I have a 2000 Windstar, enging light came on, checked codes with Actron OBD II Auto Scanner, showed P0133 and P0135. Replaced O2 sensor on the upstream Bank 1 (I believe this is the one by the firewall correct?), cleared the error, drove the van for apporx 30 miles and the light came back on, rechecked the code, it shows P0135 02 sensor heater circuit fault
Any suggestions? I noticed just before it came on when at a stop the idle got a little rough then smoothed out again.

Here are some possible causes from the Ford Workshop Information Manual.

1. Short to VPWR in harness or HO2S.
2. Water in harness.
3. Open VPWR ciruit.
4. Open Ground circuit.
5. Low Battery voltage.
6. Corrosion or poor mating terminals and wiring.
7. Damaged HO2S heater.
8. Damaged PCM.