I've just been searching the forum for info and it seems the MK1 was not OBD compliant. The oxygen sensor output could still be graphed but not, I think, using
MES? You'd need something like an automotive oscilloscope.
Maybe someone who knows more about the Mk1 could join in with info here?
Just for your info these sensors behave like a wee battery. They produce voltage between around 0.1 volts and 0.9 volts depending on how much oxygen is present in the exhaust gasses going through the exhaust manifold. Rich mixture (low oxygen content) gives a higher voltage reading - tending towards the 0.9v) lean mixture (high, unburnt, oxygen content gives a low voltage reading - tending towards 0.1v) The ECU is trying to maintain a 14.7 : 1 air/fuel (by weight) mixture - at which all the hydrocarbons in the fuel are consumed by the oxygen in the air so you get a very "clean" exhaust gas with minimal pollutants. The voltage being generated by the sensor is constantly trying to trim the fuel delivered to suit the air entering the engine so you'll see it going from rich to weak to rich to weak roughly once or twice a second (at idle - does it a bit faster at higher engine speeds) you can see this displayed in graphic form via
MES on your computer screen. It's very useful to be able to see this but I've only ever done this on cars with 16 pin sockets so I'm a bit lost if you can't connect.