[Bobby78]

Code 43 is ECS Electronic Spark Control (failure)

Code 45 O2 Sensor (Rich)

Pretty sure code 45 the rich condition is coming from from your spark problem. It means the fuel mixture in the exhaust is rich as it passes the O2 sensor.
So fix the spark problem and the O2 code 45 should clear.

 

[Bobby78]

Robert
I made a mistake above. Looking at V6 codes.
Here's the 5.0 V8...
http://www.troublecodes.net/GM/86-88_50H_c.shtml

Code 43 is "Knock Sensor" But that could throw the O2 sensor code too I think when it detects a "rich" condition

 

[Bobby78]

OK Not trusting info on the internet, I dug out my 87 GM Pontiac service manual. Code 43 states for "Hard Failure" (not intermittent) scan displays knock retard in numbers that are constantly changing (0-255) Faulty ESC circuit.

It also states "Code 43 Electronic Spark Control (ESC) retard signal for too long a time causes retard in EST signal.
So maybe the Knock Sensor triggers the ESC code 43 then the O2 code 45

 

[Bobby78]

Yeah that's what I'm thinking. The knock sensor is sending a bad signal to the ESC and that's causing a rich condition triggering the O2 sensor code.

 

[Bobby78]

78 doesn't have a computer! That's another reason I love mine!

 

[Bobby78]

Yeah that would work. Might also chip your paint. But the issue might not be that the sensor isn't working. The values for a 305 might be different than a 350 as Woody stated above.