CEL throwing P0420 code

[AFCEA]

OK, I live in Virginia. In order to renew your plates you have to go through an emissions testing process. I swear most of it just benefits the shop not all shops that test are a certified repair facility either! It’s a 750.00 threshold to spend to get the waiver and many shops won’t tell you that either. So my CEL comes on and goes off randomly. It's sporting P0420 - Catalyst System Efficiency Below Threshold (Bank 1). What are we talking? Upstream 02, downstream 02, both 02’s, or catalyst, or all three? Vehicle runs fine. My previous run with a CEL is the shop milks you for all your worth starting off with the small stuff and then hitting you for the big ticket items. I’m not interested in paying the repair shops rent just getting my wheels to pass the dreaded emissions with CEL off! Before I start playing duck, duck, goose on what part to start out with replacing what does the code really mean and why can’t a good shop tell the difference either before my wallet goes dry?

[johnceggleston]

the good news is that it is probably not the cats, so your retirement money should be safe.

 

how many miles on the car? how many miles have you had it? have you ever replaced the o2 sensor, either front or rear?

 

there is a good chance it is the front o2 sensor. it should be a subaru oem item, not after market or generic.

 

it could also be an exhaust leak , i think. but usually it is the front o2 sensor.

 

if the front o2 does not fix it, there is a work around. but you, the car and the environment would be happier if the front o2 was working correctly.

[brus brother]

I agree entirely with John but here are some considerations.

What year/model car.

My recent experience with P0420 was with my 05 Outback follows.

At 104,000 miles I got CEL. Autozone checked (for free) P0420 with the description you note. They erased the code as well. Car ran like crap as soon as I restarted it but smoothed out once the computer re-learned the engine.

CEL came back again at about 60 miles but before I drove back to Autozone and having researched the code on-line I feared the worst that the passenger side catalytic converter was dead.

I bought a cheap code reader on-line for about $20. After I made the purchase, I read that others suggest that a real time data-logger program with connector running off your laptop for not much more $ is the way to go with a better diagnosis but so far I have had adequate results with the code reader.

OK now that I didn't have to drive to Autozone every time it threw a code I could experiment and with the help of the web I tried various "cheats". There is a spark plug anti-fouler ($5 for a pair at Autozone in the self help area part# 42009) that can be drilled out and used to back the 02 sensor out of the cat and fool the computer into thinking the catalytic converter is working at it's desired efficiency.

I tried one then two piggybacked but my code came back (and even got a different code... ) so after a while I gave up and removed all of the cheaters and then no CEL until last week at 1085 miles later.

Is your car well tuned? Have you started using cheapo gas? The dealer implied there would be a different code for O2 sensor but John's O2 suggestion is tempting my wallet's sensibility.

I cleared the code again but I am prepared to get an aftermarket cat with a 5 year warranty for a third of the price of an OEM cat with a 12 month warranty.

Note that my 05 Outback has a particularly expensive design in that there are 3 catalytic converters and 5 O2 sensors! This particular setup appears to NOT work well with the cheaters.

I performed the modifications on the O2 sensor immediately behind the first cat on the passenger side.

When I was tinkering around with the cheaters, I got yet a different code.

People swear by the fix and a wink from Autozone sales seems to indicate it's common knowledge.

So if you just want to pass emissions and simulate the rich man's $750 emissions conscience, buy a cheap code reader. Simple to learn and operate. Once you clear the code, the computer will read certain systems NOT READY (which would also red flag your car at testing). There is a certain method of driving at 60 mph for a few cycles to set the systems as ready. Keep checking the code reader. Timed right, when codes have been cleared and all systems are "ready" you can get right to testing and you might pass before throwing the first "pending" code and then the second code which lights up the CEL.

Like the outcome of a rain dance, sometimes it's just a matter of timing.

NOTE: I have read that this friggin' code can return even after you change the cat so while I am certainly in favor of clean environment, the difference between readings that are AOK and operating just "below threshold" is a gray area for you and your wallet to mull over.

Edited April 1, 2011 by brus brother

[AFCEA]

I'm the original owner. It's a 2000 Outback Ltd Sedan. It's got 120K on her. A proud Subaru owner and our third one too! I'd buy another one after this one dies without a doubt and recommend one too. Never had an ounce of trouble out of any of them except this one’s CEL light within the past year. It always seems to go off just in time for emissions inspection and returns shortly thereafter. Like someone is watching out for me. Virginia actually hooks up to the ECU and reads the codes and inspects the readiness of the drive cycles. It does escape the dyno test as the drivetrain would engage the rear wheels and rocket it off the equipment. I just got done with the CEL from H#@@ on my Montero. One thing after another needed replacement and I spent a healthy amount of money too. Front O2, Back O2, cat, plugs, wires, MAF and the list went on and on. And the only thing that originally coded was the front O2. It was like a domino effect, replacing one thing led to another and each time a different code popped up. It got so bad I bought my own code reader as I was believing the garage. It was like Pandora’s box and I was trying avoid the repeat of the same with the Subaru.

[unibrook]

I have a 2001 Forester with 106k on it. My front 02 sensor has been crapping out every 40k miles. Mostly city driving. Just fyi.

[davebugs]

I've had luck with seafoam. If you do a search here there should be plenty of reading.

 

Feed it through a vacuum line. Makes a HUGE cloud of white smoke.

 

Under 10 bucks a can. Worth a shot to me.

 

I've done this with several and put another 1k or so on them without the code coming back.

 

The old GF's 06 Impreza now gets this code when she gets cheap gas. Try a major refiners station for a while.

