Powertrain Controls - ECM/PCM

Diagnostic Trouble Code (DTC) 65:

Oxygen Sensor Circuit:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a voltage of about 450 mV between terminals "C8" and "C7" (If measured with a 10 megohm digital voltmeter, this may read 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) 65 WILL SET WHEN
Signal voltage remains above 700 mV for 50 seconds, throttle angle is greater than 4 percent and system is operating in "Closed Loop."

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 refers to circled number on the diagnostic chart.

1. Engine must be at normal operating temperature.

DIAGNOSTIC AIDS
Using the Tech 1 scanner, observe the long term fuel trim values at different RPM and load 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 65 may have been set. If the conditions for DTC 65 exist, the long term fuel trim values will be near 108.

^ Fuel pressure. System will go rich if pressure is too high. The PCM can compensate for some increase. However, if it gets too high, a DTC 65 may be set. Refer to Chart A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

^ Rich injector. Perform "Injector Balance Test" using the Tech 1 scanner or refer to Chart C-2A Injector Coil Test Procedure. Chart C-2A Injector Coil Test Procedure

^ Leaking injector. Refer to Chart A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

^ Check for fuel contaminated oil.

^ Evaporative Emission (EVAP) canister purge. Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to Chart C-3 EVAP Cannister Purge Solenoid Check. Chart C-3 EVAP Canister Purge Solenoid Check

^ Mass Air Flow (MAF) sensor. Disconnect the MAF sensor and see if rich condition is corrected. If so, check for proper installation. If installation is correct, replace MAF sensor. If the MAF sensor is installed backwards, the system will go rich. The plastic portion of the sensor has arrows cast into it indicating proper air flow direction. The arrows must point toward the engine.

^ Check for leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for fuel.

^ Throttle Position (TP). An intermittent TP sensor output will cause the system to go rich, due to a false indication of the engine accelerating.

^ Exhaust Gas Recirculation (EGR). An EGR valve staying open (especially at idle) will cause the oxygen sensor to indicate a rich exhaust, and this could result in a DTC 65.

^ An oxygen supply inside the HO2S is necessary for proper operation. This supply of oxygen is supplied through a hole in the HO2S wiring loom grommet. All HO2S connectors should be inspected for contamination that could prevent oxygen from reaching the HO2S.