Chasing a problem on my '93 240 wagon. The car has 77K, is very clean, and the previous owner was a little old lady who kept it meticuliously maintained at the local dealer. Service records show timing belt O2 sensor, fuel filter, etc.
The error codes are consistently 221 and 231 in that order after clearing codes/driving for at least 20 minutes. Am experiencing a slight hesistation and pinging (lean) under light load. Starts easily and has a smooth idle. So far I have checked the following:

*Tested ground, resistance and voltage at the MAF.

*Cleaned and silicone grease at MAF connector.

*Tested all fuel pressures, fuel regulator, etc.

*Checked for vacuum leaks including evap control, etc.

*Disconnected hose from air box to rule out stuck thermal valve in air box ( although thermal valve might possibly be bad hesitation still exists and codes still latch with hose disconnected).

*Checked air filter.

Tomorrow I will double check for grounds and signals at the ECU and clean/check the signal grounds at the engine.

I'm running out of ideas. Anyone have any similar experiences or ideas?