Can MMo cause p0420 code? Got one using MMO

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Aug 8, 2005
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stamford, CT
Hi again all! 98 lesabte.had a new AC Delco upper O2 sensor replaced over 6 months ago.the reason? Code 0420,bank 1,sensor 1. All was running good until,last night. I put 8 ozs. Marvel mystery oil in with a empty tank. Its. Suppos d to be an 18 gallon tank..though in the orange,near empty,it will take 12_13 gallons fuel.directions are 4 ozs. Per 10 gallons.so,I put 8.
Got a cel code half hour ago, same thing.p0420. I have a scanner,so I erased it. Car runs fine.
I think,the MMO I put in in last nifht,some has gotten on the sensor. What should I do? Discontinue use of MMO? Put another tank of gas in without MMO after this,see if code dosnt come back?
Hopefully I won't have too have it replaced again,the sensor.seems the code showed, 24 hours in and 40 miles driving ,after putting MMO in.
 
So...do you know think a tank of no MMO,might just make it so he code doesn't come back? Clean the MMO off via combustion?
I US d Lucas upper cylinder punt cant,all spring and summer and fall..the big gallon and a half sized one.3 ozs.per 10 gallons.no code.
MMO seems too have coated the sensor. Would a tank after this now,but without MMO,clean the MMO thief mow,off the O2 sensor?
 
What should I do?
If your Lesabre runs Ok, leave it alone. Next time you get the urge to do it again, follow the directions on the can. However, if you use a quality fuel, you don't need to be adding chemicals in your fuel tank.
 
I had a LEXUS that didn't like any additives in the gas tank even when following directions/dosage. A lower dosage may have been OK but not optimal. Once a new take of gas went in, the CEL went away.
 
Back in the day I put Techron in my gf's 1998 Delta 88 with a fresh full tank of gas and immediately got a check engine light. From then on, no more additives of any kind.
 
Obviously you've got several MMO related variables at work here. Can't change what you did now. Going forward, quit the MMO. Fill with pure gas, drive and repeat. Hopefully after several full tanks, no more CEL. If not, may have to replace sensor again. Do I know if MMO caused CEL? No. But timing is interesting.

Good Luck.
 
Don't blame the MMO, after only 40 miles it hasn't had time to do anything good or bad. You might have gotten a bad tank of gas.
 
Don't blame the MMO, after only 40 miles it hasn't had time to do anything good or bad. You might have gotten a bad tank of gas.
It’s had plenty of time if he put in several times the recommended dose!

He added 8oz to a couple gallons. Call it 2oz/gallon, to be generous, it was probably higher.

MMO is recommended to be 4oz/10gallons. Or .4oz / gallon. So, best case, he’s at five times the dosage, and probably higher.
 
Take your car on a few 'long-legged' runs and get back to us.
There are too many unknowns.
Did you add other products?
Do you generally buy CGT (Cheapest Gas in Town)?
Could you have replaced the fuel cap incorrectly?
Could the Buick have vent lines on the filler neck where the MMO could've gone...resulting in a clog NOT UNLIKE brimming a modern gas tank?
ADVICE: Eat peppermint candy if you want. Don't buy peppermint scented sewing machine oil.
 
MMO has a higher specific gravity than gasoline. It may have had a slug of it while driving with the ratio you used.
What you should have done was add the MMO at the gas station prior to pumping.

I recommend filing with just gas and driving it. I doubt the O2 sensor is "coated". There are many oil burners out there that smoke more than my grand father with no codes. Just fill and drive and give us an update in 100 miles.
 
MMO has a higher specific gravity than gasoline. It may have had a slug of it while driving with the ratio you used.
What you should have done was add the MMO at the gas station prior to pumping.

I recommend filing with just gas and driving it. I doubt the O2 sensor is "coated". There are many oil burners out there that smoke more than my grand father with no codes. Just fill and drive and give us an update in 100 miles.
I did this.I added mmo on near empty tank.took.13 gallons. I'm assuming,the tank is 14_15 gallons? Car book says 18 gallons. So this I don't know,the actual tank capacity.on virtually empty,needle all the way down,it will take 14.5 gallons.did it once.
I USD Lucas ucl all spring,summer,fall,big bottle.no issues. But mmo this use,seems too have upset the sensor/CAT
 
I used Lucas ucl, big gallon and half container all spring,summer,fall. No hi ssues.one tank mmo...set off the upstream sensor /CAT. It is an aftermarket cat,I had a new exhaust system put on 3_4 years ago. Old cat was shot. Mechanic showed me old one..the comb in it,was completely broken ,cracked,holes in hi t from pieces missing,and rust galore. I do know aftermarket ones are nowhere near the quality of factory..won't last as long I have out at least 37,000 miles on it since installed.
Made me think.maybe the cat is on way out and not the mmo?I believe the cat,new one installed was a bosal.
I question it,because they did not install new muffler tailpipe.though it was on receipt.the clamps.were not new...I looked under it after.mechanics often do things like this. Why I try to do mY lol an stuff..so not ripped off.
 
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