The MGA1200 also has the 3 potentiometers on the Andros analogue board for setting.
There's a test point on the board you can monitor to aid setting those pots (interesting waveform BTW, it makes the operation of the bench really clear) but there are more than three pots, the bench is calibrated for span and zero so with a 'scope attached you set the zero gas levels to be equal, then feed it cal gas (find a friendly SPX/Bear user if they're not extinct and borrow their tank (1 year cal interval and only £150 or free with service contract but my UKAS lab is now in the hands of someone else) or beg some from a cal engineer's UKAS bottle) and adjust the other pots for correct values on the display/computer screen.
There's also reference voltage, 'black' level, speed of the 'wheel', servo for the IR source (you can fry the source) and a bunch of other 'stuff'. You can get yourself in a proper mess if you 'twiddle' randomly.
I preferred the later Andros 6100 benches as used in the later machines because the early bench electronics didn't have the analogue resolution for accurate work on catalytic converter equipped vehicles, the later Andros 6000 benches with the larger piggyback board were really good.
Have to agree on the real engineering quality there 
The first GA single Co meter i used was a Sun analogue 'needle' 
type in 1984/5 and that was about $6000 
at the time in the U.K. 30 years on......and a far better one is thrown in the skip 
Criminal throw away tech world 
But Hey, good for us that can appreciate what we have gained
Advise for the O2 chemical cell......it has a limited life if left in the gas it's detecting, i have read that if it is not used much.....to place it in a small sealed container, better still to vacuum it or fill will OFN, to extend its useful lifespan. Hope this may help you?
Cheers
.
Meh to engineering quality, they were built heavy but were shit, unreliable and that six month cal was as much for drift as well as regulatory requirements.
O2 sensor, nope, don't seal it in a bag, starving it of oxygen can ruin it as it won't 'restart', they're an electrochemical fuel cell, you're best just to purge the analyser with fresh air through an activated charcol filter.
The cells are usually supplied in a packet with an air port (unless someone has repacked it to sell it on form a bulk pack) and should have a life of over 2 years in atmosphere but it's shortened by exposure to hydrocarbons and various organic compounds.
City technologies have a bunch of useful information on their site about care and feeding of O2 and NOx cells.
There is an unlock on most if not all MOT analysers, but if it's a standard Andros 6x00 bench then there's a lot of software out there that doesn't have a cal interval enforced (obviously it can't be used for certification)
*edit* I am reliably informed the calibration bypass is a firmware change on the MGA1200, not an unlock as such, my source is probably correct but YMMV, I've asked how much it costs for the EPROMs.
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