I bought a 2004 Honda Civic LX the other day. It ran and drove beautifully for the cost (1700) and miles (174,000). The check engine light was on but we were 90% sure it was the upstream (wide band) air/fuel ratio sensor.

I drove the car home and noticed the breaks jumped slightly, like the fuel line was leaking but it wasn't too bad. I went out and bought a Denso o2 Sensor for my car (Denso part number 234-9005) and installed it. Check engine light went away and the car started right up.

Drove out to get gas and noticed that braking was even harder, like the car was fighting itself to drive and brake at the same time. When I parked the car, it was idling at 2500 RPM and I couldn't shift out of park. I unlocked the shifter with my key and turned it on in neutral (Still idling at 2500 RPM) and drove home. The car was holding 30mph on it's own and even accelerated up to 40mph without any pressure on the pedal.

Once home, I tried troubleshooting and ended up turning the car off and on a few times but now the car won't even take my key. I can't push my key into the ignition without meeting resistance. The only feedback I get from the car is beeping, which you can do by pushing a screwdriver into the ignition gently.

I'm desperate for help.