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Catalytic converter/Subaru Forester

On my 2002 Subaru Forester (137,000 m), the check engine light came on. They diagnosed and repaired O2 sensor. About 300 miles later it came on again, this time they said the catalytic converter was cooked, and they blamed whatever caused the O2 sensor to fail (or the O2 sensor failure itself). After the repair (about $1300 and 30 miles later), I discovered that the engine oil was so low that it was not showing up on the dipstick (tho the oil light was not yet on). The shop (after admitting that they did not check the oil while it was in there) said it was related to catalytic converter failure. Does this make sense? I have only gone another 60 miles since the oil was refilled, but so far it looks OK. Should I be on the look-out for anything else?

I’d be on the lookout for another repair shop.

We’ve had EXACTLY the same issues but have replaced two catalytic converters and have spent nearly $5,000 in repairs! Can’t wait to follow this discussion . . .

Oil has nothing to do with the catalytic converter. They are blowing smoke. Definitely find a new mechanic.

Get into the habit of checking your oil regularly. That a car with that kind of mileage loses some is a given but It really should not be allowed to get that low. If you see the level drop, add a bit to top it off. That’s just part of owning a car.

Would you know what O2 sensor they replaced? It likely threw a code when the check engine light came on so would you know what that code is? (it looks like PXXXX with XXXX being a number).I believe your model/year has two O2 sensors, maybe three if you’re in CA. Someone that knows how they work should be able to tell which one is bad.
After they replaced it, they likely reset the system and it takes about 200-300 miles for the computer to determine whether the car’s systems are working properly. That change out likely was unnecessary.
I wouldn’t be too quick to replace the catalytic converter. Because they don’t want to see you back, a dealer will replace both O2 sensor and the CAT to make sure it is fixed but, more often than not, only one needs to be replaced.

Since oil is a hydrocarbon, if is passing out the exhaust it will pollute the catalytic convertor to failure. It is an effect of the car using oil but not the cause. Watch the oil level carefully and if it is using more than a quart in about 500 miles you will need to find a solution. It may be a stuck ring that feed itself if the oil consumption returned to normal after the cat replacement. It likely will come back.

If it still uses a lot of oil, try adding Marvel Mystery oil to the oil during one oil change and see if that clears things up either way. Make it a short oil change, no more than 3000 miles. Check your oil frequently from now on. If you run the car to low on oil that $1300 for a cat will be a fond memory after you see the bill for a new engine.

Good point, @Mustangman. Good clarification.
It can cause the cat to foul but the cat doesn’t cause the oil to run low.

“If you run the car to low on oil that $1300 for a cat will be a fond memory after you see the bill for a new engine.”

Somebody who is so negligent that he/she never bothers to check the dipstick, and instead (incorrectly) assumes that a warning light will save the day, is going to wind up with really big repair bills on this car and on all future vehicles–if this type of negligence continues.

Note to OP:
Now is the time to…
…read the Owner’s Manual and Maintenance Schedule in order to see what types of maintenance you have skipped…
…get into the habit of lifting the hood once a week and checking all fluids. Correct the oil & trans fluid levels as soon as you see a drop of 1/2 qt.

Usually an oil warning light means the engine it toast.

Excessive oil consumption CAN damage a catalytic converter

I had the same thing with my 2003 Forester at 90K–one catalytic converter replacement later and the dealer says he didn’t know what to do. Now with 190K and no problems. Doing all routine maintenance and up till that catalytic converter issue all done at the dealer.

If the oil warning sensor is an oil pressure switch – that’s what it is on both of my cars – it won’t turn on until the oil pump starts sucking air. As long as there is enough oil in the pan to completely fill the oil pump inlet, the oil pressure will be enough to keep the warning light off. And the oil can be quite low and still cover the inlet. This doesn’t mean there was no engine damage or abnormal wear due to the low oil though. Maybe so, maybe not. When the oil is very low, it recirculates back into the engine as fast as it drains out, and by being overworked like this can overheat or become very contaminated and gritty, both of which speeds engine wear or could damage some internal engine part outright.

My Oldsmobile Silhouette van blew its engine recently. The mechanic said it was bone dry yet I had checked the oil a week before, added oil, and when they towed the van back to me, the dipstick had oil on it. There was no oil draining out because I would have seen in in the snow in the driveway. I looked at a used Caravan with the engine light on. The owner had the catalytic converter replaced and it came on. Two mechanics told him they could not fix the light and not to worry as it was working right. I didn’t buy it because how would I know if anything else went wrong that works off that light? And the catalytic converter burns all the oil so you don’t see black smoke anymore if there is a problem. How do you ward off problems since sensors can’t seem to talk to components, especially if repairs have been done, even by the dealer?

@"Sled Doggie"‌ It would be better to start your own thread with a complete description of your problem. Tagging onto an old string means a lot of readers won’t see your problem because they’ll assume your post has something to do with that old thread about a completely different car. Good luck with your problem.

The root cause of the oxygen sensor failure and the catalytic converter failure is a small amount of antifreeze entering the engine combustion chamber over a long period of time. The silica in the antifreeze contaminates these parts. Plan on a head gasket job. Do not replace the converter until after the head gaskets are replaced or the converter will re-fail.

Yes, apparently the famous Subaru head gasket problem has appeared, leaking oil but not yet mixing fluids according to the shop. May have been leaking for a while, but was either never seen or not pointed out to me. I’ve never run the car low on oil and never overheated it. They want $2000 to replace gasket/machine head. I think that is more than the car is worth, but I need the car to live another 2 years. Still trying to decide what to do and praying it won’t blow soon. Sigh. I love this car, and the new one is too big!

@2000 is not chump change

However, it should buy you a few more years with this car

A few years ago, I fixed my brother’s older car, which wasn’t worth much. I provided free labor, and he paid for the parts. It served its purpose . . . cheap transportation, while he saved money to buy a newer car with low miles

What would concern me is the part about oil consumption and allowing the engine to run low enough on oil that it wasn’t showing on the dipstick.

The oil light does not have to be illuminated for an engine to suffer damage due to low oil levels.

If the problem with the head gaskets (plural) is simply oil leakage then maybe you could talk someone into trying to snug the headbolts up a little tighter without actually going in and replacing head gaskets.