There are presently two main types of cylinder deactivation used today, depending on the type of engine. The engine P1635 code is found for the pushrod design which uses solenoids to change the oil pressure carried to the lifters. In their collapsed state, the lifters are powerless to raise their companion pushrods under the valve rocker arms, resultant in valves that cannot be activated and remain closed. Solve the problem from the car engine as soon as possible if you want drive the car safely. The catalytic converter has an oxygen sensor what is in front and behind it. When the car is hot and running continuously, the upstream oxygen sensor waveform reading should alter.