Phosphate materials lock onto and contaminate the catalyst surface. These and other poisons collect and deactivate the catalyst surface over time. 

Gradually covering the active catalyst surface over time; these materials masks the active catalyst surface and block interaction with exhaust gas.  Contaminants and catalyst poisons also infuse deeper into the catalyst coating which caused further deactivation. 

In some cases, the DOC will clog and restrict exhaust gas flow. 

When DOC performance falls below a certain threshold, it cannot keep itself clean, or support reliable particulate filter regeneration. 

The DOC is a key exhaust emissions control device, which is also critical to Reliable Particulate Filter Regeneration regimes. 

Low DOC performance results in poor emissions, incomplete filter regenerations and ultimately leads to particulate filter failure. 

One role of a DOC in the system is to scrub diesel exhaust emissions. In the case of Particulate Filter equipped exhaust systems , the DOC also raises exhaust gas temperature and increases nitrogen dioxide concentration in the exhaust stream. These combined effects are crucial for reliable particulate filter regeneration and performance. 

Typically, filter regeneration events begin when the electronic fuel control system injects excess fuel into the exhaust stream. The excess unburned fuel reacts strongly inside the catalyst which then creates substantial heat, and increases the nitrogen dioxide in the exhaust. 

When the DOC is operating at a low rate of performance, insufficient heat is generated. Without the required high exhaust temperature, the Particulate Filter will not Regenerate, and fails.