| Table 1: | HO2S Voltages |
Circuit Description
The wide band heated oxygen sensor (HO2S) measures the amount of oxygen in the exhaust system and provides more information 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 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 voltage in the oxygen sensing cell. The ECM monitors the voltage variation in the sensing cell and attempts to keep the voltage constant by increasing or decreasing the amount of current flow, or oxygen ion flow, to the pumping cell. By measuring the amount of current required to maintain the voltage in the sensing cell, the ECM can determine the concentration of oxygen in the exhaust. The HO2S 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 fuel system is lean, the oxygen level will be high and the lambda signal will be high or more than 1. When the fuel system is rich, the oxygen level will be low, and the lambda signal will be low or less than 1. The ECM uses this information to maintain the correct air/fuel ratio.
DTC Descriptors
This diagnostic procedure supports the following DTCs:
| • | DTC P2251 HO2S Reference Ground Circuit Bank 1 Sensor 1 |
| • | DTC P2254 HO2S Reference Ground Circuit Bank 2 Sensor 1 |
Conditions for Running the DTC
| • | Before the ECM can report DTC P2251 or P2254 failed, DTC P0030, P0031, P0032, P0050, P0051, and P0052 must run and pass. |
| • | The engine is operating. |
| • | The ignition 1 voltage is between 10-18 volts for more than 30 seconds. |
| • | The HO2S heater control is enabled. |
| • | The HO2S heater is at operating temperature for more than 20 seconds. |
| • | The calculated exhaust temperature is less than 900°C (1652°F). |
| • | The internal HO2S sensing element resistance is more than 570 ohms. |
| • | The ECM detects the internal HO2S signal voltage is between 1.47-1.53 volts. |
| • | A fuel cut-off event has not occurred in more than 2 seconds. |
| • | DTC P2251 and P2254 run continuously once the above conditions are met for 10 seconds. |
Conditions for Setting the DTC
| • | The ECM detects the internal HO2S voltage signal change is more than 10 millivolts as the heater control switches. |
| • | The above condition occurs 20 times within 10 seconds. |
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 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. |
Diagnostic Aids
| • | The front wide band sensors do not toggle or switch like a switching HO2S. The front HO2S signals will be relatively stable for an idling engine. |
| • | High resistance in any of the HO2S circuits could cause this DTC to set. |
| • | Use the J 35616-A/BT-8637 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector. |
| • | The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Engine Controls Component Views . |
| • | For an intermittent condition, refer to Intermittent Conditions . |
| • | The following table illustrates the typical voltages for the HO2S circuits. |
HO2S Circuit | Voltage | ||||
|---|---|---|---|---|---|
| |||||
Heater Control | 4.6-5.0 V | ||||
Heater Supply Voltage | B+ | ||||
Reference Voltage | 2.6-3.1 V | ||||
Low Reference | 2.2-2.7 V | ||||
Pump Current | Less than 0.5 V | ||||
Input Pump Current | Less than 0.5 V | ||||
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step determines if the condition exists. If there is a condition with the HO2S circuits, the ECM commands open loop for the applicable sensor.
-
This step tests for a bias voltage across the reference voltage circuit and the low reference circuit of the HO2S. The bias voltage that you are measuring, is the voltage difference between the reference voltage circuit and the low reference circuit. If there is a condition with the reference voltage circuit or the low reference circuit of the HO2S, the bias voltage will be more than the specified value, which indicates there is a condition with the circuits.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |
|
Important: DTC P2251 is for bank 1 sensor 1. DTC P2254 is for bank 2 sensor 1.
Does the scan tool display Closed Loop? | -- | Go to Step 3 | Go to Step 4 | |
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids |
4 | Is DTC P0131, P0132, P0151, P0152, P2243, or P2247 also set? | -- | Go to Step 5 | |
Is the voltage within the specified range? | 350-550 mV | Go to Step 7 | Go to Step 6 | |
6 | Test the low reference circuit of the HO2S for an open or for a high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8 |
7 | Test for an intermittent and for a poor connection at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 9 |
8 | Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10 |
9 | Replace the HO2S. Refer to the appropriate procedure: Did you complete the replacement? | -- | Go to Step 11 | -- |
10 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. 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 | |
