99' Legacy -- catalytic converter

I have a '99 legacy with roughly 112,500 miles. Back in October, the ‘check engine’ light came on for the first time since I’ve owned the car (just over a year). Soon thereafter, I brought the car into my mechanic, who found a P0420 code. He told me that he frequently often sees Subarus with CELs on in early winter as gas stations switch from their summer to their winter fuel. He cleared the code, asked me to keep an eye on it, and let him know if it came on again.

Earlier this month, the light came on again, and sure enough, it was a P0420. He suggested a few less expensive treatments before replacing the catalytic converter, including going up temporarily to a higher octane fuel. He also mentioned that the cats on older Subarus are sensitive to fuel that contains ethanol, and I could try something like Lucas Ethanol Fuel Conditioner.

The trouble is that, when I recently looked at the car’s records, I saw that a new catalytic converter was installed at 95,000 miles. Could it be that the cat is failing after less than 20K? Are there other things I should look at before putting in a brand new one?

Your thoughts would be most appreciated!

P0420 Is One Of The Most Common DTCs That Cars Acquire.

I read your story, but I’m not convinced that a new cat would solve the problem. That code isn’t necessarily saying the converter is bad and there are lots of things that can set that code.

Your mechanic was wise to hold off on the new converter. Many of them are sold to people who come in with P0420 and a short time later the code reappears.

Link to info:


I suggest you have a shop check out what the O2 sensor data is showing. Check the fuel trim levels and see what they say. You may have a faulty O2 sensor. You shouldn’t have to replace the CAT after such a short time period. If coolant is leaking into the CAT that can damage it or excessive oil. If the CAT is clogged up you could try a fuel additive product by BG Products. It is called 44K Power Enhancer (part no. 208). It might help to clean it out.

I’ve read that the sensors are often off. I’ve got 218K on my 97 and get this code about once a year. I clear it and go on my way.

I have a 2002 Subaru Legacy that I purchased this year with a similar problem. The place I bought it from first replaced one O2 sensor, but the light continued to come on. Then they replaced the other O2 sensor. Same thing. Finally they replaced the catalytic converter. About a month later the light came on again, but the place said there is nothing else they can replace. The light came on about once a month after that, until the cold weather set in, and now it doesn’t come on any more. Recently, I found out from another owner that the plug wires might need replacing, so that is what I intend to do next.

Checking the O2 sensors are a good idea as posted above. However, there’s a good chance the replacement cat converter was one that didn’t meet OEM specs. Since OEM cats are quite expensive, there’s a market for cheaper ones. (You may read about people who leave their car on the street overnight and wake up to find the cat is gone. That’s due to the same reason.) And there’s replacement cats available for considerably cheaper than OEM version, but they don’t contain as much of the expensive metals that make cats work, and so they don’t last nearly as many miles before they stop working. So hold on to your wallet. You may in fact need a new OEM cat to get you back on the road and w/out the CEL.

Thanks for your input, everyone! Fortunately, I’m not due for inspection until March, so I have a little time to experiment, and see whether it’s something besides a bad cat that’s triggering the CEL.

One follow-up question: Am I right in thinking that the car is still safe to drive?

Yes, you can still drive it but things need to be looked at since damage to the CAT could occur over time if you ignore it.

Good News: Drive And Pay Attention To How Long Between CEL Illuminations.

Bad News: Don’t Drive If The CEL Ever Begins Flashing (See Your Owner’s Manual.)

Good News: Some national chain type auto parts stores will read your codes (and many will reset the CEL) for you free of charge. Good idea.

Bad News: Driving with the CEL is bad because you will not know if any new fault codes are present the would ordinarily illuminate the CEL to alert you.

Good News: Also, now that you’re “getting into it” you might want to consider 50 bucks to buy an inexpensive DIY code reader / CEL turner offer. Actron Pocket Scan, available at Sears or auto parts stores is one of many to choose from that will read your OBD2 system.

Bad News: Some folks pay a technician more than $50 to diagnose a CEL alert caused by a loose cap that could be caught with a DIY reader!


I’ve had some CEL challenges with my 99 Legacy SUS. I replaced one oxygen sensor on top of the engine. It was a pretty easy once I found a YouTube video where a woman swaps it out in about 5 minutes. Bought a new sensor off ebay for $20-30 if I recall. But as someone else pointed out, spending another $30 or so (ebay!) for a cheap code reader/clearer is a good investment, I think, if you’re going to drive an older Subaru and fiddle with it from time to time.

Thanks again, everyone!