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The code p20c9  is an informational code that the DEF deisel exhaust fluid system controller has detected a fault and logged a code.in isuzu 2016 mean the problem is in : 

1. Faulty Reductant Control Module
2. Reductant Control Module harness is open or shorted
3. Reductant Control Module circuit poor electrical connection

and to solve it you must fix it in dealer 
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Engineer mohammad

Dear Sir
Welcome, and thank you for trusting us
you are welcome
I hope my answer was helpful, please let me know if you have any follow up questions. If you feel I've answered your question already, please rate my answer from top of the page. Your feedback helps to ensure only the best experts are available to help you.
You can also ask questions of our other specialists in the areas of nutrition, therapists, IT specialists, medical doctors, and many more.
We are privileged to communicate with you, and you can always consult many experts on Jawabkom like Doctors, Nutritionists, and Engineers
Kind Regards
Engineer mohammad