Alexander- Posted June 11, 2017 Share Posted June 11, 2017 Hello, My car keeps getting a P0420 CEL. I have no idea what the culprit might be since both O2 sensors were changed not so long ago. What could be the probable cause? If this might be of any help. When I drive at low speeds and lower RPMs. When I barely hold the gas pedal, there seems to be some kind of surging happening (RPMs go up and down sporadically) I don't know if this is related but this is also a problem I haven't managed to fix yet. Thank you, Alexander Link to comment Share on other sites More sharing options...
MilesA Posted June 11, 2017 Share Posted June 11, 2017 It's been seen here many times that Denso oxygen sensors (the OEM brand) are the best match for the car. So, did you use compatible Denso sensors? Other than that, the service manual has a diagnostic tree you can go through for each code. P0420 is one of the more complex ones. The system is basically looking for a pattern by comparing front and rear 02 sensors. Anything that does not match that pattern throws P0420. Could be sensors, wiring, bad catalytic converter(s), exhaust leaks, etc. See the service manual and try to go through the troubleshooting procedure they give. http://legacygt.com/forums/showthread.php/vacation-pix-171435.html Link to comment Share on other sites More sharing options...
Alexander- Posted June 11, 2017 Author Share Posted June 11, 2017 It's been seen here many times that Denso oxygen sensors (the OEM brand) are the best match for the car. So, did you use compatible Denso sensors? Other than that, the service manual has a diagnostic tree you can go through for each code. P0420 is one of the more complex ones. The system is basically looking for a pattern by comparing front and rear 02 sensors. Anything that does not match that pattern throws P0420. Could be sensors, wiring, bad catalytic converter(s), exhaust leaks, etc. See the service manual and try to go through the troubleshooting procedure they give. http://legacygt.com/forums/showthread.php/vacation-pix-171435.html Yes, the replacement O2 sensors were: DENSO 2344445 and DENSO 2349123. Thanks for your input, I'll have a look in the troubleshooting procedure. Link to comment Share on other sites More sharing options...
relative4 Posted June 11, 2017 Share Posted June 11, 2017 At the risk of oversimplifying, P0420 is the code for a bad cat, and 90% of the time that's exactly what it is. Link to comment Share on other sites More sharing options...
Alexander- Posted June 11, 2017 Author Share Posted June 11, 2017 At the risk of oversimplifying, P0420 is the code for a bad cat, and 90% of the time that's exactly what it is. Is there a way to confirm before paying for the new part? Link to comment Share on other sites More sharing options...
relative4 Posted June 11, 2017 Share Posted June 11, 2017 The FSM procedure. A scanner with live data is critical, you wanna see those downstream numbers. Link to comment Share on other sites More sharing options...
MilesA Posted June 11, 2017 Share Posted June 11, 2017 Is there a way to confirm before paying for the new part?Process of elimination -- once you have ruled out everything else, then the catalytic converter is the only thing remaining. Since that is a really expensive repair, so you want to check the other possibilities first. We have seen people on the forum avoid spending hundreds of dollars replacing the cat by fixing exhaust leaks or using the most compatible sensors. Link to comment Share on other sites More sharing options...
Max Capacity Posted June 12, 2017 Share Posted June 12, 2017 I see you have a 2.5i, yea its the cat. Seems to be an issue with them. Look around the web and on here, there have been aftermarket cat's that may work well for less money. IDK as I don't really follow 2.5i's. 305,600miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. (SOLD). CHECK your oil, these cars use it. Engine Build - Click Here Link to comment Share on other sites More sharing options...
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