Gatecrasher

1996 Ford Thunderbird, V8 4.6L
104,000 miles

My car didn't pass the Smog Check because the Check Engine Light (CEL) was on, showing OBD codes PO 172 and PO 175 meanng "System Too Rich (Bank 1) and (Bank 2)". Car runs perfect, and passed the emissions test.. but the damn light failed me!!!!!

Around 8 months ago, I went to the mechanic after my Check Engine Light had turned on, and he did a diagnostic, turned off the CEL, and said "nothing's wrong, don't worry about it". I know that the CEL can turn on for something as simple as a gast tank cap that is not screwed in tight enough.

The CEL can manually be turned off if you disconnect the car battery for minutes. And I guess that if it is a "real" problem, it will turn back on. (That seems to be the recommended advice to double check if the problem is real, or if it is just the CEL being annoying).

MY QUESTIONS:

So now I have to fix the "problem" and then go to get retested. Should I manually turn off the CEL to see if it comes back on? Does the fact that I turned it off light that considered "tampering" with my SMOG check? Would the SMOG test person see that? After how much driving does the CEL come back on, if the problem is really there?

Or, if "system too rich" is in indeed an ongoing problem in my car... how much will it cost me to fix? (estimate?) and/or is there anything I can double check myself?

Thanks