3. I’ve got an inspection due next month. If I clear the codes right before, any guesses on my chance of passing inspection?

Thanks for any input.

Been told that Subie cat conv are weakly constriucted and the active element can become loose and rattle around in the housing. I had a real vibration there, and that's what apparently had happened some 50 - 75,000 miles ago. Fortunately it vibrated into a non-rattle condition; hope it stays there No harm to drive it.

On clearing the codes just before inspection, that's what I'd try - good luck. But I live in a no inspection state so haven't had any emission system inspection on my Subie. My car is high mileage '99 in good condition, but I'm trying not to put big bucks in further repairs.

 

If the cat is not functional you will probably not pass emissions testing. The ECU runs programs called monitors on the different emissions systems. The code clearing resets these monitors. Most states will allow only 1 monitor to not have run successfully to pass. If you go with out all the monitors having run, they will tell you to come back after they have run.

 

Question about the logic of shutting off the cruise control and VDC when there’s a cat converter code.

Why? What’s the benefit of turning off those two systems? Seem unrelated to the cat.

 

1. A week before the lights appeared, I’d had one of the mufflers replaced. Any chance this could be related?

2. I just dropped $1800 on the car for various issues and I’d like to hold off on spending more. What’s the danger of running like this for a few months?

1. What part of the muffler did you have replaced? Was it before or after the CATs? When people say "muffler", they often mean something under the car related to the muffler, like pipes, etc - so that's why I'm asking.
You may have a leak in your muffler, after it has been jossled around, that caused this code to pop up. I'd get that checked. Or they could have caused one of the connections to one of the O2 sensors to become intermittent.

2. You may not want to because if this truly is a P0420 code due to a bad connection or leak and not a bad catalytic converter, but it could cause the CAT to go bad in that time. An exhaust leak is also not particularly healthy.

I'd get it checked, if you are not comfortable enough to know what you're looking at under the car.


The VDC and Cruise Control turn off any time there's a Check Engine Light (CEL) thrown on a Subaru. It is by design.

 

@Remco I had one entire muffler replaced. I’ll get it checked at some point.

Snowing today in NH and the roads are a mess. I had to do a 90 minute drive with a number of intersections. When I go from a stop and turn the car was fishtailing in a manner I’m unaccustomed to. I didn’t realize the VDC was helping that much. Not really a fan of Subaru’s design here.

 

The monitoring parameters for the P0420 code are quite narrow. It's so common that there actually is a pinned thread about it when you first open the "Problems, Maintenance" section, and it's at the top. (considering that it took 3 days for the light to come back on, I'd say that it is "mostly doing fine". If it came on immediately, I'd be concerned)

(All Years) - P0420 - Catalytic Converter Efficiency...

Forester 2001 L - Stock 101k Miles P0420 - Catalytic Converter Efficiency Below Threshold The forester has been throwing this code for a few months now, typically shows up on long highway runs (but to be fair, that is the vast majority of my driving). I bought a scanner from scantool.net...

www.subaruforester.org

 

''The VDC and Cruise Control turn off any time there's a Check Engine Light (CEL) thrown on a Subaru''
Is this actually so? Seems strange that safety systems would shut off for something like an EVAP system leak, or a failing O2 sensor.
I saw this comment on a Legacy forum and I think it indicates more what is actually happening
''Don't let the brake, trac and vsc lights bother you. Many oems use this tactic to push you into repairing the car. They found that many people will look past a CEL if the car is running well but its hard to look past a christmas tree on the dash. Drive it in and have them check it over. I work as a master tech for a toyota dealer and 3/4 of our vehicles have this computer logic built in. When the tech goes to pull any code other than the CEL it will give a generic code to check powertrain ecu. If it fires up and drives fairly normal don't worry and take it in. All manfacturers have built in fail safes for their ecus. If the car won't accelerate above 10 miles and hour then you have a big problem and the vehicle has went in to maximum fail safe. If the CEL starts flashing then get to the dealer asap as the codes in the ecu can cause fatal damage to components. Hope this helps. And don't worry if it runs relatively normal just take it in. Probably just a bad sensor. DO NOT DISCONNECT THE BATTERY! Doing this basically ties the technicians hands. All codes and freeze frame data will be lost and will leave the tech with no way to repair your car. ''
Source--https://legacygt.com/forums/showthread.php/check-engine-light-blinking-brake-cruise-control-light-vdc-light-186174.html
Personally I think it's a good move by anyone to buy their own cheap code reader. They are readily available for under $20 on Amazon. Especially in places where free code reading services are unavailable, it saves time and money figuring out the problem. If your check engine is blinking steadily, I would not keep driving the vehicle as it indicates there is a severe fault that will cause further damage if the vehicle continues to operate.

