2002 Outback P0420 code AFTER new CAT converter

[thedoctor]

Mom's 2002 Outback keeps throwing the code p0420.   I'm told this means "catalytic converter". 

 

History (within the last six months): 2002 Outback, odometer 165,000, purchased from used Subaru sales/repair business.

 

- Engine bearing went out. Highway breakdown within 1000 miles of purchase.

 

- Short block replaced.

 

- Check engine p0420. Shop replaced cat saying maybe the engine failure ruined the cat.

 

- Check engine p0420. Shop says 2002 Outbacks have this problem, not 2001, not 2003. Shop says we could 1) try various fuel additives, 2) replace cat with genuine Subaru part for around $1000, or 3) replace cat with a high performance cat Magnaflow for $500.

 

- Installed Magnaflow

 

- Check engine p0420. Shop says again its just this year for some reason. Options: 1) replace magnaflow with Subaru cat, and receive $500 credit, 2) swap 2002 computer out with a 2001 Outback, since its really the computer that is having the problem, or 3) keep trying different additives.

 

When I mentioned o=o2 sensor, the shop said there's another code for that, so that isn't the problem.

 

So what to do? Is this 2002 Outback cat problem a well known problem? Because I'm not finding a lot of talk about it. How exactly does information about the catalytic converter flow to the computer? How are o2 sensors related?  How to get this check engine light to go away so my poor mother can drive in peace?

[zombieforce]

I would try a rear o2 sensor. If its reading slow it will see cat as inefficient and flag a 420 code. They need to be watchi g the data streamon a scanner and make sure the rear sensor is working. Its only job is to monitor the cat.

[john in KY]

Had the same code in my 95 wagon last month. Replaced both O2 sensors and no more code. 

[idosubaru]

this is not a "2002" problem. 

 

best fix is to ignore it.  if you have emissions - clear it and see how long it goes before it comes back - you may be able to just clear, pass emissions, and do that ever time.

 

you can install a $5 spacer between the rear O2 sensor and the exhaust, that will often alleviate the code.  lots of info on web about it, they're readily available on amazon and ebay.

 

P0420 codes are very tricky to diagnose - which is why your mechanic is guessing and throwing the wrong parts at it.  nearly any component related to the engine can trip this code.  mechanics and dealers don't diagnose this code.  replacing the converter is not the proper fix, it may "cover it up" like a band aid for awhile, but that's not the actual fix...obviously in your case. converters last the life of the vehicle, they are amazingly robust on Subarus - regardless of all the backseat chatter like "maybe your converter is clogged" - wrong. that's a false notion widely perpetuated. installing a new Subaru converter is probably a bad idea.  the converters were probably never bad to begin with, but even if they were - whatever caused the others to fail, will eventually cause the Subaru one to fail again too.  the cause needs addressed.

 

 

P0420 is one of many things engine related (but not the converter itself). 

 

it would be good to know if it's running lean or rich, but there's no easy way to check that except - what kind of gas mileage are you getting

 

here are a few items that are good to have in good repair/condition anyway, so i generally go through these:

 

1.  exhaust leaks need fixed, particularly any in front of the converter

2.  seafoam the intake and fuel

3.  tune up - stock OEM NGK plugs, Subaru wires (or high quality wires), air filter, PCV valve

4.  adjust the valves - note any that are particularly tight

5.  replace O2 sensors - they could be lazy - but this is just a guess and shot in the dark - i figure i'm replacing them once during the 100,000+ miles i own the car anyway so i just replace them as preventative maintenance and to prevent lazy sensors/reduced gas mileage.

6.  some folks have commented that heat shields being missing from the exhaust can cause the code, since that interferes with temperature characteristics of the exhaust.

 

outside of that you need to connect to the OBDII data port and find out what is tripping this code - some parameter is out of spec.

[thedoctor]

OK. That's a good list to work on.

 

I think I need to purchase a device to connect to OBDII data port... one that will allow me to clear this code without disconnecting battery. Also one that will allow me to read o2 sensor data. 

 

Any recommendations on what device would serve the budget minded amature mechanic?

[1 Lucky Texan]

yeah, most of the simple definitions for the various DTCs that mention a car part or system, don't really mean "replace that part" - they just point to some sensor reading that is out of allowable range.

 

P0420 can have many sources and it rarely is a bad cat conv.

 

exhaust leaks,vacuum leaks, even bad wiring from rodent infestations, etc.

 

always best to start with tune-up and inspection type stuff.

 

I have an older Innova 3100 handheld scanner, but if you have a smartphone, or a laptop, there are devices/cables and programs that are very affordable - free.

Edited June 5, 2014 by 1 Lucky Texan

[zombieforce]

U scan will work with a smart phone think its about 69 dollars its decent. I used one a friend bought and other than it being slow it will clear your codes just fine. If theres not emissions testing I

where you live dont worry about the code. Tons of cars around here in east Tennessee with check engine light on for that very code.

[idosubaru]

buy the cable and you can get free software, etc.

FreeSSM

http://www.subaruoutback.org/forums/65-parts-accessories-performance/39426-freessm-complete-access-your-ecm-tcu.html

 

Diagnosing the data is tricky, would be nice to know how quickly your O2 sensor is responding and if it's running lean or rich and which cylinders.

 

I like the cheap ebay OBDII scanners just for reading/clearing codes. i don't have to worry about breaking them, grease, taking them on trips, etc.  break and replace.

