1989 Buick Regal Code 45 Rich exhaust

Not enough air or too much fuel.

Check the vacuum hose from the fuel pressure regulator for fuel. Do this with the engine off. Sometimes these leak only with vacuum applied, hence no leak down at engine off. You may also want to try applying vacuum to the regulator and see if any fuel leaks then. This problem will cause an intermittant stall when the engine gets a gulp of fuel from the vacuum hose.

Have you checked for a sticking egr solenoid or carbon under one of the egr valves seats?

This is a run down on code 45:
Code 45 is set when the O2 sensor signal voltage or circuit 412: Remains above 0.75 volts for 2 minutes and in "Closed Loop"Throttle angle between 0.7 and 1.4 volts. Engine time after start is 1 minute or more.

Check the following: Fuel pressure - System will go rich if pressure is too high. Rich injector - Perform injector balance test. Leaking injectors - see Chart A-7 Canister purge - Check for fuel saturation. If full of fuel, check canister control and hoses. Mass Air Flow Sensor - An output that causes the ECM to sense a higher than normal airflow cab cause the system to go rich. Disconnecting the MAF will allow the ECM to set a fixed value for the sensor. Substitute a different MAF sensor if the rich condition is gone while the sensor is disconnected. Check for a leaking fuel pressure regulator. Remove vacuum line. TPS - An intermittent TPS will cause the system to go rich due to a false indication of engine accelerating. EGR - An EGR staying open(especially at idle)will cause the O2 sensor to indicate a rich mixture and could cause a Code 45.

If all is checking out, I would replace the pcm. Have seen a fair amount of these go bad and cause this.

Taht is correct on the o2.

Lets treat this as a no start. 45 may be misleading on this , may or may not be related to no start.

This chart assumes that battery condition and engine cranking speed are OK, and there is adequate fuel in the tank.

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

A "SERVICE ENGINE SOON" light "ON" is a basic test to determine if there is a 12 volt supply and ignition 12 volts to ECM. No ALDL, may be due to an ECM problem and CHART A-2 will diagnose the ECM. If TPS is over 2.5 volts the engine may be in the clear flood mode which will cause starting problems. The engine will not start without reference pulses and therefore the "Scan" should read rpm (reference) during crank.
For the first two seconds with ignition "ON" or whenever reference pulses are being received, PPSW should indicate fuel pump circuit voltage (8 to 12 volts).
Because the direct ignition system uses two plugs and wires to complete the circuit of each coil, the opposite spark should be left connected. If rpm was indicated during crank, the ignition module is receiving a crank signal, but no spark at this test indicates the ignition module is not triggering the coils.
The test light should blink, indicating the ECM is in control of the injectors. How bright the light blinks is not important. However, the test light should be a J-34730-3 or equivalent.
Use fuel pressure gage J-34730-1 or equivalent. Wrap a shop towel around the fuel pressure tap to absorb any small amount of fuel leakage that may occur when installing the gage.
This test will determine if the ignition module is not generating the reference pulse or if the wiring or ECM are at fault. By touching and removing a test light to 12 volts on CKT 430, a reference pulse should be generated. If rpm is indicated, the ECM and wiring are OK.
This test will determine if the ignition module is not triggering the problem coil or if the tested coil is at fault. This test could also be performed by using another known good coil.