Vehicle: '93 Ford E350 RV cutaway, 152K miles, 7.5 l engine, automatic trans. Some of you are familiar with my getting this thing ready for a trip to Alaska. Fixed broken exhaust manif bolts, new fuel pump, water pump, etc. Anyway just came back from a 9300 mile, seven week trip and NO PROBLEMS, engine ran just fine,climbed some steep but not high elevation grades. The steepest and hotest were "the Grapevine" and the Siskiyou Summit on I5. I do have a question on a Check Engine Light occurence. It happened several times on this trip. It also had happenned other times over the last 15 years. It ONLY happens when I start up in the morning,not particularly cold, and drive very slowly( 5-10 mph) for several minutes, maybe 5, mainly getting out of the campground, with hardly any throtle opening (foot essentially off the gas pedal). Sometimes the CEL would come on and then go out after resuming normal driving mode. Sometimes go out right away, sometimes a little later. No driveability issues. The code that I pulled was always 172 (OBDI) "HEGO sensor indicates lean condition,cylinder bank no.1". NEVER happens on normal startup and driveaway. NEVER happens when even on prolonged idle after startup, as long as the driveaway is normal, giving enough throtle to achieve normal steet speeds. I didn't think much about this over the years since there were no driveability issues. I'm just curious, what makes this happen?