Hi Ray,
The fault codes you're getting are 13 & 44, but it should flash code 12 as well :confused
Code 13 is "Oxygen Sensor Circuit"
Code 44 is "Oxygen Sensor Lean"
so it looks like you're narrowing it down
The code that was there before, but not now (23), is "Mixture Control Solenoid Circuit", so either disconnecting/reconnecting the computer plugs has cleared it (could have been a bad connection?) or it's just gone! Have you touched the connector on the carb for the M/C solenoid?
Back to the O2 sensor: How old is the sensor? It could just simply be that the sensor needs replacing. Is the sensor in the stock location just down from the manifold? If the sensor is fairly new then I'd very carefully check all connections from the sensor back to the ECM. Pay particular attention to the connector at the sensor itself as it can get covered in a lot of oil/crud. The voltages in this circuit are low (up to 1V?), so something as simple as a dirty/oily connection can wreak havoc.
I can't remember if low voltage from the sensor is a lean or a rich condition. Hopefully somebody will jump in & say (Hib?), but if low voltage = lean then bad connections could easily be the problem.
This is a long shot & a complete guess, & I could get shot down in flames for it!
I've fitted Headers on mine which meant moving the O2 sensor downstream. Doing research on Headers & O2 sensors I found out that if the sensor is further downstream from the exhaust ports than it was designed to be, then it can very easily cool to below its operating temperature at idle, giving bad readings. People have had this problem & clear it by raising the revs for a while to heat it back up. Could your sensor be doing the same due to carbon build up insulating it? Just a wild guess
I'll go through my manual & see if it gives any easy methods of finding the fault.
Good luck
Paul