Powertrain Controls - ECM/PCM

Trouble Code 44 Chart:

Heated Oxygen Sensor Schematic:

CIRCUIT DESCRIPTION
The heated oxygen sensor (HO2S) has 450 millivolts applied to it by the Powertrain Control Module (PCM) on circuit 412. This voltage may vary slightly when measured with a 10 megohm (minimum) impedance DVOM. As the HO2S warms and becomes active, it also produces a voltage which will vary from 100 to 1000 millivolts. The PCM then detects whichever voltage is greater at the HO2S signal input. If the HO2S voltage is less than the 450 millivolts reference, the sensor acts as a ground and functionally reduces the voltage level at the input. This effect causes the Tech 1 scan tool to display a 100 to 1000 millivolt operating range. The heater in the HO2S allows for faster sensor warm-up and better temperature stabilization during engine operation.

DIAGNOSTIC TROUBLE CODE (DTC) 44 WILL SET WHEN
Vehicle is operating in "Closed Loop" and circuit 412 is below 250 millivolts for greater than 60 seconds.

ACTION TAKEN (PCM) WILL DEFAULT TO
Vehicle will operate in "Open Loop" and Malfunction Indicator Lamp (MIL) will become illuminated.

DTC 44 WILL CLEAR WHEN
A current DTC will clear when the HO2S voltage rises above the calibrated lean exhaust voltage threshold. A History DTC 44 will clear after 50 consecutive ignition key cycles without a current DTC 44 being stored.

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

1. The engine must be operating in "Closed Loop" to store DTC 44.

DIAGNOSTIC AIDS
Using the Tech 1 (or equivalent) scan tool, observe the long term fuel trim at different RPM and air flow conditions. The Tech 1 (or equivalent) scan tool 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 around 150.

Heated Oxygen Sensor Wire
Sensor pigtail may be mispositioned and contacting the exhaust manifold. Check for intermittent ground in wire between connector and sensor.

Lean Injector(s)
Perform injector balance test, refer to Computers and Controls / System Diagnosis / Procedures / Diagnostic Charts / Chart C-2A. Chart C-2A Fuel Injector Balance Test


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 HO2S signal 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 correct fuel pressures. Refer to Computers and Controls / System Diagnosis / Procedures / Diagnostic Charts / Chart A-7. Related Tests, Information and Procedures


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

NOTE: If the above are OK, it is a faulty HO2S.