P0151

Schematic:

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 range HO2S functions more like an air fuel sensor. 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 signal will be high or more than 1. When the oxygen level is low, the lambda signal will be low or less than 1. The ECM uses this information to maintain the proper air/fuel ratio.

CONDITIONS FOR RUNNING THE DTC
^ The following DTCs are not set:
- DTC P0300-P0306
- DTC P0171
- DTC P0172
- DTC P0174
- DTC P0175
- DTC P0440
- DTC P0442
- DTC P0443
- DTC P0446
- DTC P0455
- DTC P0460
^ Engine at operating temperature
^ The engine speed must be between 1400 and 2400 RPM.
^ Engine load 8-12 percent
^ Mass air flow 18-22 g/s
^ Throttle position 6-10 percent
^ The secondary air diagnosis is not running.
^ Closed Loop fuel control has not reached maximum or minimum.
^ There is no Evaporative Emissions (EVAP) high loading indicated.

CONDITIONS FOR SETTING THE DTC
The ECM has determined that the HO2S sensor does not respond as expected.

ACTION TAKEN WHEN THE DTC SETS
The ECM illuminates the Malfunction Indicator Lamp (MIL) and records the operating conditions in the Freeze Frame on the second consecutive drive trip that the diagnostic runs and fails.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The ECM turns OFF the MIL after three consecutive drive trips that the diagnostic runs and passes.
^ A History DTC clears after forty consecutive warm-up cycles in which no failures are reported by this diagnostic or any other emission related diagnostic.
^ The scan tool clears the MIL/DTC.

DIAGNOSTIC AIDS
Use the J 35616-A connector test adapter kit for any test that requires probing the ECM harness connector or a component harness connector.

This DTC is most likely to set by a blocked passage that does not allow the exhaust gas to pass through the sensor. Check for any sign of contamination and the source of the contamination.

Refer to Testing for Intermittent and Poor Connections in Diagrams Inspect the HO2S 1 harness for any signs of high resistance or shorts to ground. Use the DMM if necessary.

An open fuse in the HO2S heater circuit may be caused by the heater element in one of the sensors. The condition may not be present until the heater operates for a period of time. If no fault is present in the heater circuit, monitor the amperage of each heater using the DMM to determine if one of the heater elements is the cause of the open fuse.

TEST DESCRIPTION

Steps 1 - 11:

Steps 12 - 24:

Steps 25 - 28:

The numbers below refer to the step numbers on the diagnostic table.
2. Multiple DTCs will set if the supply voltage to the sensor heaters is open.
4. Any exhaust leak even a small one can affect the operation of the HO2S. The ECM compares the reading of sensor 1 with the reading of sensor 2 as part of the diagnosis. A leak between sensor 1 and sensor 2 may set a DTC.
9. The test lamp may illuminate or flash depending on the operating conditions. This indicates the ECM and the control circuit are functioning. The engine must be running for the ECM to operate the control circuit.
14. A short between the signal circuit and the reference ground circuit or the pump current input and output circuits may not appear on the voltage tests. The ECM must be disconnected and each circuit tested for continuity with the other HO2S circuits.