I have a 1999 Accord EX, 2.3L, 5spd manual. My check engine light has recently come on and the car is idling very low (200 - 400 RPM). By jumping out pins 8 and 13 on the DLC, I found that the code set is for "System Adaptive fuel too lean" or "System Adaptive fuel too rich". It does not specify which.
I have checked the primary O2 sensor and it seems to be generating the required 0.1 - 0.9 volts as well as 17 ohms on the heater.

Can anyone tell me what might be causing this problem?

Thanks in advance for any help!

 

Try some kind of fuel addative to clean the system. Dirty injectors/lines/etc. could cause the drop