I’m curious what suggestions people might have. Here’s my scenario:
I have a 2001 Subaru Forester. 186,000 miles. Mostly highway miles. I love the car.
I bought it at 90,000 miles. Great condition. I do all the regularly suggested maintenance, including oil changes every 2,500-3,000 miles.
The Check Engine light has come on a few times. Usually it’s been either the front or rear O2 sensor.
However, I had to replace the catalytic converter the first time back in 2010 (at 120,000 miles).
Now, at 186,000 the Check Engine light came on again, and it indicated another catalytic converter failure. I also needed a new front O2 sensor.
(The light had been coming on and going off every few weeks for months.)
My questions are:
I had a cracked radiator about 1,000 miles ago and had to replace it. It was leaking antifreeze. Is it possible the leaking coolant might have corrupted the catalytic converter this time?
Given that I’ve had to replace the catalytic converter twice, is it likely that there’s a deeper cause, some other kind of engine malfunction? (My mechanic said that most catalytic converter failures are due to ‘bad gas,’ but he wasn’t able to tell me how to distinguish ‘bad gas’ from ‘good gas’, just that the good stuff usually comes from the well-known brands like Shell, Exxon, etc.)
Is it time to sell it before this third catalytic converter fails?
I would get a second opinion and then a third. My theory is that half of all catalytic converter replacements as well as 02 sensors are done needlessly. The mechanic has to be good at troubleshooting the CEL system because a CEL code does not necessarily point to the defective component. There are a lot of reasons that can cause a check engine light to come on.
The only way a leaking cooling system can possibly affect a cat converter is if it gets into the cylinders, and I don’t see anything to suggest that here.
What actually happens to most cat converters is that the platinum-palladium coating inside becomes coated with carbon from oil burning. The coating must come into contact with the NO2 molecules for the cat to do its thing. It cannot if it’s coated. As cars get older, they tend to burn more oil, so it wouldn’t be at all abnormal for the second cat to fail in fewer miles than the original. The mileage figures in your post aren’t at all unusual.
How much oil does the car use? I’ll gong to make a wild guess that it’s perhaps a quart every 1200 to 1500 miles? That’s a perfectly normal and acceptable usage in a Subie of your mileage. If you’re uncomfortable with it, we can suggest some tests, but in all honesty everything sounds perfectly normal to me. Unless there are other problems not mentioned in your post, I see no reason at all not to just replace the O2 sensor and, in the O2 sensor doesn’t do the trick, the cat converter.
By the way, I disagree with your mechanic on this one.
Agreed with missleman that you consider another opinion or two. I do not agree with the statement that most converters are killed by “bad gas”. Most converters die because of prolonged heavy oil consumption, chronic and severe engine misfires, excessive fuel, or contamination by engine coolant, etc.
This kind of makes the converter diagnosis a bit suspect.
“Bad gas” does happen (rarely) but it’s also a catch-all phrase sometimes used when the answer is not really known.
A cracked and leaking radiator won’t kill a converter but if a head gasket is weeping coolant into a combustion chamber the converter can be affected.
You might consider getting a chain type auto parts store to at least give the car a quick scan for codes (they will do this free) and post the exact codes here for discussion. It’s possible that some more help could be given based on what those codes are.