[SJRhino]

Hello all, newbie here. New to this site and the Subaru Forester family. I recently needed to replace plugs and 2 or the 4 appeared to have melted center electrodes. For the last month or so I have pulled the P0420 Cat sys efficiency below threshold bank 1 code. After resetting, it would usually stay off for a few days. Eventually, in addition to the P0420 code, I got a cyl 3 misfire code, clear that and in a couple of days I got a cyl 1 misfire instead of the cyl 3. So I changed out the four plugs. Plugs for cyl 2 & 4 for, while not great, looked much better and had gaps that were close to correct

. Plugs for cyls 1 &3 looked to have melted (rounded and very low) center electrodes with huge gaps. I looked online for possible causes of the plug problems, but didn't see much. After installing new plugs the vehicle ran great, no codes for about 2-3 weeks. Now the P0420 is back, but no misfire codes. I haven't had the opportunity to pull one of the problem plugs to see if there is an ongoing issue. What could have caused the plug problem? And would the plug issue, probably not burning all fuel, cause the P0420 or is it unrelated. Thanks for any and all input.

 

[ichabod]

It almost looks like you have 2 different types of plugs in the photo. It could be the angle but the ceramic around the electrode looks to have varying length. Anyway before going after the P0420 it's important to make sure the engine doesn't have any other issues. Do you have access to a basic scanner. I'd be curious to see what your long term and short term fuel trim is showing at idle, 2K and 3.5K rpm to eliminate intake problems as a cause. Cat's going to be expensive so make sure it gets diagnosed correctly. Here's an informative little video.

 

[Makingforestergreatagain]

How old were the plugs?

If ignition is incomplete that let's more unburned fuel downstream to the cat. Pretty sure that will lead to ovrhtd cat and the code you have.
What is odo ,plug coil miles?

Add a signature to your control panel with year etc... see mine?

 

[SJRhino]

How old were the plugs?

If ignition is incomplete that let's more unburned fuel downstream to the cat. Pretty sure that will lead to ovrhtd cat and the code you have.
What is odo ,plug coil miles?

Add a signature to your control panel with year etc... see mine?

2010, 150k, no idea how old the wires and coil pack is.

 

[Makingforestergreatagain]

Might want to research troubleshooting the coil (s)? Or replace outright and the wires
O sensor(s)? might also be suspect.

 

[UhOh]

Strange, but I'd just replace plugs with iridium type plugs and worry about it again in another 120K.

 

[Makingforestergreatagain]

Could be coil failure, though ,,,,replacing plugs is first step, to be sure.

 

[Even_Stephen]

What year is your Forester? I ask because older ones that still have high voltage spark plug wires fired half of the plugs "backwards". That is, the voltage polarity was reversed to that of the other two plugs. If one left the plugs alone for a long time, the center electrodes of the "backwards" plugs would show more wear than the other two.

So there may be nothing at all wrong. Just put in a new set of plugs.

 

[SJRhino]

It almost looks like you have 2 different types of plugs in the photo. It could be the angle but the ceramic around the electrode looks to have varying length. Anyway before going after the P0420 it's important to make sure the engine doesn't have any other issues. Do you have access to a basic scanner. I'd be curious to see what your long term and short term fuel trim is showing at idle, 2K and 3.5K rpm to eliminate intake problems as a cause. Cat's going to be expensive so make sure it gets diagnosed correctly. Here's an informative little video.

Sorry, been away on vacation and just backing back to the issue. All four plugs are the same brand and # plug. I don't have a scanner. I pulled the codes using Advanced Auto's code scanner.

 

[SJRhino]

How old were the plugs?

If ignition is incomplete that let's more unburned fuel downstream to the cat. Pretty sure that will lead to ovrhtd cat and the code you have.
What is odo ,plug coil miles?

Add a signature to your control panel with year etc... see mine?

Sorry, I went on vacation and am just getting back to the issue. 2010 Forester 2.5 non-turbo PREV, 150,000 miles. I don't know how old or the miles on the plugs/wires; I had just gotten the vehicle. I assume they were very old. The vehicle was driven mostly on highways between Philly and DC.

 

[SJRhino]

What year is your Forester? I ask because older ones that still have high voltage spark plug wires fired half of the plugs "backwards". That is, the voltage polarity was reversed to that of the other two plugs. If one left the plugs alone for a long time, the center electrodes of the "backwards" plugs would show more wear than the other two.

So there may be nothing at all wrong. Just put in a new set of plugs.

2010