The car engine P2131 DTC exactly mentions to the camshaft timing. In this situation, if the cam timing is over-retarded, the engine light will be illumined and the code will be set. Do not make more trouble in the car engine, you may face terrible problem in the car engine. Every parts of the car engine should be solved if any problem is located. An automobile engineer knows how to solve the car engine problem properly. Each car manufacturer uses a dissimilar techniques to check, but the most often it is some type of fuel tank compression sensor. I hope, you have found the car engine fixed properly.