This is so true!

I did a carbon cleaning on my 2014 CX-5 GT, and I'm never going to do it again. Granted it came with 158k miles and severe misfires. New plugs, a replaced coil, and premium gas fixed 99% of that, but I believe the carbon cleaning did help - at great cost.

The reason why I'd never do it again is because I destroyed my AFR sensor and O2 sensor, costing me over $200 on parts , and hours of time diagnosis and research. I did not have a check engine light before the job, but it sure lit up immediately after!
I had a P0421:00-27 Efficiency Of Catalyst 1 Is Lower Than Limit - Bank 1and thought I killed the catalytic converter. I just installed my second rear O2 (swapping out the crap Bosch I'd just bought and replaced with NTK) and after dozen miles and monitoring the live data, I think I'm safe.

I believe my issue was that I used Berryman's B-12 and Gumout carb cleaner. Too much of this stuff entered the combustion chamber and some unburned crap got into the exhaust.

Next time, I would gladly pay a shop to do a walnut blast instead of using petroleum products to get the carbon out.

 

We use and install BOSCH o2 and more specifically HO2 ( I wish people would get what oxygen sensor they have on their vehicles correctly named on forums) all the time without any going bad. They are after all the founder of the highest quality HO2 (and o2s) in the industry. When I read that DIY have trouble it is generally 99% they damaged the sensors unknowingly when installing it.

NEVER NEVER use any chemicals around an engine with sensors unless the cleaners and additive state clearly SENSOR SAFE.

For those that are better DIY or us techies we all know that a PO421 often times is a simple exhaust gasket leak and not to always jump at the first internet youtube informational on how to solve it. As with ALL DTC's there is a proper way to diagnosis the cause and randomly replacing unconfirmed parts is a great way to create more issues then the original. Not to mention costly.

As for carbon build up inless an engine by way of a scope indicates an excessive amount, your EGTs are higher then normal, the power is distinctly lower, as well poor throttle response at lower engine speeds, the temperature of coolant seems to be slightly higher then norma.... (and all these can be a cause of there problems not necessarily carbon build up, the labor to have a shop perform this should be carefully considered and talked to thoroughly with your "TRUSTED" shop Technician to know if there is a truly beneficial service that will benefit you in the long term of your daily driver?
My strong advise is to have the engine completely health test first. You may find that the cause of some or most of the carbon build up may not be because your engine is a DI and that there is other things causing it like a normal none DI engine?

 

@sstearns2 were you using a vacuum following the walnuts while spraying?

 

Since the above post is very recent, I'm wondering if the recommendation against cleaning intake valves with Sea Foam or CRC intake valve cleaner also applies to the 2.0L that I have in my 2015 (with manual transmission)?

I'm nearing 100K miles and am tackling a few maintenance items and would like to PM intake valves if possible.

 

Seafoam can somewhat help cars with carbon buildup, but it's really a case by case basis.

 

you can not introduce either in the intake runner realistically. The other thing is most of these products really require soaking the runner with the intake vale closed for a day or more. Things users don’t tell you after spending 7-15 dollars for a can of those cleaners. I am talking about a DI engine .
For fun I tried soaking an intake and exhaust port on a SOHC engine.After letting both cleaners you mention for a couple days it barely even change the color of the cleaners. I also tried 100% Xylene and it at least started to make improvements after a day. It was also summer and the ambient was about 100+f.

 

Thanks i was considering doing that over walnut blasting.