If you see any black smoke or soot residue at the stack or tail pipe outlet, your DPF has failed and is allowing Diesel Particulate Matter (PM) to bypass the DPF and must be replaced. There are several other types of DPF failures that are not as easy to detect because there are no visible signs. Some other common conditions that may indicate that your DPF may not be functioning properly are frequent DPF plugging or your engine may be experiencing shorter intervals between regenerations,