And the heated oxygen sensor informs the ECM of the LEAN condition (low voltage, i.e. less than 0.45 V).
When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio, the oxygen will be vanished from the exhaust gas. And the heated oxygen sensor informs the ECM of the RICH condition (high voltage, i.e. more than 0.45 V). The ECM judges by the voltage output from the heated oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection time accordingly. If the malfunction of the heated oxygen sensor causes an output of abnormal voltage, the ECM becomes unable to perform accurate air-fuel ratio control.
The heated oxygen sensor include a heater which heats the zirconia element. The heater is controlled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low), current flows to the heater in order to heat the sensor for the accurate oxygen concentration detection.
Active Air-Fuel Ratio Control
Usually the ECM performs the air-fuel ratio control so that the A/F sensor output indicates a near stoichiometric air-fuel ratio. This vehicle includes "active air-fuel ratio control" besides the regular air-fuel ratio control. The ECM performs the "active air-fuel ratio control" to detect deterioration in front catalyst and the heated oxygen sensor 2 malfunction. (refer to the diagram)
"Active air-fuel ratio control" is performed for approximately 15 to 20 seconds during a vehicle driving with a warm engine. Under "active air-fuel ratio control", the air-fuel ratio is forcibly regulated to go LEAN or RICH by the ECM.
If the ECM detects a malfunction it is recorded in the following DTCs: DTC P0136 (Abnormal voltage output), DTC P0137 (Circuit open) and P0138 (Circuit short).
Abnormal Voltage Output of Heated Oxygen Sensor 2 (DTC P0136)
As the ECM is performing the "active air-fuel ratio control", the air-fuel ratio is forcibly regulated to go RICH or LEAN. If the sensor is not functioning properly, the voltage output variation is smaller.
Under the "active air-fuel ratio control", if the maximum voltage output of the heated oxygen sensor 2 is 0.6 V or less, or the minimum voltage output is 0.2 V or more, the ECM determines that it is abnormal voltage output of the sensor (DTC P0136).
Open or Short in the Heated Oxygen Sensor 2 Circuit (P0137 or P0138)
Under "active air-fuel ratio control", the ECM calculates the Oxygen Storage Capacity (OSC) * in the catalyst by forcibly regulating the air-fuel ratio to go RICH (or LEAN).
If the heated oxygen sensor 2 has an open or short, or the voltage output by the sensor noticeably decreases, the OSC will indicate extraordinary high value. Even if the ECM attempts to continue regulating the air-fuel ratio to go RICH (or LEAN), the heated oxygen sensor 2 output does not change.
Under "active air-fuel ratio control", although the targeted air-fuel ratio is RICH but the voltage output of the heated oxygen sensor is 0.25 V or less (LEAN), the ECM determines that it is an abnormal low voltage (DTC P0137). Also, the targeted air-fuel ratio is LEAN but the voltage output is 0.59 V or more (RICH), it is determined that the voltage output of the sensor is abnormal high (DTC P0138).
*: Oxygen Storage Capacity (OSC): A catalyst has a capability for storing oxygen. The OSC and the emission purification capacity of the catalyst are mutually related. The ECM judges if the catalyst has deteriorated based on the calculated OSC value.
HINT: The ECM provides a pulse width modulated control circuit to adjust 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 (Mode 06):
The sensing portion of the heated oxygen sensor has a zirconia element which is used to detect oxygen concentration in the exhaust. If the zirconia element is at the proper temperature and difference of the oxygen concentration between the inside and outside surface of sensor is large, the zirconia element will generate voltage signals. In order to increase the oxygen concentration detecting capacity in the zirconia element, the ECM supplements the heat from the exhaust with heat from a heating element inside the sensor. When current in the sensor is out of the standard operating range, the ECM interprets this as a fault in the heated oxygen sensor and sets a DTC.
Example:
The ECM will set a high current DTC if the current in the sensor is more than 2 A when the heater is OFF. Similarly, the ECM will set a low current DTC if the current is less than 0.25 A when the heater is ON.
Wiring Diagram:
Step 1:
Step 2 - 3:
Step 4:
INSPECTION PROCEDURE
HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data records the engine conditions when the malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.