Toyota Repair: previa check engine light, pb blaster, fuse panel

Question
I have a 91 Previa. The check engine light is on and i retrieved codes 12, 21, 25 and 26. I have tools and am pretty handy with cars, but this has me stumped. I did check the distributor and rotor, both needed and have been replaced. The van runs smoothly now when I first start it, but gradually runs rougher. I was going to try removing and cleaning the O2 sensors, both are rusted and soaking with pb blaster. Any guidance or input is appreciated. Also, I'd like to know how to reset the engine light so I can tell if I am heading in the right direction, and eliminating any of these codes as I tackle this. Thanks in advance,
John

Answer
The two codes that are of concern are the 25/26, both are O2 sensor codes as you probably know. You can clear the codes by removing the EFI fuse in the fuse panel on top of the dash under the cover above the radio. The rough running after it warms up could be a leaking headgasket, this will trigger the O2 sensor codes. I would do a chemical block test, you casn get the kit at most autoparts stores at a reasonable cost, it's a bottle of blue test fluid and a clear tube that is filled with the fluid and attached to the radiator opening, if the fluid turns green or yellow it indicates a leaking head gasket. By the way, have the spark plugs and wires been checked, these engines also had a tendency to leak oil through the spark plug seals into the spark plug tubes causing the plugs to short out. Keep in mind that all engine misfires will affect the O2 sensor operation and turn the check engine light on, the O2 sensor may just be reacting to the rich/lean condition caused by the misfire and if it can't keep up and readings go outside the sensors parameters then the light comes on.