ok, first off this isn't a "turn on the car and code sets" kind of code. it requires a specific amount of time to set and only if the conditions are right.

OBD1 systems will only turn the "check engine" light on when there is a current problem .... but will always store a code in the pcm. just because the light didn't come back on when you started the car, the problem isn't necessarily gone.

now the issue.
let's say that you had a completely stock single cam setup --- no mods at all and factory pcm. the system will operate as efficently as possible and in a manner the will make the catalytic converter work its best for cleanest emmisions and longest converter life. now let's say that you changed only the fuel injectors to a size 10% larger flow than stock, with no other changes ... still same pcm and everything else. the programming of the pcm still thinks it has the smaller, original injectors and tries to operate like that ... but the injectors are now spraying in more fuel than the whole system really needs to be most efficient. the engine is only taking in a certain amount of air so only part of the fuel can now be burned and the rest is going out the exhaust on its way to the converter. the converter's job is to burn off any extra unburned fuel so that the emmisions are clean --- but this is too much unburned fuel. if allowed to continue trying to burn off more than it can handle, the converter will overheat and become damaged. so to protect the converter, the pcm monitors the exhaust by using the oxygen sensor.

ok, the oxygen sensor. while the latest cars use a "wideband" type sensor to precisely determine how clean the air/fuel mixture is burning, the older OBD1 systems use a relatively simpler type sensor. the older oxygen sensors can't tell how overly rich or lean the exhaust stream is. the older type can only really tell above or below a point called stoichiometric. the "stoich" point is the air/fuel mixture where the most efficent, cleanest burn takes place.
( notice I didn't say the most horsepower. that is a different point. )
the oxygen sensor reports back to the pcm when driving along or idling. but when under accelleration, the pcm richens up the mixture to make more power available. so the pcm ignores the oxygen sensor reports when accellerating but listens to the sensor at all other times. when the pcm is listening, that is called "closed loop" ... when the pcm is ignoring the oxygen sensor in the interest of more power, the is called "open loop".
[tuners who modify the pcm programming sometimes will set the pcm to be in "open loop" all of the time, ignoring the oxygen sensor. while it allows the ability to make more power, lower fuel economy is usually also a result. that is one of the reasons that leaving the programing in "open loop" all of the time is not the best idea for a street driven car.]


ok, now back to the example. the larger injectors are spraying in more fuel than the engine can burn and that unburned fuel is on its way tothe converter. as that exhause goes by the oxygen sensor, that sensor sees there is no oxygen left in the exhaust ... all of the oxygen was used to try to burn all of the fuel. the sensor reports that information back to the pcm. the pcm thinks that this isn't quite right and shorten the pulse on the injectors a small bit and waits for the oxygen sensor to report if that is enough of an adjustment to "clean up" that overrich condition. this is call a "fuel trim" adjustment. the pcm is trimming the time that the injector pulse is open to adjust the amount of fuel going into the engine. remember that the pcm is interested in fuel economy and clean emmisions during "closed loop". so the pcm "trims" the fuel back and waits for the next report from the oxygen sensor. but because the injectors are 10% bigger, that little adjustment wasn't enough. so the pcm "trims" some more back until the oxygen sensor says the exhaust is back to its correct mixture. now this trimming happens continuously and requires alot of monitoring to keep up on the quick adjustments. these quick adjustments are made on a "short term" basis so they are called the "short term fuel trim".
now the pcm can see that because those injectors are spraying more fuel in than origianlly expected. and it keeps having to trim back the fuel to keep it clean. so after a few minutes of this triming back, the pcm figures out that the injector are going to keep putting in too much fuel so it adjusts its "longterm" type thinking. so it adjusts the program a little so that the "short term fuel trim" doesn't have to make as must of an adjustment. just a % at a time until the short term is only making little adjustments. the short term is designed to make little quick adjustments while the long term makes the bigger, slower adjustments. and since the injector size isn't changing, that is a "long term fuel trim" adjustment.

now the pcm can compensate for a certain amount of change from its original programming. this is there so that the pcm can keep it clean as injector lose some flow due to age or clogging ... or if the fuel pump loses some of its pressure. but what there is a limit as to how far it can compensate for. that long term adjustment is limited so that is something is causing a big change from the original settings. so those 10% larger injectors it may be able to adjust for.... but 20% may be too much and push that long term adjustment too far and making the pcm realize that something isn't right. it gives up on trying ... and then the "check engine" light is tripped. on OBD1 that dtc is 43.

now, dtc 43 is saying that the long term fuel trim is too far for the pcm to compensate. but why is it too far? this is where it gets a little bit of a pain to diagnose

if there is too much fuel long term ... either too big of injectors, too much fuel pressure, or even the programming in the pcm (remember that has been "tuned" by someone else now)

or on ther other side, not enough fuel long term can set the exact same dtc ....caused by injectors too small, not enough pressure, or that programming again.

and a third possibilty ... the oxygen sensor (or an issue in the wiring) could be giving bad information to the pcm ... lying about the exhaust the whole time when everything could be just fine.



and there is the issue ...how does one determine exactly where the problem is located?
if you had the ability to monitor the oxygen sensor reading, then you could determine if the pcm is being told the the exhaust shows rich or lean. monitoring the sensor while making adjustments to fuel pressure would give you feedback as to if the correct adjustments are being made and if the oxygen sensor is responding in the manner it should. a proper scan tool datalist would show immediately what the long term fuel trim is.

but without the ability to see what the pcm sees, then you can only make guesses.
adjsuting the fuel pressure and then resetting the pcm by disconnecting the battery or pcm and driving "long term" to see if the code returns. if going down in pressure doesn't help, then maybe it needs to go up. and the possibilty the the sensor is lying to the pcm remains also.