Mercedes CLS DPF Fix: Diagnosing and Repairing a Faulty Pressure Sensor (With On-Car Clean)

Mercedes CLS DPF Fix: Diagnosing and Repairing a Faulty Pressure Sensor (With Safe On-Car Clean) A Mercedes CLS Shooting Brake came in with the engine light on and poor performance. The culprit, a faulty DPF pressure sensor that froze the fill level at 400%. Here’s how the fault was diagnosed, the sensor replaced, and the filter cleaned safely on the car. Spotting the signs of a DPF problem This CLS runs the 2.1 diesel. It arrived with the engine management light on and a lack of power. Common signs: * Engine light on * Limp mode or low power * Fault codes for DPF and sensors Plugging in the diagnostic tool The OBD port sits under the dash. The X431 Euro from Launch connected to the ECM with the ignition on. Reading the fault codes Main codes found: P245414 differential pressure sensor circuit P246309 Soot content of the diesel particulate filter is not ok P244800 the pressure differential in the particle filter is too high P244A00 the differential pressure in the particulate filter is too low P244A7A the differential pressure of the DPF is too low, a leaky seal was detected * DPF differential pressure sensor faults * Soot accumulation * Oxygen sensor fault The owner had a burst intercooler hose fixed earlier, so the O2 fault likely came from that air leak. Why the oxygen sensor fault might be linked Air leaks skew readings, so the oxygen sensor can log a fault. Live data helps confirm it’s healthy. Checking live data for clues Key PIDs: engine speed, soot content, DPF fill level, DPF pressure. The fill level was stuck at 400%, and the pressure value jumped from 900 to minus 50 mbar. Parameter Before fix DPF fill level 400% (Locked in) Pressure at idle ~900 to -50 mbar (erratic) Graphing the data for better insight Revving the engine did not move the sensor reading. The data was locked, pointing to sensor failure. Getting under the bonnet Removal steps: 1. Lift off the engine cover. 2. Release the air pipe clips and disconnect the plugs. 3. Undo the ECU bracket bolts, then lift the airbox. Parts can be hot after running. Accessing the DPF pressure sensor With brackets eased back, the sensor sits below. Use a magnet tray to keep bolts safe. Tools used: E10 bits and clip pliers. Removing and inspecting the faulty sensor The sensor has two hoses, one pre-DPF and one post-DPF, to measure pressure difference. Electrical failure is common. Several of these get replaced each week. Comparing old and new sensors The original had one skinny port, the replacement had two equal-size ports. Spares were on hand to match. Installing the new DPF pressure sensor The new vs. old unit was fitted, hoses reconnected, and wiring clipped back. Verifying the sensor with live data * Idle: about 30 mbar * 3,000 rpm: about 200 mbar The sensor now responded cleanly. The importance of full diagnosis before cleaning A quick note. Cleaning a blocked DPF without fixing the cause wastes money. Always fix the root cause first. Many mobile cleaners do not diagnose, which leads to repeat faults. Risks of improper DPF reset Forcing a regen on a full DPF can push temps toward 1,000°C, which can melt or crack the core. you would fit a new DPF at this point. Preparing and performing the DPF cleaning Launch UK fluid was injected into the larger pressure hose with about 8 bar (120 psi) air. The foam fills the DPF and breaks down soot at low temperature, around 100°C. Why this method is safer No risky ECU tricks. No high-heat regen on a loaded filter. Monitoring the cleaning process Hold 3,000 rpm and watch pressure drop. Foam exited the tailpipe as it worked. Target pressure after cleaning sits in the 30 to 60 mbar range. Resetting the system and clearing codes Drift compensation was reset for the differential pressure sensor and oxygen sensor, then the DPF fill level was set to zero. All plugs were refitted, codes cleared, and live data checked. Final live data check Engine off: 0 mbar. Idle: reading present. 3,000 rpm: about 60 mbar, dropping toward 40 as it cleared. Lambda readings looked healthy. Test drive and power restoration The car now revved past 3,000 rpm with full power. Post-drive, pressure settled around 40 mbar at 3,000 rpm. Mileage was about 177,000 miles. No codes present and the engine light was out. Conclusion A stuck DPF fill level often points to a failed sensor, not a dead filter. Replace the sensor, clean the DPF safely on the car, reset adaptations, and verify with live data. If you see 400% fill and erratic pressure, start with the sensor, then clean. Want more fixes like this on modern diesels? Stick around for the next one.