E Test

I have a 02 extra cab Taco with 277,000 on it, its completely stock and I just failed my etest. So here is my story,My cel has been on for a while, I have scanned it and it was a 0136 code, I looked it up and it said 02 was bad so I naturaly assumed it was the front air fuel sensor which is common for these, I ordered it paid $210 for it(Bosch) and threw it in, erased the cel and and thought I was ready to test. Well the day befor I was going to go the light came on again, I scanned it again and quickly realized that I bought the wrong one cause I didnt read the code completely ,anyway I talked to my mechanic buddy and he said dont worry about it, the second one has no bearing on the test because its after the cat. So I cleared the code and had it tested, well surprize surprize it failed, it failed on the CO by alot, I called him back and he said, clean the Maf and check the plugs, make sure they are denzo, sure enough the plugs were NGK with the single electrode. So I went and bought some Denzo plugs, Ive cleaned the Maf and Im going to run some Seafoam through it and hope that solves the problem, What do you guys think. I know I should just get the second O2 but I dont have time to wait for it to come in.