Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
DTC Descriptors
DTC P2251: HO2S Reference Ground Circuit Sensor 1Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Heater Control | P0031 | P0030, P0135 | P0032 | -- |
Heater Supply Voltage | P0013, P0023, P0031, P0458, P2088, P2092 | P0030, P0135 | -- | -- |
High Signal | P0131 | P0135, P2243, P2251 | P0132 | P0130 |
Low Signal | P0131 | P0131, P0135, P2251 | P0132 | P0130 |
Pump Current | P0131 | P2243, P2251, P2626 | P0132 | -- |
Input Pump Current | P0131 | P2237 | P0132 | -- |
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.
Conditions for Running the DTC
| • | Before the ECM can report DTC P2251 failed, DTC P0030, P0031, P0032, and P0135 must run and pass. |
| • | The ignition 1 voltage is between 11-18 volts for greater than 30 seconds. |
| • | The HO2S heater control is enabled. |
| • | The ECM has not commanded fuel cut-off for greater than 2 seconds. |
| • | The HO2S heater is at operating temperature for greater than 20 seconds. |
| • | The internal HO2S sensing element resistance is greater than 570 ohms. |
| • | The calculated exhaust temperature is less than 900°C (1,652°F). |
| • | The ECM detects the internal HO2S signal voltage is between 1.47-1.53 volts. |
| • | DTC P2251 runs continuously once the above conditions are met for greater than 10 seconds. |
Conditions for Setting the DTC
| • | The ECM detects the internal HO2S voltage signal change is greater than 0.12 mV as the heater control switches. |
| • | The above condition occurs 20 times within 10 seconds. |
Action Taken when the DTC Sets
DTC P2251 is a Type B DTC.
Conditions for Clearing the MIL/DTC
DTC P2251 is a Type B DTC.
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References for scan tool information
Diagnostic Aids
The following table illustrates the ECM connector locations:
ECM Connector Location | |
|---|---|
X2 | X1 |
Front of Vehicle | |
| • | The front wide band sensor does not toggle or switch like a switching HO2S. The front HO2S signal will be relatively stable for an idling engine. |
| • | 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+ | ||||
High Signal | 2.6-3.1 V | ||||
Low Signal | 2.2-2.7 V | ||||
Pump Current | Less than 0.5 V | ||||
Input Pump Current | Less than 0.5 V | ||||
Circuit/System Verification
- Ignition ON, observe the scan tool DTC information. Verify DTCs P0131, P0132, or P2243 are not also set.
- Engine idling at operating temperature. Observe the scan tool Loop Status Sen 1 parameter. The parameter should display Closed Loop.
- Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.
| ⇒ | If any of the above DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis. |
Circuit/System Testing
- Ignition OFF, disconnect the harness connector at the HO2S 1.
- Ignition ON, test for 350-550 mV between the high signal circuit terminal 1 and the low signal circuit terminal 5.
- If all circuits test normal, replace the HO2S 1.
| ⇒ | If less than the specified range, test the low signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for engine control module replacement, setup, and programming |