Edited June 5, 2014 by grossgary

[zombieforce]

I have a genuine vag com cable will that work with free ssm?

[86BRATMAN]

Gary, that thread says it'll only work with 99-09, do you know of one that would work like that for the 95-99 ecus

[leapdog]

Need some serious help. I have to get this fixed. This car is going to my girlfriends kid when he passes his drivers test and it has to pass inspection. Turning off the code just before inspection is not the answer.

2003 forester x-  I have had P0420 code since June(ish) 2019. No other codes except the occasional fan rationality check code which I'm pretty sure I resolved with the new Coolant temp sensor. 

An engine with 120K(ish) was put in the car in December of 2018 after the original blew up from catastrophic head gasket failure. It has about 155K(ish) on it now.  My odometer says 310K it read 275K when the engine was replaced. 

Since  first getting P0420  I have done the following:

New PCV valve, new NGK spark plugs, new NGK wires, New fuel filter in the tank and under the hood, new IAT, new coolant temp sensor,  new denso A/F sensor and 02 sensor, new eastern CAT.  No exhaust leaks and from what I can tell no vacuum leaks. Most of the vacuum line were replaced anyway. 

I have done the seafoam intake cleaner. I did the high concentrate on a low tank of gas injector cleaner twice. 

My generic scanner says Cat monitor incomplete. 

Fuel economy seems ok. It's not excessively bad. Performance is decent.  Don't know about emissions since my state doesn't do vehicle inspections. My profile says I'm in NC but actually live in SC. 

 

The Scan tool shows the A/F sensor at 2v +- and the 02 Sensor bouncing but hovering at .5v. I have read that the A/F sensor is supposed to read .1-.9 and fluctuate and the mixture is adjusted by the ECM. However, the scantool shows you a voltage that is read from the ECM concerning the A/F sensor and converted so I the 2.1- 1.9 ish readings are good. 

Freeze frame from the last code set after resetting. 

 

RPM 2288

Load calc % 12.99

MAP Hg 12.4

Coolant temp 192

ST Ftrm -0.06

LT Ftrm 7.8

MPH 57

Closed loop

 

Edited March 9, 2020 by leapdog

[brus brother]

I have had P0420 on my 05 OB for the last ~100k miles.

I live in an emissions testing state.

I have tried inserting the resistor trick on the O2 signal wire but while the P0420 went away, I started throwing P0298 immediately. Uninstalled the resistor and back to the P0420.

While P0298 comes on almost immediately after clearing the codes, I can clear the P0420 and then have enough time to get all the systems "ready" to pass inspection.

Procedure for readiness:

I use an old Autel Maxiscan LINK listed from China for cheap if you can wait for delivery of there are similar US sellers for $18-20

It's cheap enough to keep in the car and plug into the OBD port without fiddling around with opening programs on your phone to mate with the other types. But, both types will do the same thing.

Best done with 1/2 tank of gas

1. Clear codes

2. Start car without stepping on the gas (it will take a few cranks as the ECU is starting from scratch).

3. Let car idle for 15 minutes without touching the gas

4. Drive car about 10-15 minutes at highway speed (55 mph) trying to keep speed constant (no braking or hard acceleration)

5. Check for system readiness and if all set as ready, drive to inspection station.

I check for inspection stations nearby and plan my route accordingly. Last time it took a few tries to hit the sweet spot but I am now good for another year and a hlaf.

I have been doing this for the past 7 years through three inspection cycles.

[leapdog]

8 minutes ago, brus brother said:

I have had P0420 on my 05 OB for the last ~100k miles.

I live in an emissions testing state.

I have tried inserting the resistor trick on the O2 signal wire but while the P0420 went away, I started throwing P0298 immediately. Uninstalled the resistor and back to the P0420.

While P0298 comes on almost immediately after clearing the codes, I can clear the P0420 and then have enough time to get all the systems "ready" to pass inspection.

Procedure for readiness:

I use an old Autel Maxiscan LINK listed from China for cheap if you can wait for delivery of there are similar US sellers for $18-20

It's cheap enough to keep in the car and plug into the OBD port without fiddling around with opening programs on your phone to mate with the other types. But, both types will do the same thing.

Best done with 1/2 tank of gas

1. Clear codes

2. Start car without stepping on the gas (it will take a few cranks as the ECU is starting from scratch).

3. Let car idle for 15 minutes without touching the gas

4. Drive car about 10-15 minutes at highway speed (55 mph) trying to keep speed constant (no braking or hard acceleration)

5. Check for system readiness and if all set as ready, drive to inspection station.

I check for inspection stations nearby and plan my route accordingly. Last time it took a few tries to hit the sweet spot but I am now good for another year and a hlaf.

I have been doing this for the past 7 years through three inspection cycles.

I'm not willing to play this game. The issue needs to be fixed. There is a cause and it can be found properly.

[brus brother]

Check over at subaruoutback.org

Cardoc has a posting (about 1981 posts, 100 pages worth) HERE

He is very generous with his time and will help interpret your readings.

I tried replacing passenger side Air Fuel and front O2 sensor and the cat converter without success. My car has 3 cats, 2 AF sensors, two front O2 sensors and one rear O2 sensor. At 200K miles, I am not going to throw my wallet at any more fixes.

Good luck and let us know how it works out.

Edited March 9, 2020 by brus brother