Tired of the Christmas tree - P0420

Before anyone says did you do a search, the answer is yes and it yields many different possible problems.

 

Back story, bought a 2009 Legacy 2.5i and it ran great....shortly after purchasing I noticed a growl coming from the rear of the car. Replaced both rear wheel bearings and sound is gone. Within a month I had to replace the clutch. These are not the cause of the trouble code, just giving some insight on my frustrations on what I thought would be a great car.

 

When I bought the car I also decided to do an oil change and spark plugs as I had no idea when they had been done last. I opted to go with a plug that I was unfamiliar with but seemed to have good reviews. They are the E3 Diamond Fire Spark Plugs. Shortly after this is when the Christmas tree started happening. Needless to say since then apparently there was an oil leak that just poof drained the car and the engine needed replaced. The mechanic replacing the motor was instructed to swap the plugs from the old motor as they weren't that old and weren't cheap. The code was still present after the engine swap and since then I have replaced the upstream O2 sensor as I was told that was the culprit. Unfortunately the code still persists.

 

So I guess the question is, could the cause be the E3 plugs messing with the fuel/air mixture readings by the O2, or is a bad downstream O2 or catalytic converter the issue? Thanks for any insight

It would be a good idea to change the plugs first. Do you have any exhaust leaks? You'll need to be able to monitor fuel trims and o2 data to determine if the cat has failed.

Get rid of those Damn E3 what ever plugs man. Use stock NGK plugs for your 2.5i PLEASE. Also what 02 sensor are you using? Denso, NTK, or Subaru are the ones to use. I used Denso, and NTK with no problems I had a 2008 2.5i, and loved it.

 

Also what shop did you take it to and do they work on Subaru's mostly or farm trucks and minivans? It's better to take this to a shop that works on them then to local joes. If you can't figure it out find a good dealer to tell you the issue and then go from there and fix it your self.

 

2.5i cars are not hard to work on.

All three potential culprits are valid reasons to throw that code. Start with cheapest to most expensive.

1) Check for any vacuum or fuel leaks.

2) Get a Bluetooth dongle for like $15-$25 to look at AFR and maybe fuel trims if that’s possible (I’ve never done this personally before, can’t tell you what works or not because I use my Cobb AP for the turbo model which you can’t use for yours)

3) Replace the spark plugs with NGK or Denso.

4) Test the catalytic convertor(use an infrared temp gun to compare post and pre cat temps, if post cat is not hotter or the margin is very slim then likely the cat has failed but any of the other problems you looked at could have caused this failure. Alternatively you can pull off the cat and inspect the cells visually if it’s blocked.

5) Replace O2 sensor with Bosche or Denso

 

My bet is that the cat has failed. All that oil that you mentioned earlier that disappeared had to have gone somewhere and likely out the exhaust and through the cat which clogged it or fouled all the metal. Original reason for the code could have been something else but the second time around it’s a safe bet that cat is the problem.

OBD2 scanner has been ordered and should be here by the weekend. O2 sensor is a Denso. Will probably be picking up different plugs today or tomorrow. Any particular NGK better for Subaru?

Only the ones recommended for your model and year car.

Well, I went and bought a set of NGK platinums and found out a big issue. Apparently the mechanic that replaced the motor didn't listen about putting original plugs back in. In fact I'm not even sure they replaced the plugs at all. It was running Autolite garbage gapped at .046. Scanner will be in tomorrow so I will be able to have more info on how it is actually running.

 

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Edited March 6, 2020 by horsementech

I don't think those plugs are the problem, they're the correct plugs for the car, and .046" is just barely outside the correct range. I've had worn oem ngk's with a gap bigger than that run just fine.

 

It's not the smartest thing to do, but I'd wing a downstream o2 sensor at it. If that doesn't fix it, it's probably the converter.

Change the plugs, reset the code and see what happens. Who in gods name runs autolite plugs .

 

Like said above throw an 02 at it. I had this code pop up on my 2008 2.5i and I changed the 02 and the car never had a problem.

So as of now, plugs have been changed & upstream O2 replaced. Codes have been cleared and lights are out.....sort of. CEL is out and cruise is no longer on, however the VDC light is. Also reflowed the solder joints on the overhead console to get rid of the airbag light issue that showed up. So as of now the problem seems fixed, but usually takes a day or so depending on driving for lights to come back. So now I get to try and figure out the other lights. I also just recently had new tires put on and ended up replacing two bad TPMS sensors, however the light flashes and goes solid still although all sensors have been re calibrated and are showing good. It keeps changing, sometimes it is on and sometimes off within a matter of a few minutes.

 

I have read that the VDC could be something as simple as needing brakes which is next on my list, but TBH I haven't checked them out yet. So, the saga continues. Thanks for all of the suggestions so far.

Edited March 7, 2020 by horsementech

I'm thinking one or both of the other tpms sensors are bad too. Would make sense that they'd all go bad at around the same time.

 

All four of the aftermarket tpms sensors in my winter wheels are about 6-7 years old, and they all show a "good" battery when I check them with my tpms reader. However, I still started to get the flashing tpms light, which I think eventually turns solid. I'm thinking after that much time (for aftermarket sensors), they're probably just done.

 

Also, my problem was somewhat temperature dependent. Above 40 or 50F and I wouldn't have a tpms light. Below those temps, and it'd come on, guaranteed. I know that sounds like it could be tire pressure, but tire pressure is always fine, well above 29psi or whatever the light threshold is. Makes sense to me, I can't remember why, but I think batteries perform better when the weather is a little warmer and above freezing?

P0420 = Downstream O2 sensor or cat

VDC = wheel speed sensor on one of your wheels

TPMS = tire pressures or TPMS sensor only

 

Those are the basics. Since you threw the parts cannon at it, it can be a number of things at this point.

 

For the P0420, a plug fowler will get rid of the light, but if you want to address the cause, you can get a new cat or a new downstream sensor. The car will probably run the same if you do any of these things. The plug fowler will cost around $10, a new cat will run you ~$500, and a new sensor will be in the ~$50 range. The plugs don't have anything to do with it, but be aware that NGK are recommended and E3 are not recommended.

 

VDC light comes on when there is a major problem with the VDC system (TCU error code) or if one of the wheel speed sensors is acting up. On my car, if I heat up the front end a lot at a track day, then the heat from the front brakes probably freaks out the front wheel speed sensors in the knuckles, and they trigger the light. In my case, it the light shuts off when the car cools down. In your case, I would doubt that you're seeing the same thing. But since you had your rear bearings replaced, I would guess that they messed with the rear wheel speed sensors while doing the bearings. I would start by removing them out of the rear hub and seeing if the wires look alright, and if there is a ton of gunk or debris on the end that gets inserted. If the rears look fine, then look at the fronts. They're relatively cheap to replace if you need to. They are in the neighborhood of ~$50-75 if you need to get new ones. Or you could go to an ebay salvage yard listing and fine a takeoff set for cheap for diagnostic purposes and just change them out and see if that makes the light go away.

 

TPMS sensors are really finicky, if I ever have any problems with them on my cars, I just deal with the light until I need new tires and get new ones. If you have a lot of tread on your tires, then just get new ones on the two that you didn't change. That one depends on how much the TPMS light bothers you. For TPMS sensors, buy new, reputable ones. Don't buy used or cheapies since it's such a hassle to go back to the tire shop to have them swapped out if you have any problems down the road.