Shoutout to the $60 timing light that told me what the internet couldn't
My 94 Civic kept throwing a code for the O2 sensor on the highway, so I bought a $60 timing light and in 20 minutes saw the base timing was off 8 degrees, which cost me $0 to fix. Everyone on the forums told me to just swap the sensor and $180 later the code came right back. What's the dumbest cheap tool that ended up saving you a real repair bill?