new fault code 1664 where to start ?

[JimT]

First, if you don't have free access to Cummins Quickserve, go sign up. Once logged in you can look up the fault code and then pull up the corresponding troubleshooting steps.

According to the troubleshooting steps you're supposed to:
1) look for other DOC or DPF fault codes that could cause this.
2) Insect, Clean and Test the aftertreatment injector.
3) Check for exhaust leaks between the aftertreatment injector and the DPF.
4) Inspect the DOC for damage or face plugging.

The DPF inlet temp is supposed to become hotter than the DOC inlet temp if the DOC is working properly. So, that means that the fuel being injected by the aftertreatment injector isn't being oxidized by the DOC resulting in a lack of combustion at the DPF (possibly occurring at or in the SCR can instead), or the fuel isn't reaching the DPF and is burning at the DOC face, or the fuel isn't being injected properly in the first place.

So, first pull the DPF doser, clean and test it to ensure it's working properly. If that checks out then look for exhaust leaks (the bellows pipe between the doser and the DOC inlet is a good candidate). If that doesn't yield any results your next step is to pull the DOC and thoroughly inspect it for damage.

My money would be on the doser injector or the fuel delivery to the injector.

Speaking of fuel delivery, have you serviced/updated your fuel pump?