Asbestosis

Hello. I have just fitted a tow bar to my vehicle, I was advised to disconnect the ECU and the battery from the electrical circuit when welding on the vehicle which I did, when completed I reconnected the ECU and refitted the sensor on the air inlet tube just beyond the air filter (which I had to disconnect to gain access to the ECU). The vehicle is now very low on power (it has had a stage 1 performance upgrade) and the brakes are no longer boosted, the pedal is hard. All I have done to the wiring is mentioned above. Can anyone enlighten me on possible causes of the faults I have listed???