P0172

DTC P0172 Rich Exhaust:

Heated Oxygen Sensor (HO2S) Circuit:

Circuit Description
The VCM applies a bias voltage of approximately 450 millivolts (350-550 mV is normal) between terminals "BL23" and "BL24". The heated oxygen sensor varies the voltage within a range of about 1 volt if the exhaust is rich to .100 volts if lean. DTC P0172 will set if HO2S voltage remains high too long.

DTC P0172 Will Set When
DTC P0172 will set when HO2S voltage is greater than .698 volts, TP is greater than 0% but less than 99% for 70 seconds.

Action Taken (VCM will default to)
VCM will default to "Open Loop."

DTC PO172 Will Clear When
Conditions for fault are no longer present and Tech 1 clear DTCs function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and Fuse #9 (ECM B) fuse is removed for 10 seconds.

DTC Chart Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Oxygen sensor voltage above .69 volt indicates that the fault is still present.

2. By disconnecting the HO2S, the scan tool should display the bias voltage from the VCM.

Diagnostic Aids
Using the Tech 1 scan tool, observe the long term fuel trim values at different RPM and air flow conditions to determine when the DTC P0172 may have been set. If the conditions for DTC P0172 exist, the long term fuel trim values will be around 115.

An oxygen supply inside the HO2S is necessary for proper HO2S operation. This supply of oxygen is supplied through the HO2S wires. All HO2S wires and connections should be inspected for breaks or contamination that could prevent reference oxygen from reaching the HO2S.

^ Fuel pressure - System will go rich if pressure is too high. The VCM can compensate for some increase. However, if it gets too high, a DTC P0172 may be set.

Refer to "Fuel System Diagnosis" chart. Related Tests, Information and Procedures

^ Check for fuel contaminated oil. Refer to "Fuel System Diagnosis" chart.
^ HEI shielding - An open ground CKT 453 (ignition system reference low) may result in EMI, or induced electrical "noise." The VCM looks at this noise" as reference pulses. The additional pulses result in a higher than actual engine speed signal. The VCM then delivers too much fuel, causing system to go rich. Engine tachometer will also show higher than actual engine speed which can help in diagnosing this problem.
^ MAP sensor - An output that causes the VCM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the VCM to set a fixed value for the MAP sensor. Substitute a different MAP sensor if the rich condition is gone while the sensor is disconnected.
^ Pressure regulator - Check for leaking fuel in pressure regulator diaphragm by checking for presence of liquid fuel.
^ TP sensor - An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating.
^ ECT - Check for a shifted sensor that could cause a rich exhaust, but set DTC P0117. Refer to chart for DTC P0117. P0117