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06 Elantra 5 spd
New upstream o2 sensor

keep getting p0420 after about 20 miles after resetting computer
also getting a slight miss under load usually under 2k rpms

this car has taken me all over the states even with the miss its never let me down with that being said do we think its the Cat or is there something im missing?


TIA
 

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Likelihood of a bad cat is low...they generally last forever. P0420 can be a pain to diagnose but it's usually combustion related. Low cat efficiency just means that your downstream O2 sensor output is not as smooth as expected.

My experience has been that this is due to unburnt fuel getting sent down the exhaust. You might see "pending" codes for a misfire if your scan tool supports that (these won't make your CEL flash as they are intermittent). But basically anything that results in poor combustion can lead to this code...and unfortunately that is a long list of possible causes. I'll highlight a few easy first steps to get the ball rolling:

Check for dirty oil.
Check for dirty coolant.
Clean/replace your air filter.
Clean MAF sensor.
Check for vacuum leaks around intake manifold, throttle body, and valve cover.
Clean throttle body.
Clean idle air control.
Check/replace spark plugs (and/or plug wires).
Test/replace coil pack.

I've also seen this code with a bad connection at the ignition coil...and it would only present when the engine was hot. Go for a drive to get the engine bay warmed up nicely. Then park and wiggle the plug like mad and see if you can make the engine skip a beat.

If you have the gauges, it's never a bad idea to check fuel pressure and cylinder compression.
 
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