The general meaning of the car engine P2172 Code is p for Powertrain Code Problem is related engine, transmission and emissions systems. 2 for MFG – Manufacturer Specific. 1 for Transmission. 7 for Vehicle Speed Output Circuit and 2 for A/C System High Pressure High Temperature. The valves of every cylinder are remained closed by using a pair of arms what substitute the conservative engine’s roller-type rocker arm. The car engine P2172 code is one of these arms what follows the cam profile while the 2nd arm joins to the valves. During normal engine operation the 2 major arms are linked by a locking pin.