The catalyst/DOC may need to be replaced over time and especially if any of the following upstream engine issues have occurred (replacement is dependent on the extent and length of the failures noted below):
EGR failure and/or coolant leak into the exhaust system
Turbo failures that lead to lube oil contamination into the exhaust system
In-Cylinder injector failure that causes poor engine operation and incomplete combustion
When there has been an upstream engine issue such as those mentioned above, the engine will need to be repaired accordingly. However, depending on the extent of the failure, a DPF and/or DOC replacement may also be required. Please note: frequent active regeneration cycles may also be an indication that the DOC may be failing.
The constituents of coolant and engine oil are catalyst poisons. In low amounts the constituents are harmless to the catalyst, however as the amount increases, they become highly poisonous to the catalyst. If the DPF/DOC has been severely exposed to coolant or engine oil, then it's likely that cleaning the DOC may not aid in salvaging and restoring the chemical activity of the catalyst.
To determine whether the DOC is still chemically reactive, it is recommended to perform an active or forced regeneration cycle. Please note: this test does not conclusively determine whether or not the DOC is the root cause of the problem, however it does provide some very useful information and troubleshooting feedback.