P420-Cat or something else?

[Nick Bruns]

Hey! First let me tell you what’s wrong with the car…

Passenger side head gasket leaks coolant/oil (no overheating or combustion leaks that I know of) (yes, it’s a 2002 Legacy 2.5)
Transmission shifts weird
Strange noise in the front end like something rubbing when backing up or going forward after the car’s been sitting for a while (overnight, a few hours at the store, etc.)
Negative battery cable connector is disintegrating and I need to tend to that, but it starts fine, no electrical issues for now

I had a CEL come on and it stayed on for a couple days, then turned off. It came back on two days later. I just took it to a AutoZone and had them pull the code for me. Only one code came up P0420- bank 1 catalyst efficiency below threshold. They listed possible causes as: Rich A/F ratio, Ignition system malfunction, or faulty catalytic converter. The car runs fine. It does have intake noise that seems to be too loud when I tromp on it at certain RPM’s when driving.

I did some reading and learned that it’s not always the catalytic converter going bad that causes that issue. Could be HO2S? Vacuum leak? Cheap gas (I run cheaper gas from Fred Meyer more than I used to). The rear 02 sensor is 13 years old. Had a major service 2 years ago to fix a P0171. I don’t want to replace the cat(s) as it comes as a system and is about $1000 or more from Subaru :pinch: I heard that they last the life of the car provided there’s no other mechanical issues that ruins them.

Any suggestions?

Thanks!

Topic

[zero]

Considering the age of the car, it is possible that the O2 sensor is starting to go. Without knowing what kind of voltages the sensor is reading it’s hard to tell. That being said, considering the cost of a new cat if you can replace the sensor yourself it wouldn’t be the worst idea.

[Nick Bruns]

Thanks! I don’t have a way to check voltages because I don’t have a scanner and don’t want to pay a mechanic to do it if I don’t have to. Will it be bad to run it like this until I can swing the cost of a repair or shop visit, or will it break something?

Thanks again!

[EricTheCarGuy]

I’ve actually covered this in a video.

Also some possible cures

[Author]

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