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#1 2016-11-04 07:12:37

mawkie
Member
Registered: 2012-09-27
User Number: 7709
Posts: 4

Troubleshooting Catalytic Converter in 05 Linear

Last month the CEL came on and it was the dreaded P0420 code on my 05 Linear with 118K miles. Found no exhaust leaks, engine temp normal, no vacuum leaks (cleaned the throttle body while checking all the vacuum lines, very overdue).
Decided to replace both up and downstream O2 sensors figuring if the CAT is bad I'd be replacing them anyway. Bought aftermarket sensors and cleared the code but in less than 1k miles it's back. Also this time I noticed a sooty exhaust tip. Mileage seems to be normal or slightly low (hard to tell, lots of stop-n-go traffic here in Baltimore recently), performance normal .
So I bought and put a code reader in data log mode and took her for a drive to get up to at lease 2K RPM. Found that the downstream 02 sensor voltage is ranging from 250mV to 750mV. Upstream A/F sensor voltage swings from around 2.5V to 3.5V (hard to log with such low bandwidth on this code reader). Pretty sure this points to a non-functioning CAT as I would expect the downstream O2 sensor voltage to be steady at all RPMs if normal (correct?).
I do have access to an IR temperature gun at work so I'm going to take a measurement of the CAT input and output flange temps. Am I correct that the rear flange temp should be higher if functioning correctly and due for replacement if otherwise?

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#2 2016-11-04 11:12:17

mawkie
Member
Registered: 2012-09-27
User Number: 7709
Posts: 4

Re: Troubleshooting Catalytic Converter in 05 Linear

Got to work after a 30 minute drive and measured the input and output temperatures of the front CAT and they were within 5 deg F of each other (260 input vs 256 output at idle). Believe that confirms that the original CAT is done. Now I just need to decide if I want to take a chance on the new no-name A/F sensor or man up and purchase a Denso or other name brand sensor to install with new CAT. Just want the new CAT to last and not go up within a year or two.
Good news is that cleaning the throttle body now allows me to reset codes and it runs properly on default factory settings (yay, small victory but I'll take it), bypasses that annoying remapping.

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#3 2016-11-08 15:59:32

elislider
accept no substitutes
Registered: 2014-11-30
User Number: 25881
Posts: 356

Re: Troubleshooting Catalytic Converter in 05 Linear

be sure you are getting the actual correct aftermarket O2 sensors and by a reputable brand: Bosch, NTK, or Denso.
there are ways to possibly clean the catalytic converter, if its simply covered in soot or something.
but usually the simplest and easiest method is just to find a used one and swap it out.
try searching craigslist or local facebook forums and subaru facebook groups for pretty much any Impreza non-turbo cat section

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#4 2016-11-22 13:54:01

mawkie
Member
Registered: 2012-09-27
User Number: 7709
Posts: 4

Re: Troubleshooting Catalytic Converter in 05 Linear

Ended up getting an OEM Denso A/F sensor and all's well. Fuel mileage is bang-on, back to normal 28 mpg in mixed driving and the exhaust is clean. A/F and downstream O2 sensor voltages look right. When I get the chance I want to repeat the CAT temp test but at an elevated rpm, not at idle. I suspect the temps I saw at idle were too low for the catalyst to be engaged and working. Also need to verify that the short and long term fuel trims are OK but I'm confident that they are or the mileage wouldn't be so good.

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#5 2017-01-15 15:03:38

carsonbradley7
Member
Registered: 2017-01-15
User Number: 39481
Posts: 1

Re: Troubleshooting Catalytic Converter in 05 Linear

I believe I am in the same boat. My CEL light is on and my cruise control light is flashing and is not functioning. I had Autozone pull the code and said that there is a misfire on 4 and 1. I went with the downstream O2 sensor by Bosch. the part doesn't come in for at least two more days. is this a good starting point? I'm clueless when it comes to automobiles. Soi apologize for the ignorance. Thanks

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#6 2017-01-15 20:34:32

elislider
accept no substitutes
Registered: 2014-11-30
User Number: 25881
Posts: 356

Re: Troubleshooting Catalytic Converter in 05 Linear

carsonbradley7 wrote:

I believe I am in the same boat. My CEL light is on and my cruise control light is flashing and is not functioning. I had Autozone pull the code and said that there is a misfire on 4 and 1. I went with the downstream O2 sensor by Bosch. the part doesn't come in for at least two more days. is this a good starting point? I'm clueless when it comes to automobiles. Soi apologize for the ignorance. Thanks

a rear/downstream O2 sensor has literally nothing to with how the car operates, it is only used to emissions purposes to measure if the catalytic convertors are doing their job or not.
You could unplug it and it wouldn't do anything other than give you a check engine light and prevent you from passing a state emissions check

the front/upstream O2 sensor is VERY important, and gives feedback to the engine on how to run properly.
other things to check/replace when trying to fix a misfire issue:
- spark plugs
- spark plug wires (if applicable)
- knock sensor, not repairable, just replace it after 100k or so
- MAF sensor (if applicable), usually just cleaning it helps
- MAP sensor (just clean/check it)

usually unrelated but maybe could cause issues:
- fuel filter
- air filter
- throttle body

Also  look for any loose/cracked/petrified hoses in the engine bay, which could indicate air leaks

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