Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 44:

Left Oxygen Sensor Circuit:

CIRCUIT DESCRIPTION
The Powertrain Control Control Module (PCM) supplies a voltage of about 450 mV between terminals "C20" and "C19" (If measured with a 10 megohm digital voltmeter, this may display as low as 320 mV.). The Heated Oxygen Sensor (HO2S) varies the voltage within a range of about 1000 mV if the exhaust is rich, down through about 10 mV if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 315°C (600°F). An open sensor circuit or cold sensor causes "Open Loop" operation. Whenever the ignition is "ON" voltage is supplied to the HO2S heaters on CKT 439. Even with the engine not running, the HO2S will reach operating temperature and indicate the oxygen content of the exhaust gas at the sensor. Typically the signal voltage will be near 450 mV at key-up. The signal voltage will then rise or drop as the sensor becomes active, depending on the oxygen content of the air near the sensor.

DIAGNOSTIC TROUBLE CODE (DTC) 44 WILL SET WHEN
System is operating in "Closed Loop," not in power enrichment, and signal voltage remains below 200 mV for 50 seconds.

ACTION TAKEN (PCM DEFAULT)
The Malfunction Indicator Lamp (MIL) will illuminate and the fuel control system will go into "Open Loop."

DTC CHART TEST DESCRIPTION
Number below refer to the circled number on the diagnostic chart.

1. Engine must be at normal operating temperature before performing this test.

DIAGNOSTIC AIDS
Using the Tech 1 scanner, observe the long term fuel trim values at different RPM and air flow conditions. The Tech 1 scanner also displays the fuel trim cells, so the long term fuel trim values can be checked in each of the cells to determine when the DTC 44 may have been set. If the conditions for DTC 44 exist, the long term fuel trim values will be near 160.

^ Heated Oxygen Sensor wires. Sensor pigtail may be mispositioned and contacting the exhaust manifold.

Check for intermittent ground in signal wire between connector and sensor.

All HO2S wires and connections should be inspected for breaks or contamination that could prevent reference voltage from reaching the HO2S.

^ Lean injector(s). Perform "Injector Balance Test" using Tech 1 scanner or refer to Chart C-2A Injector Coil Test Procedure. Chart C-2A Injector Coil Test Procedure

^ Fuel contamination. Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC 44.

^ Fuel pressure. System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to Chart A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

^ Exhaust leaks. If there is an exhaust leak, above the oxygen sensor, the engine can cause outside air to be pulled into the exhaust stream and flow past the sensor causing lean condition. Vacuum or crankcase leaks can cause a lean condition.

^ AIR system. Make sure the secondary air pump is not operating in "Closed Loop." While monitoring long term fuel trim, squeeze the air hose going to the exhaust ports (left bank). If long term fuel trim values go down, refer to Chart C-6 Secondary Air Injection (AIR) System Check. Chart C-6 Secondary Air Injection (AIR) System Check

^ If the above are OK, the oxygen sensor may be at fault.