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P0420 Persistant little B!tch

5K views 3 replies 4 participants last post by  jdmeaux2002 
#1 ·
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
 
#3 ·
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.
 
#4 ·
Simply resetting the computer codes doesn't necessarily clear these odd codes completely. Once you have completed your repair and cleared the codes, try it out and see if the problem comes back. I found that you sometimes have to completely have to reset the computer. I have done all my own repairs after the first year of ownership.

Disconnect your battery ground/negative cable from the battery. Protect/cover the negative post to stop it from making contact with the cable. Now connect a jumper wire between the positive and negative cables. You can keep the positive connected to the battery. Let it set for 5 to 10 minutes undisturbed. This will cause the computer to get rid of all those electrons that seem to hide everywhere in it. Disconnect jumper and reconnect negative cable. Now the computer has to relearn your driving patterns. This usually corrects the problem.

I used to have a persistent P0171 and P0420 OBD-II set of codes pop up after about 81,000 miles. A friend suggested replacing the injectors. I purchased rebuilt and CERTIFIED injectors, and send my old ones back for a rebuilt. The old ones got flushed and retested when they rebuilt them. They were with 1% of flow on each other, making basically a new set within OEM specs. Now I have spares with a warranty.

The P0420 was not so simple. I checked for continuity and ground & power on each O2 sensor, and found the secondary sensor (the one behind the cat) was not heating properly. So I purchased an OEM O2 sensor to replace it. While installing the new one, even after tightening it down, the sensor felt "loose" in the bung. It felt like it would wobble every so slightly. Off came the exhaust and I found a bung with cracked welds. Once I repaired that, the sensor sat in there tight. Everything was put back together, I cleared the codes, and then drained off those crazy electrons. Once completed, the engine started every easily, had no miss, and ran like a top all the way to 6700 rpm. Never have replaced the cat and she purrs and wants to run.
 
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