11i/R12 Concurrent Program/Request : Pending Standby or Inactive No Manager

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When I run a Concurrent request I get Phase – Pending and the Status – Standby. What does pending standby means and how to fix it ?
When I run a request in GL,the status appears Inactive and phase as No Manager. Please tell me what the problem could be ?
These are some of common questions I receive in my mail box so I decided to dedicate a post on concurrent request life-cycle.
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Concurrent Request Lifecycle
Concurrent Request/Program can be under one of four Phases . Each phase has different Status .
Phase : Pending, Running , Completed, Inactive

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Status :
Pending - Normal, Standby, Schedules, Waiting
Running – Normal, Paused, Resuming, Terminating
Completed – Normal, Error, Warning, Cancelled, Terminated, Disabled
Inactive – Disabled , On Hold, No Manager
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A. Pending Standby  - Phase Pending and Status Standby means Program to run request is incompatible with other program(s) currently running.
How to check Incompatible Program/Request for any Program/Request ?

• Login with Application Developer responsibility
• Click on Concurrent > Program
• Query Program
• Click on Incompatibilities button

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B. Inactive – No Manager
i) No manager is defined to run the request
or
ii) All managers are locked by run-alone requests
or
iii) Concurrent Manager Service is down
or
iv) No concurrent manager process because of workshift
To check Work Shift for any Concurrent Manager From System Administrator responsibility > Concurrent > Manager > Define > Work Shifts > Processes