Regen Failures and Derate on a Route Bus
A 2017 transit bus with a 6.7 L diesel and DOC/DPF/SCR aftertreatment keeps setting a DPF restriction fault and going into derate. The bus runs a stop-and-go city route with frequent short trips. The driver reports the regen lamp comes on and the bus sometimes goes into a low-power mode before the driver can complete a parked regen. Soot load estimated by the ECM is 180% of the trigger threshold. Exhaust backpressure at high idle is 8 in-H2O above the clean-filter baseline. The DPF differential pressure sensor reads 0.2 psi at key-on, engine off.
1. What are the likely reasons the bus cannot complete a regen, and how would you verify each?
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A city route with short trips is a classic passive-regen failure mode. The ECM sees soot load at 180% of trigger, so active regen is being commanded but not completing.
Likely causes and verification:
- Insufficient exhaust temperature: Short trips may not allow the DOC to light off. Verify DOC inlet and outlet temps during a commanded regen; a small rise across the DOC indicates a failed or poisoned DOC.
- Excessive soot load beyond active regen capability: At 180% of trigger, a parked regen may not be enough. A dealer-level forced regen or DPF removal/cleaning may be required.
- Sensor error: The DPF differential pressure sensor reading 0.2 psi at key-on, engine off is a red flag. It should read near zero. A biased sensor can falsely trigger restriction faults. Verify with a known-good sensor or a manometer.
- Leaks or restrictions: Check for exhaust leaks upstream of the DPF and for a restricted DOC.
- Driver behavior: Short trips may never allow a complete regen. Consider a route change or scheduled highway runs.
2. The differential pressure sensor is replaced and reads 0.0 psi at key-on. Backpressure is still high. What is the next step?
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With a corrected sensor and confirmed high backpressure, the restriction is real. Next steps:
- Perform a dealer-level forced/parked regen if soot load allows, monitoring DOC inlet/outlet temps and DPF outlet temp. If temps do not rise as expected, the DOC is suspect.
- If forced regen cannot reduce backpressure, remove the DPF for cleaning or replacement. Ash loading from oil consumption can permanently raise backpressure and cannot be burned off.
- Check oil consumption and use of the correct low-ash oil; excessive ash is a common cause of irreversible DPF restriction.
- After repair, verify backpressure returns to baseline and clear adaptations.
- Address the root cause (route duty cycle, oil consumption, or a failed DOC) so the fault does not recur.