Is it really the catalytic converter?

Maintenance/Repairs
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1

I have a 99 Subaru Legacy with 200,000 miles. It has given me very few problems until the last few years. All my repairs have been done at a dealer.



The Check Engine Light came on, and the scan came up with a P0420, that the catalytic converter is working below optimum. They cleared the code to see if it happened again because I couldn’t afford a new converter. The light didn’t come back- but my mileage is less than it was.



Various places- including the Subaru website- say that bad O2 sensors will affect mileage. So far it seems like a preferred fix- but I had the front O2 sensor replaced just 18 months ago.



Could the front sensor really be bad again? Can they be checked? Will the rear one affect the mileage? Or is it really a bad/clogged catalytic?



2

You replaced the front sensor (upstream of the cat, I assume). Is there one after the cat? I assume so, only way to know if it’s ‘working below optimum’. If you haven’t, then it might need replacing.

3

Is it possible that the front sensor could be bad again? Absolutely. A friend who worked for a major car company says that one of their suppliers for these (a major aftermarket supplier, too) was having high rates of failure on brand new vehicles… so there is no reason to believe that the same problem couldn’t hit aftermarket parts.

And yes, they can be checked - but the test equipment is generally only owned professional mechanics… so you have to find an honest mechanic who will check the function of the O2 sensors before assuming the cat is bad. I’ve seen too many places see a code that a cat is bad and assume the cat is bad without checking to see if the sensor telling them that the cat is bad is the actual problem. Happened to me once - it was a $35 part that was bad, not the cat… but the cat was a much more profitable repair for them to try to push off on me.

I’d find a trustworthy independent mechanic with a well-equipped (computerized) shop and take the car in to them and report just the bad mileage. Don’t tell them that the dealer says you need a new converter. Heck, don’t even tell them about the P0420 code. Most mechanics will immediately hook up the car to the proper test equipment and check things like O2 sensors when given a vague problem like bad gas mileage.

4

It could be the rear O2 monitor, but that wouldn’t cause any decrease in MPG. Of course, the MPG decrease could be imagined or be due to some unrelated issue.

5

Thanks. I track my mileage, so I’m sure there’s a drop.

6

Thanks. I just moved, so I don’t have a history with anyone here, and I wanted a little guidance before walking into a place.

Guess I’ll have to poke through the Mechanic’s Files…