Code 44
OXYGEN SENSOR CIRCUIT - DTC 44
The ECM looks for poor closed loop fuel metering control while correcting for a rich or lean mixture condition.
The heated oxygen sensor, located in the exhaust gas stream, compares exhaust oxygen content to ambient air that is drawn through the heater wires. The heater is powered by the fuel pump relay (1990 - 992 MY vehicles) or oxygen sensor heater relay (1993 MY ON vehicles).
The ECM lengthens or shortens the injector pulse duration in response to the oxygen sensor detecting a RICH or a LEAN mixture. The fuel metering can vary ±12.5 % without flagging a code. If a RICH mixture cannot be corrected by the ECM, the oxygen sensor feedback voltage (monitor with JDS or PDU) falls to 0 volt; if a LEAN mixture cannot be corrected by the ECM, the oxygen sensor feedback voltage (monitor with JDS or PDU) rises to 5 volts. A further 37.5 % fuel metering adjustment is then made. If the oxygen sensor signal voltage does not respond, DTC 44 is flagged. In addition, the ECM will flag DTC 44 if the oxygen sensor signal voltage (raw) is not sensed at the ECM (open or short circuit).
Conditions Required For DTC 44 Flagging
Throttle position - Less than 3 volts.
Engine coolant temperature - Greater than 75 °C (167°F).
Closed loop fuel metering (oxygen sensor feedback) - Not in control (rich and/or lean).
Oxygen sensor signal voltage (raw) - Not switching.
Condition monitored for - 450 consecutive engine revolutions.
Limp Home Default
If DTC 44 is flagged, closed loop fuel metering is canceled and no dynamic fueling correction is applied to the fuel metering strategy. The oxygen sensor feedback voltage will remain stationary at the mid point (2.5 volts).
Possible Faults
- Fuel pressure fault.
- Stuck open purge valve.
- Poor ground connections.
- Poor oxygen sensor connection.
- Open or short circuit oxygen sensor wiring.
- Open or short circuit oxygen sensor heater wiring.
- "Lazy" or defective oxygen sensor.
- Defective air injection pump or circuit.
- Intake air leak.
- Blocked or inoperative fuel injectors.
- Defective mass air flow 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); if the feedback voltage stays above 4 volts, look for causes of lean running (DTC 26). The PDU Datalogger can be set to trigger upon DTC 44 being flagged.
2. Test for a DTC reoccurrence by bringing the engine to normal operating temperature then holding the speed at 2000 rpm for 1 minute.
3. 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.)
4. 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.
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 H02S RAW:
Circuit Diagram
Circuit Diagram:
Flow Chart - DTC 44
k �Flow Chart - DTC 44:
