CIRCUIT DESCRIPTION:
The ECM supplies a voltage of about .45 volt between terminals "GE 14" and "GE 15". (If measured with a 10 megohm digital voltmeter, this may read as low as .32 volt.) The O2 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 316'C (600'F). An open sensor circuit causes "Open Loop" operation. The heating element in the O2 sensor causes the sensor to heat up quickly, allowing for quicker closed-loop operation.

TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.

1. Code 45 is set when the O2 sensor voltage:
   • Remains above .75 volt for 50 seconds; and the system is in "Closed Loop."

DIAGNOSTIC AIDS:
Using the "Scan," observe the block learn values at different rpm and air flow conditions. The "Scan" also displays the block cells, so the block learn values can be checked in each of the ceHs to determine when the Code 45 may have been set. If the conditions for Code 45 exists, the block learn values witor)l be around 115.

• 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. See Fuel System diagnosis CHART A-7.
• Rich Injector. Perform injector balance test CHART C-2A.
• Leaking Injector. See CHART A-7.
• Check for fuel contaminated oil.
• HEI Shielding. An open ignition ground CKT 453 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 canister for fuel saturation. If full of fuel, check canister control and hoses. See "Purge Valve Operation" Section "C3."
• MAP Sensor. An output that causes the ECM to sense a higher than normal manifold pressure can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to set a fixed value for the sensor. Substitute a different MAP sensor if the rich condition is gone while the sensor is disconnected.
• Check for leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for fuel.
• TPS. An intermittent TPS output will cause the system to go rich, due to a false indication of the engine accelerating.
• EGR. An EGR staying open (especially at idle) will cause the O2 sensor to indicate a rich exhaust, and this could result in a Code 45