P0138
DTC P0138 HEATED OXYGEN SENSOR CIRCUIT HIGH VOLTAGE (BANK 1 SENSOR 2)CIRCUIT DESCRIPTION
DTC Detection Conditions:
The rear Heated Oxygen (HO2) sensor is located behind the Three-Way Catalytic Converter (TWC), and detects 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 voltage changes, the ECM adjusts the fuel injection time so that the air-fuel ratio remains as close as possible to stoichiometric levels.
The rear HO2 sensor generates a voltage that ranges from 0.1 V to 0.9 V according to the oxygen concentration in the exhaust gas. When the oxygen concentration in the exhaust gas increases, the HO2 sensor voltage drops below 0.45 V. The ECM interprets this as the air-fuel ratio being lean. Alternatively, the sensor voltage rises to more than 0.45 V when there is no oxygen in the exhaust gas. The ECM interprets this as the air-fuel ratio being rich.
MONITOR DESCRIPTION
Monitor Strategy:
Typical Enabling Conditions:
Typical Malfunction Thresholds:
Component Operating Range:
O2S Test Result (Mode 05 Data):
The ECM monitors the rear Heated Oxygen (HO2) sensor to check for the following malfunctions. If any one of the malfunctions detected, the ECM illuminates the MIL and sets a DTC:
- The HO2 sensor output voltage remains above 0.5 V (rich) or below 0.4 V (lean) while the vehicle is accelerated and decelerated for 4 to 8 minutes.
- The HO2 sensor output voltage remains at extremely low, below 0.05 V for a long time period of time while the vehicle is driven.
- The HO2 sensor output voltage remains at extremely low, below 0.2 V (extremely lean condition) soon after fuel-cut is performed while the vehicle is decelerated. The ECM interprets this as the sensor response having deteriorated.
- The HO2 sensor output voltage exceeds more than 1.2 V for 10 seconds.
CONFIRMATION DRIVING PATTERN
This confirmation driving pattern is used in step 6 of the following diagnostic troubleshooting procedure.
CAUTION: Strictly observe posted speed limits, traffic laws, and road conditions when performing the driving pattern below.
a. Preconditions
The Heated Oxygen (HO2) sensor monitor will not run unless:
- 2 minutes or more have elapsed since the engine was started.
- The Engine Coolant Temperature (ETC) is 75°C(167°F) or more.
- Cumulative driving time at a vehicle speed of 30 mph (48 km/h) or more exceeds 6 minutes.
- Air-fuel ratio feedback control is performed.
- Fuel-cut control is performed for 8 seconds or more (for the rear HO2 sensor).
b. Driving Patterns
1. Connect a hand-held tester or OBD II scan tool to the DLC3.
2. Turn the ignition switch to ON.
3. Turn the tester or scan tool ON.
4. Clear DTCs.
5. Start the engine and warm it up until the ETC reaches 75°C (167°F) or higher.
6. Drive the vehicle at 30 mph (48 km/h) or more for at least 40 seconds.
7. Allow the engine to idle for 20 seconds or more.
8. Repeat steps (6) and (7) described above at least 8 times in one driving cycle.
9. Change the transmission to 2nd gear.
10. Accelerate the vehicle to 30 mph (48 km/h) or more by depressing the accelerator pedal for at least 10 seconds.
11. Soon after performing step (10) above, release the accelerator pedal for at least 10 seconds without depressing the brake pedal, in order to execute fuel-cut control.
12. Allow the vehicle to decelerate to less than 6 mph (10 km/h).
13. Repeat steps from (10) through (12) described above at least twice in one driving cycle.
Wiring Diagram:
Step 1 - 2:
Step 3 - 4:
Step 5 - 6:
Step 7:
INSPECTION PROCEDURE
HINT: Hand-held tester only:
Malfunctioning areas can be identified by performing the A/F CONTROL function provided in the ACTIVE TEST. The A/F CONTROL function can help to determine whether the Heated Oxygen (HO2) sensors and other potential trouble areas are malfunctioning.
The following instructions describe how to conduct the A/F CONTROL operation using a hand-held tester.
1 .Connect a hand-held tester to the DLC3.
2. Start the engine and turn the tester ON.
3. Warm up the engine at an engine speed of 2,500 rpm for approximately 90 seconds.
4. On the tester, select the following menu items: DIAGNOSIS / ENHANCED OBD II / ACTIVE TEST / A/F CONTROL.
5. Perform the A/F CONTROL operation with the engine in an idling condition (press the RIGHT or LEFT button to change the fuel injection volume).
6. Monitor the voltage output of the HO2 sensors (O2S B1S1 and OS2 B1S2) displayed on the tester.
HINT:
- The A/F CONTROL operation lowers the fuel injection volume by 12.5% or increases the injection volume by 25%.
- Each sensor reacts in accordance with increases and decreases in the fuel injection volume.
NOTE: The front HO2 sensor has an output delay of a few seconds and the rear HO2 sensor has a maximum output delay of approximately 20 seconds.
- Following A/F CONTROL procedure enables technicians to check and graph the voltage outputs of both the front and rear HO2 sensors.
- To display the graph, select the following menu items on the tester: DIAGNOSIS / ENHANCED OBD II / ACTIVE TEST/ A/F CONTROL / USER DATA / AFS B1S2 and OS2 B1B2, and press the YES button and then the ENTER button followed by the F4 button.
NOTE: If the vehicle is short of fuel, the air-fuel ratio becomes lean and HO2 sensor DTCs are recorded, and the ECM illuminates the MIL.
HINT:
- If other DTCs relating to different systems that have terminal E2 as the ground terminal are output simultaneously, terminal E2 may have an open circuit.
- 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 occurred.
- A high HO2 sensor (sensor 1) voltage (0.55 V or more) could be caused by a rich air fuel mixture. Check for conditions that would cause the engine to run rich.
- A low HO2 sensor (sensor 1) voltage (0.4 V or less) could be caused by a lean air fuel mixture. Check for conditions that would cause the engine to run lean.
CHECK FOR INTERMITTENT PROBLEMS
HINT: Hand-held tester only:
Inspect the vehicle's ECM using check mode. Intermittent problems are easier to detect with a hand-held tester when the ECM is in check mode. In check mode, the ECM uses 1 trip detection logic, which is more sensitive to malfunctions than normal mode (default), which uses 2 trip detection logic.
a. Clear DTC.
b. Switch the ECM from normal mode to check mode using a hand-held tester.
c. Perform a simulation test.
d. Check and wiggle the harness(es), connector(s) and terminal(s).