Gas and Oil Home Heating Furnaces - Not the condesate drain

I have the American Standard Freedom 90 single stage gas furnace. I have had it for a few years now. The guy who installed it was a real piece of work so I have had issues with it since. One issue that I had right away(because he installed the furnace upsidedown) was the condensate drain was plugging up. Therefore I am familiar with the symptoms of that problem. Basically the ingnitor gets hot the gas comes out and lights and then quickly goes out until the furnace goes into lockdown.

Well the furnace is doing that now. I have cleared all drain lines. I got the furnace to work for about 24 Hours monday into Tuesday. Tuesday night it locked out again. I went and monkeyed with the drains again this time with no luck. I let the furnace sit over night off. This morning I went and turned it back on and it was working. My wife called me around noon to tell me it wa locked out again.

Any ideas? I have seen a lot of talk about flame sensors, so tonight I will attempt to clean that, but short f that working anybody got a guess.

One issue that I had right away(because he installed the furnace upsidedown) was the condensate drain was plugging up. Therefore I am familiar with the symptoms of that problem. Basically the ingnitor gets hot the gas comes out and lights and then quickly goes out until the furnace goes into lockdown. Was this replaced?? (Can't understand how in the heck he put this in upside down!?)

What is the code that is flashing on the board? When it goes into lock out the Red LED will be flashing X amount of time.

The code for the flash is on the back side of the blower door.

First about the guy that put it in. It worked for about, well long enough for me to write the final check and for him to leave. Literally! It took me about two minutes to diagnose that it was up-side down. I called him, he hadn't even made it to the bar yet, and he came back and I took charge and helped him install it properly. I had left him alone because I was paying him. He is the only contractor I have ever hired, and I have remodeled the kitchen, removed and rebuilt some walls, re-wired the whole house and plumbed a new bathroom suite to start the list that never ends.

On to the furnace. It was flashing twice which just means it is locked out. I took the advice I learned from reading other threads and took out the flame sensor. My wife has this mini dremel with emory wheels for her nails. I shined the sensor up and re-installed. I let the furnace get into it's cycle three times (I turned it off after two sucessful start ups) and so far so good.

Thanks for the help (on the other threads)