SPN 3490 FMI 7 Purge AIR ACTUATOR

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Hello everyone. KW T680 Cummins ISX CM2350 getting code
Fault code 3225 PID
SPN 3490
FMI 7
AFT Purge Air Actuator mechanical system not responding
Replaced air actuator, put brand new - did not work, also replaced all air fittings so there is no air leak along the way to actuator. Replaced air dryer filter. Still code coming up, stays for few days and goes away randomly.
Any ideas what else need to check?