Circuit Description
The engine control module (ECM) supplies a voltage near 450 mV between the heated oxygen sensor (HO2S) high signal circuit and the low reference circuit. The HO2S varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean.
The ECM monitors and stores the HO2S voltage information. The ECM evaluates the HO2S voltage samples in order to determine the amount of time that the HO2S voltage was out of range. The ECM compares the stored HO2S voltage samples taken within each sample period and determines if the majority of the samples are out of the operating range.
The ECM monitors the HO2S voltage for being fixed below a predetermined voltage. If the ECM detects the voltage is too high, this DTC sets.
The HO2S utilizes the following circuits:
| • | A signal circuit |
| • | A low reference circuit |
| • | An ignition 1 voltage circuit |
| • | A heater control circuit |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Sensor 2
Conditions for Running the DTC
Closed Loop Test
| • | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0141, P0171, P0172, P0201-P0204, P0300, P0336, P0337, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, and P1404 are not set. |
| • | The engine coolant temperature is more than 60°C (140°F). |
| • | The ignition 1 voltage is more than 10 volts. |
| • | The calculated airflow is more than 8 g/s. |
| • | The fuel system is in closed loop. |
| • | DTC P0138 runs continuously once the above conditions are met for 3 seconds. |
Decel Fuel Cut-Off (DFCO) Test
| • | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0141, P0171, P0172, P0201-P0204, P0300, P0336, P0337, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, and P1404 are not set. |
| • | The engine coolant temperature is more than 60°C (140°F). |
| • | The ignition 1 voltage is more than 10 volts. |
| • | DTC P0138 runs continuously once the above conditions are met for 3 seconds. |
Conditions for Setting the DTC
Closed Loop Test
| • | The ECM detects that the HO2S 2 voltage is more than 952 mV. |
| • | The condition exists for more than 4 minutes. |
Decel Fuel Cut-Off (DFCO) Test
| • | The ECM detects that the HO2S 2 voltage is more than 550 mV during deceleration. |
| • | The condition exists for more than 11 seconds. |
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the third consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views | ||||||||||||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||||||||
2 |
Does the voltage fluctuate rapidly above and below the specified range? | 350-550 mV | Go to Step 3 | Go to Step 4 | ||||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||||
4 |
Is the voltage more than the specified value? | 500 mV | Go to Step 5 | Go to Step 6 | ||||||||||
5 | Test the HO2S 2 signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8 | ||||||||||
6 | Test or inspect for the following conditions:
Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 7 | ||||||||||
7 | Test for shorted terminals and for poor connections at the harness connector of the HO2S 2. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 9 | ||||||||||
8 | Test for shorted terminals and for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10 | ||||||||||
9 | Replace the HO2S 2. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||||||
10 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||||||
11 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 12 | ||||||||||
12 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||
