Ok... A question that is probably an age old debate but I think I have found the answer...will Seafoam burn out my o2 sensor? So I bought a used 2003 Suburban 5.3l (not a flex fuel) w/ 165k miles on it. Original owner, service records, overall great shape- save for the 165k mile part. I figured I could renew it slowly, doing 1 item per month and 2 years down the road I am at the engine phase. I planned on doing plugs/wires, fix leaking oil pan, reoiling the K&N and that was really all that needed to be done, it still seemed to run great. I had heard great things about Seafoam from both reputable 1st person conversations and internet speak, and I also heard the caveats but none of those seemed to have facts to back em up...so in anticipation of changing out the plugs and wires, I put a can of Seafoam in 1/2 tank (+ or - 16 gallons) of gas. I ran that around until it was down to about 1/8th of a tank and then had about a 1/2 can in the garage I ran through the power brake booster. I know it says 1/3 of a can but I tried to kill the engine with a heavy pour the last few ounces and it got away from me. Started it up and the standard (A LOT of) white cloud of smoke greeted me. I went a couple stop lights and jumped on the freeway and drove it probably 2 miles before getting off (white smoke gone at mile 1). I have to say that post-Seafoam, I did notice a bit smoother idling and accelerating. I drove it for another 20 miles or so thinking Seafoam IS tune up in a can. At that point I thought SUCCESS and went home and proceeded to clean the engine bay for the plug/wire change. I covered the alternator, hit it with the degreaser and hosed it down. I dried what I could, AutoGlymed the engine bay and it got parked it for a day or two. Got in to go somewhere, started it up and POOF!! A big cloud of smoke from under the hood! I popped the hood and a greyish smoke was coming from under the drivers side exhaust manifold, it went away pretty quick so I thought it may be just some dressing/degreaser burning off and had no issue with how it was running, until I got in. I noticed that my mileage in the DIC was dropping .1 every 10 sec or so when it was idling (I keep it on the mileage thingie for the most part). I took it out and it seemed to run fine, except the horrible mileage. No check engine light and I thought one of the plug wire wasn't firing or something (they looked old). I changed out the plugs/wire yesterday and it made a world of difference in throttle response and power (the old plugs were VERY beat up Bosch dbl plats) but mileage still and issue. There also seems to be a rich smell coming from the drivers side of the engine bay. I am waiting on an OBDII scanner to show up in the mail but, long story short, after a bit of research it appears as though a bad o2 sensor would do exactly what I am experiencing. Runs fine, except mileage and no CEL light. That coupled with the fact I Seafoamed the engine, it has 180k miles on it now and smoke started coming from the exact location of the sensor would lead me to believe I would be about a 90% I need to replace the sensor without waiting for the reader? Thanks in advance for any help!