Code 23
OXYGEN SENSOR FEEDBACK (RICH) - DTC 23
The ECM looks for poor closed loop fuel metering control while correcting for a rich mixture condition.
The ECM shortens the injector pulse duration in response to high oxygen sensor voltage indicating rich fuel metering. The fuel metering can vary by ±12.5 % without flagging a code. If the fuel metering cannot be corrected by the ECM, the oxygen sensor feedback voltage (monitor with JDS or PDU) falls to 0 volt and DTC 23 will be flagged.
Conditions Required For DTC 23 Flagging
Engine coolant temperature - Greater than 167°F.
Closed loop fuel metering (oxygen sensor feedback) - Not in control (rich), feedback = 0 volt.
Response time - 660 consecutive engine revolutions.
Limp Home Default
None
Possible Faults
- High fuel pressure.
- Oxygen sensor heater ground loose or engine ground loose.
- ECM ground connection at bulkhead stud loose or corroded.
- Engine coolant thermostat stuck open.
- Purge valve stuck open.
- High resistance in MAFS connector.
- Incorrect mass air flow sensor calibration.
- ON vehicles (with adaptive idle fueling trim): Intake system air leak - especially between the MAFS and the throttle valve.
- "Lazy" or defective oxygen sensor.
Diagnostic Notes
1. Engine fueling correction (dynamic) can be monitored by using JDS or PDU to read the oxygen sensor feedback voltage (HO2SFB). If the feedback voltage is between 1 and 4 volts, the fueling is normal and the problem is intermittent. If the feedback voltage stays below 1 volt, look for causes of rich running (DTC 23). The PDU Datalogger can be set to trigger on flagging DTC 23.
2. 1993 MY ON vehicles (with adaptive idle fueling trim) The ECM automatically corrects for small fueling errors by adding to or subtracting from the "base line" injector pulse duration. To determine if a base line correction has occurred, first monitor the oxygen sensor feedback voltage and record the value. Then disconnect and reconnect the ECM and recheck the oxygen sensor feedback voltage. A change in the feedback voltage indicates that a correction had been applied. (Disconnecting the ECM erases the correction.)
3. A rich mixture condition can be caused by the engine coolant thermostat sticking open. At high road speed, the coolant temperature decreases, causing enriching from the coolant temperature sensor input to the ECM. To test the thermostat, check the engine setup using JDS or PDU. If the coolant temperature does not reach 88°C (190°F) (as monitored by JDS or PDU), replace the thermostat.
4. 1993 MY ON vehicles (with adaptive idle fueling trim) DTC 23 can he caused by an unmeasured air leak, especially between the MAFS and the throttle valve. The base line correction, automatically applied for the lean mixture condition at idle, can create a rich mixture condition while driving.
5. During closed loop fueling, the oxygen sensor "raw" voltage (signal voltage to the ECM) normally swings between 0.2 and 0.8 volt. If the oxygen sensor becomes contaminated or the heater is inoperative, the sensor can be "lazy" A lazy sensor may show a voltage swing of 0.3-0.7 volt or even less. The oxygen sensor raw voltage can be monitored using PDU (H02 S RAW).
Oxygen Sensor Voltage Trace: PDU HO2S Raw:
Fuel Pressure Check
42-45 psi (vacuum disconnected from fuel pressure regulator).
34-38 psi at idle (vacuum connected to fuel pressure regulator).
Purge Valve Test For A Sticking Purge Valve:
- Disconnect the purge hose at the intake manifold adapter.
- Switch ignition ON; do not start the engine.
- Apply vacuum to purge hose; vacuum should held. If the vacuum does not hold, the purge valve is stuck and should be replaced.
- Switch ignition OFF; vacuum should release after a few seconds. If the vacuum does not release, the purge valve is stuck and should be replaced.
Circuit Diagram
Circuit Diagram:
Flow Chart: DTC 23
���Flow Chart: DTC 23:
