Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Didn't last 20,000 miles

Today I removed my Autolite xp605 Iridiums trying to eliminate a misfire(at partial throttle) I've had for almost a month...and that's after $300+ on other ignition related parts and sensors trying to find the problem. I changed my plugs even though I had under 20k miles on them. I did not suspect them to be bad but I was just trying to narrow the possibilities down. Having never checked resistance on a spark plug before, I decided to try it. I found where these plugs have a reliable range of 2k to 20k ohms. Four of my six plugs ranged from 4k to 6.1k ohms, one at almost 16k, the other at 246k! I then test drove my truck... smooth as could be. I spent about $7-8 a plug right after they came out. I'm very disappointed. Stock AC Delcos are back in.