After you have successfully retrieved the fault codes from the truck and you get Fault Code SPN: 5394 FMI: 5, make sure you follow these steps to troubleshoot properly.
Check Battery connections: make sure they are securely connected.
Check wires from and to DEF Doser: look for rips and make sure it is plugged in securely.
Check your DEF filter, located underneath the DEF tank.
![](https://trucking.wiki/wp-content/uploads/2021/09/DEF-DOSER.png)
Always remember to give the truck adequate time to cool down before handling the DEF Doser.
Disconnect and Check Doser Tip: This part is the most labor-intensive, but oftentimes it is the one that will most likely need to be changed.
See the picture below of bad Doser.
![](https://trucking.wiki/wp-content/uploads/2021/09/DEF-DOSER-CODE.png)
Picture of a new Doser
![](https://trucking.wiki/wp-content/uploads/2021/09/NEW-DEF-DOSER.png)
Last Step: Delete Codes
You will need to delete codes after you have successfully installed a new Doser.
Common Symptoms Of A Faulty Doser.
If you have taken your truck to a shop and they do a forced regen and delete your codes only to have the fault codes reappear after a day or two, it might be time for a new DEF Doser.