DTC P0138 HO2S Circuit High Voltage Sensor 2. Isuzu S/T Isuzu

For 1990-2009 cars only

Makes 🢒 Isuzu 🢒 2000 🢒 S/T Isuzu - 2WD 🢒 DTC 🢒 DTC P0138 HO2S Circuit High Voltage Sensor 2

Circuit Description

The rear heated oxygen sensor (HO2S 2), located in the exhaust stream past the catalytic converter, produces an output signal relative to the oxygen storage capacity of the catalytic converter. The powertrain control module (PCM) then calculates the ability of the catalytic converter to convert the exhaust emissions effectively. Whenever the catalytic converter is functioning properly, the rear HO2S 2 signal will be far less active than the signal produced by the front oxygen sensor 1 (O2S 1).

Conditions for Running the DTC

•	DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0169, P0171, P0172, P0178, P0179, P0201, P0202, P0203, P0204, P0300, P0336, P0440, P0446, P0452, P0453, P0506, P0507, P0601, P0602 or P1441 are not set.

•	The engine coolant temperature (ECT) is more than 70°C.

•	Engine run time is more than 10 seconds.

•	The throttle position (TP) angle is between 8 percent and 50 percent.

•	Above conditions are met for 3.8 seconds.

•	Closed Loop operation.

•	System voltage is more than 11.7 volts.

•	Fuel level is more than 10 percent .

•	Fuel ethanol composition less than 88 percent.

Conditions For Setting The DTC

HO2S 2 voltage is more than 1042 mV for 50 seconds.

Action Taken When the DTC Sets

•	The malfunction indicator lamp (MIL) will illuminate after 2 consecutive ignition cycles in which the diagnostic runs with the malfunction present.

•	The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.

•	A history DTC is stored.

Conditions for Clearing the MIL/DTC

•	The MIL will turn OFF after 3 consecutive ignition cycles in which the diagnostic runs without a fault.

•	A history DTC will clear after 40 consecutive warm up cycles without a fault.

•	A scan tool can clear the DTCs.

Diagnostic Aids

Important: Never solder the HO2S wires. For proper wire and connector repair refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Inspect the following items:

•	The fuel pressure--If the pressure is too high, the system will run rich. The PCM can compensate for some increase. However, if the pressure gets too high, the DTC P0138 may set. Refer to Fuel System Diagnosis .

•	A rich injector--Perform an Injector Balance Test. Refer to Fuel Injector Balance Test .

•	A leaking injector--Refer to Fuel Injector Balance Test .

•	Fuel contaminated oil.

•	EVAP canister purge--Check for a fuel saturation. If full of fuel, check the canister control and hoses. Refer to Evaporative Emission Control System Diagnosis .

•	The TP sensor--An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving.

•	False rich indication due to silicon contamination of the heated oxygen sensor--This DTC, accompanied by a lean driveability condition and a powdery white deposit on the sensor, may indicate a false rich indication.

An intermittent may be caused by any of the following conditions:

•	A poor connection

•	Rubbed through wire insulation

•	A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

If a repair is necessary, then refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

In order to determine if the engine is rich during De-Acceleration Fuel Cut-Off (DFCO) operate the vehicle up to highway speed conditions and release the accelerator pedal, allowing the vehicle to coast in gear. Monitor the scan tool HO2S voltage and the DFCO parameter. A rich condition will cause the HO2S voltage to be more than 468 mV during DFCO.

Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor, check for rich conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S is defective, or a rich condition exists only on the suspect HO2S cylinder bank.

This step checks the HO2S HIGH signal circuit for a short to voltage.

An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of unapproved silicon RTV engine gasket material or the use of silicon-based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also become contaminated.

Step

Action

Values

Yes

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Run the engine until the normal operating temperature is reached.
Use a scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage fixed more than the value specified?

1042 mV

Go to Step 4

Go to Step 3

This DTC is intermittent.

Are any additional DTCs stored?

Go to applicable DTC table

Go to Diagnostic Aids

Turn OFF the ignition.
Disconnect the HO2S connector for the sensor that applies to this DTC.
Jumper the HO2S return circuit terminal at the engine harness side to a clean body ground.
Turn ON the ignition, with the engine OFF.
Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage within the value range specified?

351 - 551 mV

Go to Step 5

Go to Step 6

The HO2S is detecting a rich exhaust condition or may be contaminated. Check for one of the following conditions:

•	HO2S connector water intrusion

•	A silicon-contaminated HO2S

•	Fuel-contaminated engine oil

•	An EVAP canister purge condition. Refer to Evaporative Emission Control System Diagnosis .

•	Incorrect fuel pressure. Refer to Fuel System Diagnosis .

•	Rich fuel injectors--Refer to Fuel Injector Balance Test .

Repair any of the above or similar engine conditions as necessary.

Did you find a condition?

Go to Step 10

Go to Step 8

Turn OFF the ignition.
Remove the jumper from the HO2S return circuit terminal.
Disconnect the PCM connector that contains the HO2S signal circuit.
Turn ON the ignition, leaving the engine OFF.
Using a measure the voltage between the HO2S signal circuit PCM connector terminal on the engine harness side and the ground.

Is the voltage more than the specified value?

20 mV

Go to Step 7

Go to Step 9

Repair the short to voltage in the HO2S signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

Go to Step 10

Important: Before replacing a contaminated HO2S, determine and repair the cause of the contamination.

Replace the HO2S. Refer to Heated Oxygen Sensor Replacement .

Is the action complete?

Go to Step 10

Important: The replacement PCM must be programmed.

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Is the action complete?

Go to Step 10

Use the scan tool in order to clear the DTCs.
Start the engine.
Allow the engine to idle until the engine reaches normal operating temperature.
Select DTC and the Specific DTC function.
Enter the DTC number which was set.
Operate the vehicle, within the Conditions for Setting the DTC, until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?

Go to Step 11

Go to Step 2

Does the scan tool display any additional undiagnosed DTCs?

Go to applicable DTC table

System OK