P0657

P0657 means the PCM detected the Actuator Supply Voltage A circuit either open or out of range — this is the dedicated power feed the PCM uses to supply electronic actuators (electronic throttle body, EGR motor, intake-manifold runner control, turbo wastegate solenoid, and similar high-current loads). Depending on platform, the rail is regulated to either 5V (low-current actuator logic) or switched battery voltage (10-14V, high-current actuator power). On VW/Audi TSI/TDI and GM E67 ECM platforms specifically the rail powers the electronic throttle motor — a fault here will set the throttle into limp-home mode immediately. Shop-floor ladder, cheapest first: identify which actuators are on Supply A from the FSM. With KOEO, backprobe the supply wire at each actuator connector; you should read either steady 5V (regulated) or battery voltage (switched) depending on platform. If supply reads 0V at every actuator, the rail is open between the PCM and the first device — wiring or PCM internal driver. If supply reads correct at some actuators but low or 0V at others, the open or short is between the last good and first bad. Disconnect each Supply-A actuator one at a time and watch the rail; the actuator that, when removed, lets the rail return to spec is the loader. Expensive-misdiagnosis caveat: P0657 on VW/Audi will almost always be the throttle body itself (internal short pulling the supply down) rather than the ECM — VW even has TSB coverage on the electronic throttle for specific MY ranges. Don't quote an ECM until the throttle body is disconnected and Supply A measured with the throttle isolated; a $400 throttle masquerading as a $1,500 ECM job is the worst kind of misdiagnosis.

Vehicle-specific patterns: 2008-2014 VW/Audi 2.0T TSI (GTI, A4, Passat, Tiguan) and 2.0L TDI commonly throws P0657 from electronic throttle body internal shorts — the throttle motor draws excess current as the brushes wear and pulls the actuator supply rail down; the fix is a new throttle assembly with adaptation reset via VCDS. 2010-2015 GM LS3 / L99 / LSA / LT1 with the E67 ECM (Camaro SS, CTS-V, Corvette) throws P0657 from throttle body motor failure or from harness chafing at the throttle connector where the supply wire runs adjacent to the heat shield. 2009-2013 Audi 3.0T supercharged V6 (S4, S5, Q5) throws P0657 from intake-manifold-runner actuator internal shorts loading the Supply A rail. 2007-2012 GM 3.6L direct-injection V6 throws P0657 from throttle body or EGR position actuator failures. Estimated repair: $250 (throttle body DIY with TSB-covered platform discount) to $2,000 (ECM replacement + programming + throttle body when isolation goes wrong).