SES light for MAf sensor

Any help or ideas is greatly appreciated...

2001 Venture WB
gasket leak dumped antifreeze into the oil and appeared to wipe out (possibly a cam) bearing. since everything else was perfect with the car, a had the mechanic put in a used 50k mile block.

The new/used engine 'appears' to run great, but has an SES light indicating low or no electric/voltage (I believe "to") the Mass Air Flow sensor.

I do not see how anything that could have gone wrong with the engine that could account for a PCM / MAF or what seems to be an electrical issue (other then just possibly knocking the harness around when they did the install.

The PCM, harness, and intake (plenum?) and MAF are my orignal ones from the original engine so I believe they were good before the swap. Now the mechanic basically says he's done but can't get rid of (or figure out what's causing the code.

Now my (limited) understanding of a MAF is that it would noticeably run like crap if the MAF was bad, but I also thought that if it were 'disconnected' (or maybe just not reading at all) that the PCM would run in something like a 'default mode' or something similar, and the engine would probably run 'ok' (maybe rich or real crappy gas mileage etc.). Is this CORRECT??

I am afraid to take the car back with this SES light/MAF code on, and fear doing any possible damage. He's the mechanic and he says he cannot figure out what it is.....does anyone here have a clue??? If the engine runs seemingly fine otherwise, am I risking damage driving it this way??

Additional info: I think the new block was a 2000, I also believe it was CA emmissions if that makes a difference (my original 2001 was also)

Anyone know possible causes for that (MAF) SES light on what seems to be an engine that is running ok?? (sorry I don't have the exact number but I could probably get it)

bump and update....I have been reading up on all the codes for the MAF and it sounds like the van has a good chance of sounding/running 'ok' even with a 'bad' censor....

all the codes basically recommend check all wiring /connections,check voltage according to manual, then clean ro replace the sensor. One code said a heavy load on the same ground circut might trip the code.....

so I am guessing it may have gotten dirty (or even broke) during the install or the wirings not tight or grounded properly.

my question is would you drive the car or does anyone think (as it appears to run ok) it will do damage to drive with a MAF code on?