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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.
 

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P0420 1/1 Catalytic Converter Efficiency

P0138 O2 Sensor 1/2 Circuit Voltage High

should I replace both the upstream and downstream O2 sensors?
I'm trying to figure out why you didn't do this when you replaced the cat? Use NTK 02's
 

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I had a shop do it and they didn't recommend that I replace them and I just didn't think to replace them. Are the NTK 02's better then OEM? I was told to put OEM O2 sensors on. This is of topic but when I was testing the cat I decided to check my cap and rotor and they already have carbon build up on them just after 1500 miles and is causing a shudder. Any ideas?
 

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Well judging by all the posts here on the subject of 02's most choose the NTK's(including me). Bosch is one of the worst, They last about a year or so then crap out. Not sure who makes the oem ones. You never said how many miles you have on the truck. ???
 

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I have a 137,000 miles on the truck so far. I'm going to try replacing them because I added propane gas near intake and I saw no change from the upstream O2 sensor and no change when I took the propane away.
 
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