1990 civic dx 1.5l, 4spd. Had #1 & #16 ERROR CODES! I replaced the #1 oxygen sensor and ran a can of BG Engine cleaner in a full tank of gas... car started and ran for a few days but now it won't start when it's hot outside. The starting issue has been happening for over 1 year, 1st replaced distributor and car started OK for 6 months then returned to it's old habits. Any idea of the "real problem"?

Try replacing the PGMFI Main Relay. Cold solder joints may be separating, when it's hot outside. This could easily explain the Code 16.