[Fairtax4me]

You just have to register the car somewhere south of Culpeper county then you don't need to do those stupid emissions tests.

JK

 

Unfortunately you go the most fun emissions code Subaru offers. It can be anything, from a bad sensor, to an exhaust leak, to a bad cat.

We're in salt country, so exhaust pipes get nice and rusty around here. Rust eventually leads to leaks. (who knew?) Any leak before the front oxygen sensor (which is mounted to the front catalytic converter) can allow air to enter the exhaust stream and throw off sensor readings. The most common places for leaks are the flange before the cat, and the gaskets on the manifold pipes where they attach to the engine.

[brus brother]

You just have to register the car somewhere south of Culpeper county then you don't need to do those stupid emissions tests.

JK

 

Unfortunately you go the most fun emissions code Subaru offers. It can be anything, from a bad sensor, to an exhaust leak, to a bad cat.

We're in salt country, so exhaust pipes get nice and rusty around here. Rust eventually leads to leaks. (who knew?) Any leak before the front oxygen sensor (which is mounted to the front catalytic converter) can allow air to enter the exhaust stream and throw off sensor readings. The most common places for leaks are the flange before the cat, and the gaskets on the manifold pipes where they attach to the engine.

I guess you could swap sensors to see if the code switches from bank 1 to bank 2.

But do you just listen/feel for leaks? Does the leak have to be pretty big to throw a code? Is there a CEL-NoMore in a can I can spray on the hood of the car?

Edited April 2, 2011 by brus brother

[Fairtax4me]

Is there a CEL-NoMore in a can I can spray on the hood of the car?

Sure is!!!

Spray it on the instrument cluster and voila no more CEL!

 

Leaks can be barely noticeable. You might not be able to feel/hear them without help. Have someone cram a rag into the tail pipe (engine running) while you listen around the pipes for the leak. Often if the leak is large enough there will be carbon deposits around the source, but not always.

[WoodsWagon]

A 2000 legacy should respond well to a spark plug anti-fouler on the rear o2 sensor. I know of a lot of cars running with no cats, and no P0420 code because of the anti-fouler. The 16 bit ECU's aren't all that sophisticated about the monitors they run.

 

You get a flat washer spark plug anti-fouler from the Help! section of your auto parts store, drill through one with a 1/2" bit, thread that into your exhaust, and then the sensor into it. There's a few different sizes, you want the larger one with the flat, not tapered, seat.

 

If you stack them you will get a sensor slow response code. One is enough to pull the tip of the sensor back out of the exhaust stream, which slows down the sampling rate and smooths out the readings from the sensor. That makes the ECU think the cat is there and working.

 

Oh yeah, it's a federal crime to tamper with the emissions equipment on your car. Just a FYI.

[ShawnW]

Lets not forget this code CAN be properly diagnosed. It requires skills and training that most dealer technicians don't have and many aren't very good at paying attention during dealer training on this.

 

The big problem is the instructor at dealer factory training doesn't have defective vehicles to show off. They have the technicians look at properly working cars with a problem 'ghosted' on a dummy box. In other words they trick the computer into throwing bad numbers and have the tech pick the part in need of replacement based on those numbers.

 

The fact is:

1. Many of these are "fixed" by replacing the:

A. Front 02 sensor

B. Catalytic converter

 

Why does this work?

Many codes like the 420, are cleared from memory after a repair and road tested.

Many of these codes require "2 consecutive trips with the fault."

 

So:

Many times a simple clear of the codes would "fix" the problem, or in other words, it would have self reset itself.

 

Many times the fix is as simple as premium fuel and a long highway road trip on mostly flat ground. Its amazing how good your car will run after 120 miles of highway driving for most people. It cleans carbon deposits, gets the catalyst really hot, cleans the tip of the sensors, spark plugs, injectors, etc.

 

This code is also tripped by:

Loose air filter box/box not clipped in on the bottom edge behind the throttle body. Any up or downstream air leaks can contribute to these numbers. Anywhere past the MAP/MAF sensor depending on vehicle and before the rear 02 sensor.

 

Find a good shop with a high end scanner that will look at the numbers. A good tech (me included) can see o2 sensor activity, check for leaks, and visually inspect the car and be right about 90 percent of the time. A bad shop will replace both cats and both 02 sensors and send you looking for your credit card. Its possible all of these are bad too and the diagnostic is a waste of money but not very often.

 

The Front 02, or "Bank 1 sensor 1" on most Subarus is typically in need of replacement every 105K miles on US model cars. This aids in fuel economy and prevents the catalysts from being damaged by overly rich mixtures. This is a wear item and frequently original on every car I see around here. People will change 4 platinum spark plugs at 8-18 dollars a plug but they won't do 1 sensor that is 100 bucks. The outcome is virtually identical in HP and MPG improvement but nobody replaces this sensor. On many Subaru's its actually easier to replace the 02 sensor than the 4 spark plugs.

 

This code is one of the most difficult to diagnose properly. I used to hate these until I understood the system better. Now I hate EVAP codes that aren't gas cap leaks and even those can be simple.

[brus brother]

Shawn,

"The Front 02, or "Bank 1 sensor 1" on most Subarus is typically in need of replacement every 105K miles on US model cars."

There are a total of 5 sensors on my 05 OB. 2 upstream, 2 downstream behind the first cats and one behind the rear cat. At 105,000 miles and an occasional P0420 (mostly local driving / regular Citgo gas / new spark plugs) is there a preference for replacing either sensor as a "wear item" or is bank one after first cat the choice? OR is this still just throwing parts at the car and I and the original poster would be better off getting a data-logging program like THIS and then posting results?