2000 Mazda 626 P0420

I have a 2000 Mazda 626 V6. For the last couple of years the check engine light comes on with a p0420. It might come on for a couple of days and then goes off. I wonder if this could be related to the ethanol in gas since it collects water. One of the possible cause is that water could be in the o2 sinsors. I only drive this car about 50 miles a week so a tank of gas last a long time. Jf