Hello-

I just registered to this forum, but have been reading for awhile... I was just looking for some help on a code PO420 (cat below threashold...). Is there any way to figure out which cat/O2 sensor to replace without it being a "guessing-game" and seemingly (randomly) dropping money on replacing these parts one-by-one?

Thanks

 

I am a auto mechanic and have access to Alldata. If you want to give me the year and model, engine specs. I can look up the diagnostic flow chart for you and give you the probable causes. Or, you can go to the Alldata do it yourself site and register your car for like $15 and get all of that info yourself. the web-site is.

www.alldatadiy.com

It is pretty cool, you can find out just about how to do anything to your car with this site. Let me know if you check it out or if you want me to do it.[align=left] [/align]

 

I would appreciate it! It is a 2000 Honda Civic Si (w/ Magnaflow catback exhaust)-

Thanks

 

Engine Code: B16A2 Engine Code: B16A2
Engine Specs:
1.6L 16V DOHC VTEC, 160hp @ 7600 RPM[/align]

 

Cool. I will check it out when I go back to work on Monday. I have weekends off so that is why I couldnt check it today.
[align=left] [/align]

 

Dude, you only have one cat. And the sensor it's talking about is the one that is in the cat, the secondary O2 sensor. If that's the only code you're getting, the problem is your cat, and/or your O2 sensor.

 

Yup, very simple flow chart for that code. The only probable cause is the cat being bad. I would replace the cat before you replace the O2 sensor. And personally, I would just throw a high flow cat on there. They are cheaper and you will probably pass emissions witout a cat any way. In fact most states just check for codes on OBD2 vehicles instead of putting a sniffer on it. Hope this helps.

 

Here is copied text from alldata about the code. The defnition of TWC is "Three Way Cat."

The scan tool indicates Diagnostic Trouble Code (DTC) P0420: Catalyst system efficiency below threshold.

Description
This system evaluates the catalyst's capacity by means of the HO2S (Primary and Secondary) output during stable driving conditions. If deterioration has been detected during two consecutive driving cycles, the MIL comes on and DTC P0420 will be stored.

NOTE: If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot those DTCs first, then recheck for DTC P0420.

P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) Heater

Possible Cause

[*]TWC Deterioration[*]Exhaust system leakage[/ul]

 

 

Nevermind, I cant figure out how to post the flow chart. It is prey simple though, It just says to drive the vehicle and see if the code is still there. Most of the time it is the cat, which it says to inspect or replace.