Troubleshooting P0420 code

Greetings,

 

I recently inherited a 2006 Outback wagon, less than 65k, new head gaskets/timing belt/the whole 9 yards. When I got the car, it had a P0420 code (catalyst system efficiency below threshold bank 1). I replaced both O2 sensors, changed plugs (all looked very good), wires, coil pack, cleaned throttle body and top end. It ran much better after all of this. I cleared the code and went about my business. Fast forward a couple of months, and the P0420 code is back. The car still runs fine. Are there any usual suspects I should be looking for?

 

http://www.canyonero.com/files/1550939656.jpg

Occam's Razor says bad cat. A cheap infrared thermometer will allow you to compare the cat's upstream & downstream temps.

Yep cat or sensors. Since the sensors were done, it can only be the cat.

Vacuum leaks are also a cause for the code. Check all your vacuum fittings and hoses (there aren't many) including the PCV.

Thanks folks! I'll triple check for loose fittings, then start cat shopping.

I would HIGHLY Recommend a OEM Cat. I replaced my CAT with an aftermarket one a couple years back. Within a year that CAT went bad and cost me over $2k to replace a 2nd time, since it required a nearly all new exhaust system. Our Subaru's, especially those with the PZEV Motors do not like Aftermarket!

There’s also the $5 fix.

https://www.google.com.au/search?q=subaru+p0240+fix&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari

 

If you’re turbo & tuned its common to delete the code, in turn providing the opportunity to ecu log a wbo2 via the rear o2 sensor harness.

Do NOT go cat shopping until you've verified the cat is bad. That's an expensive part to pointlessly swap.

Never heard of a vacuum leak causing a P0420.

Looks like a CT plate in the background, a pzev state. It's worth talking to the dealer to see if it's covered under the 15 year/150k pzev emissions warranty. They'll probably have to run the VIN to check.

 

2nd to last page, not a lot of detail though.

http://sltg.subarunet.com/static/pdf/Quick_Reference_Material.pdf

Looks like a CT plate in the background, a pzev state. It's worth talking to the dealer to see if it's covered under the 15 year/150k pzev emissions warranty. They'll probably have to run the VIN to check.

 

2nd to last page, not a lot of detail though.

http://sltg.subarunet.com/static/pdf/Quick_Reference_Material.pdf

 

Oooh...awesome idea. Definitely worth a shot. Thank you.

 

 

There’s also the $5 fix.

https://www.google.com.au/search?q=subaru+p0240+fix&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari

 

If you’re turbo & tuned its common to delete the code, in turn providing the opportunity to ecu log a wbo2 via the rear o2 sensor harness.

 

Another great tip...thanks very much. This seems like a great way to test cat failure before I replace it like a law-abiding citizen

When at it - have the injectors checked/cleaned and make sure that they all have the same fuel flow. Mismatched injectors eats cats.

Do NOT go cat shopping until you've verified the cat is bad. That's an expensive part to pointlessly swap.

Never heard of a vacuum leak causing a P0420.

 

I have had an small upstream exhaust leak (exhaust flange) cause a P0420 code on another vehicle.

Make sure the wires are in good shape. That wire harness is close to a lot of items. In a hot area.

I have had an small upstream exhaust leak (exhaust flange) cause a P0420 code on another vehicle.

Absolutely. Vacuum leak, not so much.

Check fuel trims and make sure they are +/- 10. A vacuum leak can cause the car to falsely set a 420.

I brought it to a mechanic who diagnosed it as a bad cat. They quoted around 800 installed to replace the front cat assembly. Is that about right?

well, you could buy an aftermarket OE replacement cat for alot less money, but quality with them varies so you might have to replace it again in a couple years. in the end, its up to you.

I wonder if they were going to use an OEM subaru cat, or aftermarket for 800? If it's OEM, that sounds like a fair price to me.

I was getting P0420 on a 07 Legacy last month until I found a PCV line that was not properly seated. P0420s have now stopped.

 

Read about O2 sensors and you will find that they can be affected by vacuum leaks.

 

http://www.ffwdm.com/test/vacuum-leak.jpg

I was getting P0420 on a 07 Legacy last month until I found a PCV line that was not properly seated. P0420s have now stopped.

 

Read about O2 sensors and you will find that they can be affected by vacuum leaks.

 

 

Thank you, I'll check the vacuum hoses. I was screwing around in that area recently when replacing stuff.

 

I put a spacer on the O2 sensor for now and will see how long that lasts.