With about 40k miles on my 97SE 5spd, all of a sudden I've been getting 33, and 53 codes. (Front heated O2 sensors, bank 1 and bank 2). Sometimes both codes, sometimes one. I've measured the heater resistance on both O2 sensors and it is ok, they are receiving 12v from the ECU. I've instrumented the ECU and both sensors output vary between 0v and 1v (within spec)while driving. They do go to zero after extended idling.

Also have run the factory diagnostic Test Mode II, and the MIL light changes at least 5 times in 10 seconds.

It is unlikely that either sensor is bad (both failing at the same time very unlikely). The fact that both 33 and 53 show up tells me it is something common or upstream from the O2 sensors or the ECU itself.

I've also checked the output of the MAF sensor and it looks good.

The engine runs great.

I'm interested in further diagnostic ideas. Can bad gas cause this? I'm still on the same tank.

Thanks

Dan