A 2016 Mercedes Vito 447 III comes into the bay with the check engine light illuminated and a severe loss of power. The technician pulls three codes from the system: P246B09, P246392, and P030273.
This combination is a classic example of diagnostic misdirection. The symptom is limp mode and two of the codes point directly to the diesel particulate filter. If a technician focuses on the DPF, they will waste hours trying to force a regeneration that the engine control unit will simply refuse to perform.
Tracing the root cause of the blocked filter
The key to this job is the third code, P030273, indicating a misfire on cylinder 2. When a cylinder misfires, the combustion is incomplete or nonexistent. This drops the exhaust gas temperature significantly. A DPF requires high temperatures to perform an active regeneration.
Because the exhaust is too cool, soot accumulates rapidly, triggering code P246392. Once the soot mass reaches a critical threshold, the engine control unit registers that regeneration conditions cannot be met, logging P246B09 and throwing the van into limp mode. The DPF failure is entirely secondary to the misfire.
Testing the R9M engine piezo injectors
Our diagnostic engine ranked a defective fuel injector on cylinder 2 as the most likely cause, assigning it a 90 percent probability with high severity. The piezo injectors used in this specific R9M engine are highly sensitive to internal wear and electrical faults. A faulty injector delivers an incorrect fuel spray pattern or no fuel at all, causing the misfire.
The diagnostic path here starts with a leak-off test to measure return flow. The next step is a visusal inspection of the injector connector and wiring harness for damage. If those pass, swapping the cylinder 2 injector with another cylinder is a definitive way to isolate the fault. If the misfire moves with the injector, the component is confirmed dead.
Verifying mechanical compression and glow plugs
If the injector checks out, the next logical step is mechanical compression. We ranked low compression on cylinder 2 at 70 percent probability. Worn piston rings, a damaged valve, or a failing cylinder head gasket will bleed off the pressure needed to ignite the diesel charge. A relative compression test using a diagnostic tool, followed by a mechanical leak-down test, will confirm this without major disassembly.
A less severe but still highly relevant cause is a faulty glow plug or damaged glow plug harness on cylinder 2, ranked at 60 percent. A failed glow plug creates inefficient combustion during cold starts and specific load conditions. A simple resistance measurement of the plug and a voltage check at the harness will rule this out in minutes.
Dealing with the secondary DPF restriction
Even after locating and repairing the misfire on cylinder 2, the job is not necessarily finished. We ranked a heavily clogged DPF at 50 percent probability as a compounding issue.
The soot accumulation caused by the misfire remains in the filter. Technicians must check the real-time differential pressure and soot mass values using their diagnostic tool. If the backpressure is extreme, the filter will not regenerate even with a forced command. At that stage, the DPF requires professional off-vehicle cleaning or total replacement.
Look at your own repair orders from the last month. How often are your technicians fighting a DPF code that is actually just a symptom of an upstream combustion failure?
Reference pages for the codes in this article
- P0302, what we measure for this code across real diagnostics