P0138

DTC P0138 OXYGEN SENSOR CIRCUIT HIGH VOLTAGE (BANK 1 SENSOR 2)

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.

CIRCUIT DESCRIPTION

DTC Detection Condition:

Monitor Strategy:

Typical Enabling Conditions:

Typical Malfunction Thresholds:

Component Operating Range:

O2S Test Result:

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.

Wiring Diagram:

CONFIRMATION DRIVING PATTERN FOR READINESS MONITOR

PURPOSE






HINT: Performing this confirmation pattern will activate the heated oxygen sensor 2 monitoring by the ECM. (The catalyst monitoring is performed simultaneously) This is very useful for verifying the completion of a repair.




a. Clear the DTCs.
1. Disconnect the battery cable or remove the EFI and ETCS fuses for 60 seconds or more.

b. Connect the hand-held tester to the DLC3.
c. Enter the following menus: DIAGNOSIS / CARB OBD II / READINESS TESTS and check that O2S EVAL is INCMPL (incomplete).
d. Drive the vehicle according to the confirmation driving pattern. Note the state of the READINESS TESTS.
O2S EVAL will change to COMPL (complete) as O2S evaluation monitors operate.

e. Enter the following menus: DIAGNOSIS / ENHANCED OBD II / DTC INFO / PENDING CODES and check if any DTC (any pending code) is set.

If the READINESS CODE of "O2S EVAL" was "INCMPL" and any DTC (include pending codes) was not set, extend the driving time.

NOTE: If you do not have the hand-held tester, perform again the same confirmation driving pattern after turning off the ignition switch upon finishing the first confirmation driving pattern.

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 a 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.

Step 1 - 2:

Step 2 (Continued) - 3:

Step 4 - 6:

Step 7:

Hand-held tester

Step 1 - 4:

Step 5 - 6:

Step 7:

OBD II scan tool (Excluding hand-held tester)

CHECK FOR INTERMITTENT PROBLEMS

Hand-held tester only:
Inspect the vehicle's ECM using check mode. Intermittent problems are easier to detect when the ECM is in check mode with the hand-held tester. In check mode, the ECM uses 1 trip detection logic, which has a higher sensitivity to malfunctions than normal mode (default), which uses 2 trip detection logic.

a. Clear the DTCs.
b. Set the check mode.
c. Perform a simulation test.
d. Check the connector and terminal.
e. Wiggle the harness and the connector.