HelloCustomer my name is Mike.
This is a direct copy from the owners manual duramax supplement. If you have already read this and have a question about why this proceedure has not worked please repost and I will continue to work with you.
Diesel Particulate Filter
The vehicle has a Diesel Particulate Filter (DPF) as part of the exhaust system to reduce vehicle emissions. The DPF requires a unique exhaust tailpipe with an exhaust cooler. The exhaust cooler mixes air with the exhaust to lower the temperature before it leaves the tailpipe.
The DPF, the tailpipe, or other exhaust system components must not be altered. Inspect regularly and clean any mud or dirt from the exhaust cooler, especially where the exhaust cooler connects to the tailpipe and the openings where fresh air enters the cooler. See "Exhaust System Inspection" under At Least Once a Year.
The DPF will clean itself as part of normal operation. Several factors including fuel consumed, hours of engine operation and miles driven are monitored by the Engine Control Module (ECM). The self-cleaning occurs approximately once per tank of fuel.
Notice: Permanent damage can occur to the DPF or related components if the required Ultra Low Sulfur Diesel (15 ppm sulfur maximum) or low ash CJ-4 engine oil is not used. This damage would not be covered by the vehicle warranty.
For the filter to clean itself, the vehicle must be driven above 30 mph (50 km/h) until the warning message goes off. This will take about 20 minutes.
Caution: During DPF self cleaning or during extended idling in P (Park), the exhaust system and exhaust gases are very hot. Things that burn could touch hot exhaust parts under the vehicle and ignite. You or others could be burned. Do not park, or idle for an extended period of time, near or over papers, leaves, dry grass, or other things that can burn. Keep the exhaust area clear of material that could ignite or burn. See Parking Over Things That Burn for more information.
Notice: Extended idle should be avoided because the DPF system is not capable of self cleaning at idle. During extended idle operation, monitor the instrument panel telltale lights and Driver Information Center for messages and take appropriate indicated action. Continued idling with the warning light/message on could cause irreversible damage to the DPF requiring repair and possible replacement that might not be covered by the vehicle warranty.
The DPF warning message comes on when the DPF is dirty and needs to perform a self cleaning. You will also notice a change in the exhaust sound and engine idle speed. This is normal. See DIC Warnings and Messages.
If you continue to drive with the DPF warning message on and the exhaust filter is not cleaned as required, the Malfunction Indicator Lamp and the ENGINE POWER IS REDUCED message will come on and dealer/retailer service is necessary. See Malfunction Indicator Lamp and DIC Warnings and Messages for more information.
Vehicles with DPF have specific fuel and engine oil requirements. See What Fuel to Use and Engine Oil for more information on those requirements.
Extended idling in P (Park) can cause exhaust parts and gases to become very hot. Keep the exhaust area clear of material that could ignite or burn. See Parking Over Things That Burn for more information.
For vehicles with Power Take-Off (PTO), monitor the instrument panel cluster for lights related to the DPF.
All engines consume some amount of engine oil. Ash is the by-product of engine oil combustion. The ash will become trapped in the DPF over the life of the vehicle. Eventually, the buildup of ash will restrict the exhaust gases and the DPF will need to be cleaned or replaced.
See Accessories and Modifications for important information if you are considering adding accessories or modifying the vehicle.