On the Subaru Legacy, OBD II Code P0430 is a DTC, or “diagnostic trouble code” that means you’ve got an issue with your catalytic converter – namely that the catalytic converter associated with bank 2 of your engine cylinders isn’t operating at full efficiency. This code can cause both minor and serious problems including increased tailpipe emissions & automatic fails when getting emissions tested.
By the way, this code is extremely similar to code P0420 – which means essentially the same thing, just for the catalytic converter on the other bank of cylinders.
What does DTC P0430 mean on a Subaru Legacy?
OBD Code P0430 is a generic OBD II Code – meaning that it applies to every applicable vehicle manufactured since the year 1996. The steps associated with diagnosis & repair often vary from model to model but the code applies means roughly the same thing on every vehicle it shows up on. In this article, we focus on how to address it on a Subaru Legacy.
It’s also an extremely common code – catalytic converters can be temperamental, for starters, but the code can also be caused by sensor failures that come up reasonably often.
Catalytic converters, to the typical home mechanic, look a lot like a similar part, the muffler. However, the job it does isn’t very similar at all. Where mufflers exist to reduce tailpipe noise, catalytic converters were created & required in order to reduce automotive exhaust emissions and the smog that comes from it.
What triggers code P0430? (From a technical perspective)
Catalytic converters leverage 02 (Oxygen) sensors, or “solenoids” both in front of and behind the main converter. When your vehicle is running hot, the converter works in what’s called “closed loop mode”, which makes the upstream 02 solenoid reading change, typically as RPMs climb or fall.
Conversely, the downstream solenoid’s readings should stay mostly flat – if your catalytic converter is doing what it should, the O2 (oxygen) levels will be roughly the same at all times. P0430 comes into effect when the “upstream” and “downstream” 02 sensors are roughly in sync – this typically means that your catalytic converter isn’t functioning at desired levels.
In case it isn’t clear, the reason that sort of data similarity will trigger a P0430 is because the catalytic converter is supposed to raise 02 levels to a steady standard. If the downstream sensor is moving in sync with the upstream sensor, the converter isn’t removing sufficient non-02 gases & particulates from the tailpipe exhaust flow.
Catalytic converters aren’t typically what’s known as a “wear part” – i.e. they aren’t meant to be replaced in most cases. If you run into P0430, something is probably “actively wrong”, as opposed to it just being time for routine maintenance.
How to diagnose DTC P0430 on a Subaru Legacy:
This trouble code typically comes to an owner’s attention in one of two ways – when the check engine light (or more properly, the “Malfunction Indicator Lamp”) is lit, there’s a surprisingly decent chance that the issue is either DTC P0430 or DTC P0420 – catalytic converter issues really are quite common – so much so that this is one of the most-viewed pages on our site.
Another way you might diagnose this code is if you’ve been driving around with your check engine light on and pull into a service center to get your annual emissions testing done. This code will make you fail OBD-based emissions tests common in states like Texas, Florida, & Oklahoma.
If you’re getting a tailpipe emissions test, you’re still in trouble – if the catalytic converter isn’t functioning, you’re probably not going to pass a tailpipe test.
One final symptom that you probably won’t notice is slightly reduced engine power. If the catalytic converter isn’t functioning properly, it can lead to an exhaust backup that’ll lower your car’s effective horsepower by a small margin.
It’s generally safe for you to drive with code P0430. The catalytic converter isn’t a critical part of the engine and you probably won’t run into any directly-related problems until emissions testing. Is it safe for the environment, though? Absolutely not!
All 50 states in the US mandate emissions testing for a reason, and it’s not just to be a hassle. Get it fixed as soon as you can but feel free to drive for a bit before you make that happen.
Keep in mind, it’s dangerous to drive with your check engine light constantly on – something else could go terribly wrong without you even knowing about it.
Specific issues that can cause code P0430 on the Subaru Legacy
There’s a few things that can cause the P0430 trouble code to come into play & trigger the resulting check engine light:
- A failing or almost-failing catalytic converter
- Leaded fuel being used when unleaded is expected (You’re reading this in english so you probably don’t have this problem)
- If your upstream 02 solenoid fails, that’ll cause it.
- If the downstream 02 solenoid fails, that’ll also trigger this code.
- Check wiring to each 02 sensor – the sensor might be fine but if the wiring is damaged you’re in trouble
- Last and certainly least – spark plug misfires and ignition timing issues can cause this issue as well
How to fix a Subaru Legacy with engine code P0430
There’s a short list of options for fixing OBD Code P0430 – almost all of which have to do with the catalytic converter or the exhaust system more broadly.
You’re definitely going to want to use the list below in order – after each step, check to see if it worked by clearing trouble codes, running the engine, and seeing if the code pops back up
Step by step instructions for repair
- First, check for leaks across the entire exhaust system, starting with the exhaust manifold, and the catalytic converter itself. These are easy to spot when the engine is running – if you don’t see them, move on. It’s unlikely that this would cause it but it’s the cheapest chance you’ve got at a fix – so you might as well try.
- Use your handy-dandy emissions scoping device to check the downstream solenoid’s operating levels. While you’re examining this, keep an eye out for a stable waveform. That’s what you’re hoping to see, because it means you’ve might not have an expensive catalytic converter purchase in your future.
- You don’t generally need to check the upstream solenoid this way – it won’t tell you anything useful.
- If the sensors look fine and you’re getting an unstable waveform on the downstream 02 solenoid, bite the bullet & replace the catalytic converter. They’re not worth repairing, in spite of how expensive they are. (they typically run hundreds of dollars because they also contain rare metal)
Also, check out the trick immediately below for a “hack” to make this easier…
The “Thermometer hack” for diagnosing & repairing P0430 works like this:
Start your vehicle and let it run for a few minutes, then examine the tubes leading in & out of your catalytic converter’s second bank with a point-and-scan thermometer like this one – first on the upstream then downstream.
If the engine’s been running, gasses leaving the converter should be approximately 100F hotter than the gasses coming in. If the exiting gases are, the converter needs to be replaced. If they are not, however – it’s probably an O2 sensor malfunction.
This “trick” works because the catalytic converter, as part of its duties, lowers the temperature of exhaust significantly.
If it's time for a new catalytic converter…
If you determine that it’s time to get a new catalytic converter – first let us express our condolences. Catalytic converters are prized by street thieves because the metals within are quite expensive – and those metal prices also play into the to high price of a replacement catalytic converter.
To replace your vehicle’s catalytic converter, either go to your dealership and get a new, OEM one there – you don’t want to monkey around with a cheap non-stock converter, but… secondary-market catalytic converters are a more affordable option.
Again, avoid cheap versions – they don’t always get the job done, especially for larger engines or if you don’t have the right equipment to install them. If you’re going to spend $350+ on a part, it’d better work, right?
As a reminder, everything in the article above is suggested on a best-effort basis & we can't take responsiblity for how you use the information container therin.