In my mind, it is possible that the original poster's problem is related to an O2 sensor failing. Their performance degrades with age. Some of you might find the following information from NGK interesting
https://www.ngksparkplugs.ca/tech-info-oxygen-sensors.cfm

 

@Quadraria10 I'm pretty sure they turn all those lights on to scare the isht out of people and get them to bring their car in. On Subarus I've repaired, I've never seen just the check engine light on.
A flashing CEL, on the other hand, is the imminent danger sign. Usually related to severe misfires; things that can do damage.

Regarding the O2 sensor: yes. That's what I was thinking too. Since they farted around with the muffler just a week before, my thought it that they introduced an exhaust leak or pulled an O2 connector that is now not quite seated right.
He said he was going to get it checked.

 

Could well be the case Remco. I see you have a 2009, have you ever replaced your O2 sensors as preventative maintenance? I was thinking of replacing them on my 2011 just to ensure optimal fuel trim and keep the original front cat manifold in good operating condition for as long as possible. I did it on my previous 06 X trail at 150,000 km, and it was still fine in the fall when I sold it at 240,000 km, and those Nissans were notorious for Cat manifold problems.

 

No, I haven't as of yet but do periodically check fuel trim to make sure everything is happy by plugging my CarChip in and letting it record for a week or so.

You're right on Nissans and CAT related incidents: we had an Altima as well as my inlaws. Both went through several A/F sensors in their life. Other than that, they are good cars, tho. Theirs (1998) is still running well.

 

I've done the $5 fix on our 2009 X (2.5 non turbo) (even though it was a bit more expensive, since the $5 was years ago, and I'm paying in Canadian Dollars) and it made the P0420 go away. No performance issues. Did that like 50K Miles ago and have not seen it again. I bought the part at Napa.

And I can confirm that when the CEL comes on, Cruise, Traction Control and Stability Control all get disabled (and warning lights come one for each)

You could consider that a way to panic the driver. But considering that the CEL can mean a lot of different things (like a speed sensor) You wouldn't want your cruise to malfunction to a point where you're traveling 120 mph without the ability to slow down, because the drive by wire system thinks that you're doing 50 mph.

It's super inconvenient that all those systems get disabled. But I don't think they do it JUST to get you in the shop.

The P0420 however... It only takes one puff of exhaust to be slightly outside of the expected measurement-band and the light comes on. And doing the $5 fix will not void your emissions test as the O2 sensor can still measure the exhaust. It just gets a more "average" reading than a direct reading.

 

Had this happen twice on my 08 Impreza. First time I replaced the gas cap, they do wear out, that gasket is not impervious to chemical and friction wears on it Second time I'd replaced air filter on a planned trip I was ready to take of on and I'd not sealed that air cleaner incorrectly snapping one clip only partially. Fixed it and reset code. Sometimes though it is a cat. These cars are susceptible to air leaks flipping codes

 

How many miles do you drive each mo.? If only a few hundred to a thousand, you’re fine for a few mo.

 

Check downstream O2 sensor connection. A clogged air filter and/or bad spark plugs will trigger the code. It may be running way too rich, but I would put my money on your muffler guy failing to plug in your downstream O2 sensor. Take it back to him and ask for a check. If plugged in correctly then invest in a new O2 sensor before even thinking about a new cat. They don't go bad unless you misfire and backfire. Good luck.

 

Mount the O2 sensor with a spark plug anti-fouler...

I think that might be the $5 fix mentioned above.

 

The problem is apparently a common one and here is a Subaru service website explaining the code in depth ...probably borders on TMI ...but maybe it'll help:

Subaru Check Engine Light and Code # P0420 Explained - All Wheel Drive Auto

One of the more common codes we see on the Subaru, is P0420 Catalytic Converter Efficiency Below Threshold. I am taking a more technical approach than normal to describing what happens.

allwheeldriveauto.com

It would seem as if the problem can be caused by the after cat O2 sensor ...which monitors the outflow of the cat. The above mentioned reference talks in depth about what the after cat O2 sensor does.

Regardless the money you "dropped" on your Foz, chances are you're going to have to drop some more to clear this code. So at ten years old you probably have about 150K on it ...which is probably well past time for the two original O2 sensors. Me, myself and I toss in replacing O2 sensors in the 75K to 100K range whether I'm getting codes or not. My 86K, 2011 2.5X Foz "to do" list has having her O2 sensors replaced in the coming months. O2 sensors are like sparks plugs in that their "failure" is a slow process ...the ones in your 2010 Foz are probably not working as well as they were new ...or at 75K ...or even 100K. If it were my car I'd have already replaced them both.

O2 sensors are the prime input into the engine management computer controlling fuel mixture. Fuel mixture is what determines how effective and how quickly the cat "lights off" and operates. The environment an O2 sensor "lives" in is exceptionally severe. So while the sensor(s) may not be triggering codes indicating they, specifically, have failed in some way, their degraded performance as mileage accrues can be manifest in other ways ...possibly like this P0240 code.