I had that problem with my 86. I replaced the o2 sensor twice and yet that code always came back. Next I swapped in a spare ECU and the code still came back. Since o2 sensor is so close to the ECU, I installed a new wire to the sensor, then cut the old wire off close to the ECU and butt connected the new wire to the ECU harness. Job was easy and only took about 10 minutes. Code never returned. Fuel mileage and power increased a bit as well. That was about 4 years/60k miles ago and van is still running good with no o2 code. Tim