I had this problem for a long time on my '86 GL. I spent countless hours 
tracking it down.

In the harness just above the injectors there is a green wire that carries 
the output of the O2 sensor to the computer. This wire is actually a coaxial 
cable with an insulated center conductor that is wrapped in a braided copper 
shield and then insulated again. It is essentially a wire inside a wire, and 
it is visibly larger than the other wires.

There is a spot where this cable connects to a 'normal' black wire before it 
reaches the O2 sensor. It is a factory splice with rubber boots over the 
wire on both sides. If you pull back the boot on the green coax cable, you 
almost certainly find that the braid has worn thru the inner layer of 
insulation, thus effectively shorting the O2 output to ground. Strip back 
the outer insulation and cut the excess braided shield so it no longer 
touches the inner conductor, and your van will once again know what the O2 
sensor is doing.

FYI there is another indentical cable that sends the output of the Hall 
sensor to the idle stablizer (pulses for each firing of a spark plug). This 
one is spliced at the firewall above the transmission. I found the same 
condition on this cable too.