I had a 1997 Dodge Ram 1500 2wd extended cab short bed with the 5.9. I was on a trip and got gas and shortly after filling up the check engine light came on (within 6 miles) and the truck is fully warmed up. I pulled over and scanned it for codes and it was a P0420 which means (Catalyst System Efficiency Below Threshold). Around a month earlier the check engine ight came on but it was a P0138 and the scan tool said the downstream O2 sensor shorted to high but the ode went away. The tech said to replace that sensor. The sensor was reading the same as the upstream. The light went off so I never replaced it but the sensor still read the same as the upstream and still does. The cat is only a year old with just a little over 10,000 miles on it. I'm going to run so tests on the cat tomorrow but was wondering if that sensor set this code. If so should I replace both the upstream and downstream O2 sensors? I had checked to see if it had back pressure a month ago and it didn't. Gas mileage has gone down.