Oxygen Sensor Circuit

Circuit Description:
The ECM supplies a voltage of about .45 volt between terminals "D6" and "D7". (If measured with a 10 megohm digital voltmeter, this may read as low as .32 volt.) The Oxygen (02) sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down through about .10 volt if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below about 315°C (600°F). An open sensor circuit or cold sensor causes "Open Loop" operation.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Code 45 is set when the Oxygen (02) sensor signal voltage or CKT 412:

• • Remains above .7 volt for 60 seconds, and in "Closed Loop."
  • Engine time after start is 1 minute or more.
  • Throttle angle greater than 5% (about .2 volt above idle voltage).

Diagnostic Aids:
Using the "Scan," observe the block learn values at different rpm and air flow conditions to determine when the Code 45 may have been set. If the conditions for Code 45 exist, the block learn values will be around 115. Code 45 will be a hard code on 4.3L CPI engine. The "SES" light will remain "ON" until the ignition is turned "OFF," due to possible high fuel pressure.

An oxygen supply inside the 02 sensor is necessary for proper 02 sensor operation. This supply of oxygen is supplied through the 02 sensor wires. All 02 sensor wires and connections should be inspected for breaks or contamination that could prevent reference oxygen from reaching the 02 sensor.

• Fuel Pressure - System will go rich if pressure is too high. The ECM can compensate for some increase. However, if it gets too high, a Code 45 may be set.
• Check for fuel contaminated oil.
• HEI Shielding - An open ground CKT 453 (ignition system reference low) may result in EMI, or induced electrical "noise." The ECM looks at this "noise" as reference pulses. The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing system to go rich. Engine tachometer will also show higher than actual engine speed which can help in diagnosing this problem.
• Canister Purge - Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to "Evaporative Emission Control".
• MAP Sensor - An output that causes the ECM to sensor a higher than normal manifold pressure (low vacuum) can cause the system to go rich Disconnecting the MAP sensor will allow the ECM to set a fixed value for the MAP sensor. Substitute a different MAP sensor if the rich condition is gone while the sensor is disconnected.
• Pressure Regulator - Check for leaking fuel pressure regulator diaphragm by checking for presence of liquid fuel.
• TPS - An intermittent TPS output will cause the system to go rich, due to a false indication of the engine accelerating.
• CTS - Check for a shifted sensor that could cause a rich exhaust but set Code 15.