The OBD Code P0420 on the Subaru Legacy indicates a fairly common issue with your catalytic converter's "operating efficiency" – meaning it's not doing its job well enough. Here's how to diagnose & fix it – ideally before you need to pass an emissions test!
What does OBD II Code P0420 mean on a Subaru Legacy?
In essence, OBD code P0420 means that your O2 (Oxygen) sensors have detected that your catalytic converter isn't running well enough to meet the standards set by the EPA (Environmental Protection Agency) or CARB (California Air Resources Board).
Normally, of course, vehicles are sold with catalytic converters that are designed to meet these standards, so this code is typically a sign that something is wrong with your catalytic conversion system. It's also a sure sign that you'll fail an emissions test if you don't get it fixed.
The code itself is recorded in the electronic control unit (ECU) when certain conditions (described below) are met, indicating that you need to get your catalytic converter checked out.
What is a catalytic converter?
A catalytic converter is a part of your Subaru Legacy's exhaust system. It converts pollution & toxic byproducts from your exhaust into less harmful gases by using extreme heat & other chemical processes. It resembles a muffler, but its function is quite different.
if your catalytic converter is malfunctioning or otherwise ineffective, your engine will produce more pollution than it's legally supposed to, and your ECU will turn the check engine light on to let you know, as well as recording code P0420 in its onboard memory.
What causes OBD code P0420 on a Subaru Legacy?
Catalytic converters typically have mounts for O2 sensors ahead of & behind the converter itself. Once your engine is hot (because it's been running for a bit), your converter will start operating in "closed loop mode" – meaning that it's actively converting pollution into less harmful gases.
When this works, the front sensor should be reading low & variable oxygen levels, while the rear sensor should be reading high & stable oxygen levels.
Howeveer, if your catalytic converter isn't working properly, the two sensors will read approximately the same oxygen levels, and the ECU will trigger code P0420.
Root Causes May Include
- The direct cause is typically that your front & back O2 sensors are returning similar #'s. If you don't
- Using non-standard fuel, including leaded fuel (This is unlikely to be the issue if you're in the US, but it's still worth mentioning.)
- Issues with a failed O2 sensor, which is typically a result of damage as opposed to age.
- (Most likely) a failed or underperforming catalytic converter.
- Downstream O2 sensor wiring often causes this too.
- High fuel pressure can cause contaminated exhaust gas to enter the catalytic converter, which will often record a P0420 code.
- Contamination from oil or fuel leaks that get into the exhaust can be an issue here. (We offer specific advice on cleaning your O2 sensors if this is the case.)
- Delayed (or "Retarded") spark timing can indirectly cause below-average catalytic converter performance.
- Your engine coolant thermometer returning inaccurate values (over- or under- cooling the engine can cause PO420)
How to fix OBD code P0420 on a Subaru Legacy
The first thing to do when trying to repair P0420 is to visually examine the entire exhaust flow for leaks while the engine is running. If you find one, you're in luck – patch it up, reset your codes, & hope that does the trick. This is good news because leaks in the manifold, exhaust pipes, or catalytic converter are the cheapest & easiest fix to a problem associated with P0420.
If it's not a leak, time to take some temperatures.
If you've determined your issue doesn't stem from a leak in your exhaust manifold, pipes, or catalytic converter, it's time to pull out an infrared thermometer – this tool will let you check the temperature of the exhaust pipes flowing both in & out of your catalytic converter.
Run your engine for about 5 minutes to get it nice & warm, then shut it off & get yourself into a position with a clear shot at the cat. Point your infrared thermometer at the exhaust pipe immediately before it reaches the catalytic converter, and then at the exhaust pipe immediately after it leaves the catalytic converter, getting a reading each time.
If the readings are less than 80 degrees or so apart, you've got an expensive fix on your hands – your cat needs to be replaced. It's not generally wise to try & repair a catalytic converter, but if you're tight on cash, hold onto your old one after replacing. The rare metals within are worth a pretty penny to scrap yards (that's why they are so frequently stolen)!
Before you commit to buying a new converter, consider checking your O2 sensors & wiring to make sure they appear operational. O2 sensors are a lot cheaper than catalytic converters, so if the problem lies with them, celebrate your good fortune & replace one or both of them instead of following the directions below.
Options for buying a Subaru Legacy replacement catalytic converter.
If you need your repair to be a sure thing, we recommend going to your dealer to pick up an OEM catalytic converter. This is the priciest option but also the one you can be sure will get you past an emissions test. If you're willing to take a bit of a gamble, you can save your money by buying a replacement catalytic converter from a third-party manufacturer. These are generally fine but rarely come with any guarantee that they'll do the job well enough.
We recommend Amazon.com for third party cats – 2-day shipping on automotive parts is almost always both faster & cheaper than finding vehicle-specific parts locally. Here are a few options for your Subaru Legacy. If you are considering a catalytic converter, the only metrics that really matter much are cost, vehicle fitment, & which "supported" states will work with the emissions standards associated with the catalytic converter.
One last note regarding powertrain, emission, & bumper-to-bumper warranties.
Some manufacturers offer longer warranties specifically for emissions-related issues. We haven't done the reseach on whether the Subaru Legacy is like this, but if you're past your warranty period, check the terms – you might be pleasantly surprised & save 1k+ for no extra work. Odds of this are low, but it's definitely been known to happen.
Conclusion & Our Standard Caveat:
OBD II Code P0420 is a code on the Subaru Legacy that indicates an issue with your catalytic converter. There are a number of steps you can take to fix it, all the way up to replacing the cat. Use the information above at your own risk. This article is not meant to be a substitute for professional advice. If you're not comfortable with the steps outlined above, we recommend taking your Subaru Legacy to a mechanic or dealership to have them diagnose & repair the issue.