Catalytic converter


I own a 2003 Subaru Forester with 96k miles on it. the “check engine” warning light came on and the mechanic claimed that the catalytic converter was going bad. He turned off the warning light and told me of a inexpensive way to solve the problem. Run 5 or 6 tanks of premium fuel (91 octane in CO) the start using the mid grade (87 octane)instead of regular (85 octane). I’ve never heard of a fix like this. Is this true? Thanks.


No. I’ve never heard of that one before, but the endless list of magical fixes for bad cat’s never ceases to amaze me. The octane rating in gas is only a measure of how much of an additive the gas has which makes it more subject or less subject to preignite. (knock) I don’t for the life of me have any idea how the octane rating would be related to fixing a worn out cat. It’s actually frightening to hear how many snake-oil-type salesmen are out there. I think in this case he might not be purposely misleading you, (because he has nothing to gain by you purchasing a more expensive grade of fuel) but perhaps (giving him the benefit of the doubt) was just misinformed and passing on some bad information without realizing it. Sorry. If your cat is bad, you can do a lot of neat little tricks, but the only one that will fix your problem is replacing the bad cat. If you knew how much CO (carbon monoxide) and HC (hydrocarbons - ie: unburned fuel) that the cat converted into cleaner air during its lifetime, you would have sympathy for it and give it a proper burial.


IF the engine has been misfiring, because it has been running gasoline which has octane that is too low for it to run smoothly, higher octane could help; but, that’s a big IF.
Keeping the engine in tune, and performing the routine scheduled (by miles) maintenance, will prevent a lot of raw fuel from reaching the catalytic converter and overloading it, and the oxygen sensors, with deleterious effects. This, in the end, is the cheaper alternative to changing a lot of expensive parts later.


Octane will not cause mis-fire one way or the other. Nor will using premium fuel restore a tired cat.


Wow. And did he tell you why this was?

Catalytic converters are unaffected by the octane of the gas unless it creates an engine operational problem. And, once something like that affects a converter, which can only happen with contamination of the platinum palladium on the ceramic matrix from perhaps burning oil or by collapse of the ceramic from overheating due to, say, too lean an operating temperature (which would trigger other problems like preignition) changing octanes will not heal it. Even the downstream oxygen sensor that monitors the performance of the converter if damaged by burning oil would not heal from a change in octane.


The engine can be affected by octane levels. An engine which has carbon deposits will have a higher compression ratio, which, if high enough, can necessitate a higher octane fuel in order not to misfire and or ping. When an engine misfires, the fuel it did not burn goes to the catalytic converter. Poorly burned fuel will produce soot (carbon) which coats the inner surfaces of the exhaust, the catalytic converter, and the oxygen sensors.
An engine which is restored to a better operating condition, can help to clean up the exhaust system. A tune up, and, if needed, higher octane fuel (because of ping and misfire), could help in this cleanup.


I cannot be convinced that a cat with poor efficency can be “revived” by using high octane fuel and I cannot accept that using lower than required octane will cause a missfire, I looked through my tech books and I just can’t make it work.

I am willing to keep listening about the missfire idea.


Some perfectly healthy engines run worse on higher octane fuel, rather than, better. Try it. It’s a cheap experiment.
You can get an idea of what the catalytic converter looks like, and what may be going on in the engine by looking at the oxygen sensors. Here are photographs which show oxygen sensors which have had lead poisoning, excessive carbon / soot, and silicon contamination: Click on the stamp-sized image.


Full tank of 91 gasoline worked for my Dodge stratus in the past 10 years to pass smog test. Will it work with octane booster for 1/3 tank and 91 gasoline for 2/3 tank, because RAM van has 33 gal tank!?