P0420 code on a pair of Subaru's

[Stevo F]

Shortly after we bought our 2005 Impreza Outback Sport with 253K miles last summer it started throwing P0420 codes and after a reset, the code returns after 8- 10 miles. 

On our 2003 Forester we bought a year ago at 223k miles, it threw a P0420 earlier this year, then again about two weeks ago. I reset it and it returned in about 50 miles this time, so thius one too is getting worse.

I haven't noticed any performance issues on either car. I haven't been able to get an accurate MPG calculation on the OBS (since my son drives it and doesn't always fill the gas to full each time, but I sispect the MPG's aren't better than 21- 22 MPG.

What is the best way to diagnose the code since it can be numerous reasons other than the catalytic converter? Both are high mileaged cars and likely have the original converters. The Forester sips some oil which be depositing there as well.

We aren't required to test for emissions in our area, so I'm not in a hurry to pay big $$$ for OEM catalytic converters, especially for the Impreza, which is more of a daily beater.

Has anyone had any luck with running a catalytic converter cleaner through the system, or would that just be a waste of money?

 

 

 

[brus brother]

Hard to believe that something diluted in your gas tank and then passing through the engine burn chamber will have much cleaning oomph by the time it makes it to the cat.

If your 05 Imp is like my 05 Outback, there are three cats and 5 sensors. That's a lot of hardware to replace.

I live in CT requiring OBD-II emissions testing. I've always been curious what an old school tailpipe sniffer would actually disclose about emissions when P0420 shows up.

As you, my understanding of "catalytic efficiency Bank 1 below threshold" can result from many issues. Also, what is that actual "below" threshold and are these cars tuned so tightly for federal regulations that the slightest "below" triggers the code without actually melting the polar ice caps?

There doesn't appear to be any performance issues with my 05 Outback and I have had the code recurring for the past 100K miles starting first around 105K miles.

Car had TB and plugs changed 5K before the code first arose. I first changed Bank 1 (passenger) front (pre-cat) O2 sensor. No change. Then Bank 1 rear sensor. No change. Then the passenger front converter. This then resulted in Bank 2 below threshold code. 

Every 2 years, for inspection, I cleared the code and allowed system readiness then hurried to the local testing station to buy another 2 years of "pass".

As years have passed and the car aged, the cost effectiveness of deep diving into chasing the code has tipped to the negative.

I recently experimented with installing a resistor spliced into the post cat signal wire (I think 1 mega ohm). I no longer get the P0420 but now I get a P2098 and am not sure if it is related to my Frankenstein electrical trickery.

Google "P0420 resistor capacitor" if you just want to experiment with turning off the CEL light show and re-enable cruise control. There you'll find a YouTube video of another method using a capacitor and resistor, that I may yet try. Note that the sensor wires are stainless steel so soldering is not easy and in my case, I resorted to simply twisting the wires and covering with heat shrink tubing.

 

 

[nvu]

15 hours ago, Stevo F said:

Has anyone had any luck with running a catalytic converter cleaner through the system, or would that just be a waste of money?

Nothing cleans catalytic converters directly.  Probably injector cleaner rebadged to say catalytic system cleaner.  It does too little to work and you'd get better results cleaning the intake valves with seafoam.

[Rampage]

14 hours ago, brus brother said:

Google "P0420 resistor capacitor"

I'll have to look that up. Resistor to lower the voltage and a cap to smooth out the ripple.

Once our cars went over 270k both threw the P0420 code. Looking at the waveforms the downstream o2 sensor looked almost identical to the upstream.

Sparkplug non-foulers fixed both on them. They are on YouTube too.

I read an article that said the coating wears off the honeycomb over time and there is nothing left to work on the exhaust.

[brus brother]

12 minutes ago, Rampage said:

I'll have to look that up. Resistor to lower the voltage and a cap to smooth out the ripple.

Once our cars went over 270k both threw the P0420 code. Looking at the waveforms the downstream o2 sensor looked almost identical to the upstream.

Sparkplug non-foulers fixed both on them. They are on YouTube too.

I read an article that said the coating wears off the honeycomb over time and there is nothing left to work on the exhaust.

I tried the sparkplug non-foulers but starting in 05 they weren't effective.

[1 Lucky Texan]

the 4 primary things that will throw that code seem to be; a:f ratio off , exhaust leaks , bad rear O2 signal , bad cat. conv.

 

I know some of those are very general, but start with confirming them if possible. The temp out the back of the converter should be about 100*F more than the temp going in.

[Rampage]

2 hours ago, brus brother said:

starting in 05 they weren't effective

I didn't know about the year, but I did hear the newer ones are referred to as NOx sensors instead of o2 sensors.

[ocei77]

Check for exhaust leaks before the first O2 sensor.

What brand of O2 sensor did you buy?

Reliable brands include Bosch, NGK and Denso.

O.