It really depends. It may just be an efficiency issue; however, it may also be an issue with clogging. Clogging will rob you of power, adversely impact your mileage and cause abnormally high battery utilzation, which can wear it out.

A common practice is to pour a gallon of acetone into your gas tank, top it off and drive it until nearly empty. This supposedly purges accumlated carbon on the cat honeycomb structure. It's a cheap fix if it works. Make sure you use a funnel and don't get it on your paint.

An anti-fouler on the downstream O2 sensor can make the code go away.

 

A lot of threads on this topic suggest that the acetone isn't so good for the fuel system or the engine seals. It would be a waste of time if the cats were degraded rather than clogged. With a vacuum gauge one can test for a clogged cat. If it's really a bad oxygen sensor the acetone won't do squat.

There are commercial fuel additives that claim to clean cats, but I'm dubious. So was this guy:


and this guy actually got smog readings before and after:


This is, I think, the most informative video around, demonstrating pretty convincingly that cleaning a failed cat is a waste of time. However, if the cat was only clogged, the way this guy did it might work.

 

I already put the acetone in my car, I didn't read your last message. Let see what happens in 1 week.

 

The fears concerning the fuel system components are a throw back to the first days of ethanol. If a fuel system can be used with E10, then it can handle acetone.

 

Acetone is a stronger solvent than EtOH. It is definitely harder on various types of plastic and rubber. You might think that gasoline would always be a stronger solvent than either, but that isn't the case for some substances. For instance:

http://mykin.com/rubber-chemical-resistance-chart

LEGEND: 1 = Satisfactory, 2 = Fair, 3 = Doubtful, 4 = Unsatisfactory, X = Insufficient Data
(various rubber types ---> )
M.C. = Methylene Chloride, common brake cleaner component.

Fuel system parts are going to be chosen from substances with a "1" in the gasoline row. There are four of them listed, two of them are doubtful for EtOH and the other two safe. Presumably nowadays with E10 they would choose materials that are safe for both gasoline and EtOH. Of those two, one is also safe for acetone, and the other will fall apart if exposed to it. This is why I don't think that adding acetone to the gas is such a great idea.

If the acetone is already in try to burn through it as quickly as possible.

 

The malfunction indicator lamp turned off! Thank you very much Keith.

 

For PC, cataclean MSDS reports a composition similar to lacquer thinner...

https://www.tetrosylexpress.com/home.../KALCAT002.pdf

With a significant portion being acetone.

It may be temporary, but it's a good sign. A slight accumulation of carbon or other contaminants on the honeycomb can be cleaned this way. Those contaminants can impact cat efficiency and trip the P0420 code.

Contaminants can be due to mixture problems resulting from misfires, but you usually have associated codes when that happens (likely need new plugs or coils). More commonly, it's due to an accumulation of burned oil. If high mileage, it's probably just a result of the slight oil burn over the life of the car/cat. It's wise to monitor your oil level to ensure your oil consumption isn't high.

I wish I had tried this before replacing the cat on my 2nd 06 HCH2. Got a Magnaflow for about $500. Was pretty trivial to install, but I would rather have dropped $15...

 

Not surprising - they need to make it out of something cheap, preferably with a nice chemical odor.

Additives which work before the combustion chamber have an outside chance of doing something.

The mechanism of action for those said to work after the combustion chamber is pretty darn obscure. Put a small molecule made of C,O,H into a running car motor and it will come out as CO, CO2, H2O, NOx and a tiny, tiny, tiny amount of the original molecule. Some of the gasoline also gets through. Normally the cat then breaks down all of the remaining gas and anything else which can still be burned, including any acetone.

That assumes the motor is working normally though. If there is a cylinder which is screwed up, so that it burns incompletely or possibly burns more oil than the others, a more substantial amount of unburned hydrocarbons will get through. The glop from that cylinder might have led to the cat to become clogged, and there is a slight chance that acetone (or anything else) might get through in high enough concentration to help that cat. But the car still has an unresolved issue and once the additive's cleaning is done it is just going to foul again eventually.

I have had exactly one car's cat fail. The sequence was, the mechanic decided to clean the injectors, which magically raised the MPG from 24 to 27. Two years later the cat died. I don't think it died because the injectors were working better, but because of the several years before that they had been out of balance resulting in too much unburned gasoline getting through.

 

I'm French, I know the exact words in French but not in English. Today, the car engine stopped a few times while I was stopped. Also, sometimes, when I accelerate, there is a delay in acceleration. Maybe the right expression is loss of throttle, please let me know. Now, the weather is really hot in Miami, and my gas tank is 60% full. I hope this problem will go away once the tank is empty, and I refill it with only gas.

 

We say "stall" if it is the motor stopping because it is not running right. That word is not usually used if the car stops because of an electrical problem, as in, it stops like somebody turned off a switch.

Hesitation. Some people also say "stumble". They may be two different things though. A car where you press the gas and it doesn't accelerate right away, but doesn't make any untoward noises has hesitation but not stumble. A car where you press the gas and the engine does odd things and then slowly accelerates has both hesitation and stumble. At least that's how I use the terms.

This is with the acetone in the tank? Cars are designed to burn gas and gas mixed with EtOH, not acetone. The acetone may be just enough to mess up the effective octane rating such that the car is not firing at exactly the right time, or is running with too much or too little (effective) fuel, so the engine runs "a little off".

When an oxygen sensor went out on our Accord it was also doing both of these things. My impression is that pretty much anything that makes the motor run poorly, especially too rich or too lean, will give you those symptoms.