Code 26
OXYGEN SENSOR FEEDBACK (LEAN) - DTC 26
The ECM looks for poor closed loop fuel metering control while correcting for a lean mixture condition.
The ECM lengthens the injector pulse duration in response to low oxygen sensor voltage indicating lean fuel metering. The fuel metering can vary ± 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) rises to 5 volts and DTC 26 will be flagged.
Conditions Required For DTC 26 Flagging
Engine coolant temperature - Greater than 167°F.
Closed loop fuel metering (oxygen sensor feedback) - Not in control (lean), feedback = 5 volts.
Response time - 660 consecutive engine revolutions.
Limp Home Default
None
Possible Faults
- Low fuel pressure.
- Intake manifold air leak.
- Ignition misfire.
- Purge valve stuck open.
- Fuel injectors blocked or sticking.
- Incorrect injectors (3.6 L injectors instead of 4.0 L injectors).
- Air injection pump running continuously.
- Oxygen sensor circuit fault.
- Oxygen sensor heater ground loose.
- Engine ground loose.
- High resistance connection to MAFS.
- Incorrect mass air flow sensor calibration.
- "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 above 4 volts, look for causes of lean running (DTC 26). The PDU Datalogger can be set to trigger on flagging DTC 26.
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. 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 (HO2 S RAW).
Oxygen Sensor Voltage Trace: PDU H02SRAW:
4. 1992 MY ON vehicles (ECM part number DBC 9622 ON): DTC 26 and 44 are inhibited when the fuel level is below approximately 2.5 gallons. A fuel level signal is input to the ECM on the yellow connector pin 6 from the fuel level sensor circuit. When the voltage is above 5.7 volts, the codes cannot be flagged.
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
To 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 hold. 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 26
6M� Flow Chart: DTC 26:
