Circuit Description
The wide band heated oxygen sensor (HO2S) measures the amount of oxygen in the exhaust stream more quickly and accurately than the switching style HO2S. The wide band sensor consists of an oxygen sensing cell, an oxygen pumping cell, and a heater. The exhaust gas sample passes through a diffusion gap between the sensing cell and the pumping cell. The engine control module (ECM) supplies a signal voltage to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. An electronic circuit within the ECM controls the pump current through the oxygen pumping cell in order to maintain a constant signal voltage. The ECM monitors the voltage variation on the signal circuit and attempts to keep the voltage constant by increasing or decreasing the amount of current flow, or reversing the direction of the current flow to the pumping cell. By measuring the direction and amount of current required to maintain the signal voltage, the ECM can determine the concentration of oxygen in the exhaust. The signal voltage is displayed as a lambda value. A lambda value of 1 is equal to a stoichiometric air fuel ratio of 14.7:1. Under normal operating conditions, the lambda value will remain around 1. When the system is lean, the oxygen level will be high and the lambda value will be high, or more than 1. When the system is rich, the oxygen level is low and the lambda value will be low, or less than 1. The ECM uses this information to maintain the proper air/fuel ratio. If the ECM detects the HO2S low reference circuit is open DTC P2251 will set for bank 1 sensor 1, or DTC P2254 will set for bank 2 sensor 1.
DTC Descriptors
This diagnostic procedure supports the following DTCs:
| • | DTC P2251 HO2S Low Reference Circuit Bank 1 Sensor 1 |
| • | DTC P2254 HO2S Low Reference Circuit Bank 2 Sensor 1 |
Conditions for Running the DTC
| • | DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0131, P0132, P0133, P0135, P0137, P0138, P0139, P0140, P0141, P0151, P0152, P0153, P0155, P0157, P0158, P0159, P0160, P0161, P1137, P1138, P1157, P1158 are not set. |
| • | The engine is running. |
| • | DTC P2251 or P2254 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
The ECM detects the HO2S low reference circuit is open after 5 minutes of engine run time.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second 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 3 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 or Engine Control Module Connector End Views | ||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||||
2 |
Does the affected lambda value reach the specified value as the engine decelerates? | 1.989 | Go to Step 3 | Go to Step 4 | ||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||
4 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 5 | ||||||||
5 |
Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 6 | ||||||||
6 |
Is the voltage more than the specified value? | 2.3 V | Go to Step 8 | Go to Step 7 | ||||||||
7 | Test the HO2S low reference circuit for an open or high resistance. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 9 | ||||||||
8 | Test for intermittent and poor connections at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections , Connector Repairs , and Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 10 | ||||||||
9 | Test for intermittent and poor connections at the engine control module (ECM). 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 12 | Go to Step 11 | ||||||||
10 | Replace the affected HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 and Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 . Did you complete the replacement? | -- | Go to Step 12 | -- | ||||||||
11 | Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 12 | -- | ||||||||
12 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 13 | ||||||||
13 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK | ||||||||
