My catalytic converter was stolen this last weekend. In order to not sound like a lawn mower I got a local shop to weld a straight pipe to the muffler since I’m not legally required to have a converter where I live. After they did that though I got a check engine light. It was a code 12,21, and 51. I know the code 12 is the battery, but the 21 and 51 are with the O2 sensor. So I looked around and bought one of those things that pull the sensor out of the exhaust pipe to “trick it”. And it cleaned the code 51 but the code 21 is still there. Do I need to replace my O2 sensor, my MAP sensor? Or is it just due to running no converter?