Cummins ISB 6.7 SPN 3246 FMI 0 — DPF Outlet Temperature Very High

Code Details

Plain-English Meaning

The Cummins ISB 6.7 aftertreatment system uses a dedicated temperature sensor at the outlet of the diesel particulate filter to monitor exhaust gas temperature exiting the DPF. During a normal active regeneration, the 6th injector delivers a controlled quantity of diesel fuel into the exhaust stream upstream of the DOC, raising exhaust temperatures to oxidize and burn off the soot accumulated in the DPF. The outlet temperature rises as the soot burns and then returns to normal exhaust temperature once the regen cycle completes. When SPN 3246 FMI 0 is set, the outlet temperature sensor reading has exceeded the critical high threshold, indicating an uncontrolled thermal event inside the DPF. On the ISB 6.7, this is most commonly associated with the 6th injector (also called the hydrocarbon dosing injector) not shutting off when commanded, an injector internal leak, or an extremely high soot load regen that produced more heat than expected. The ECM responds by inhibiting regeneration and may impose a derate to protect the aftertreatment components.

SPN 3246 is Aftertreatment 1 Diesel Particulate Filter Outlet Gas Temperature; FMI 0 indicates valid signal data above the normal range at the highest severity level. On the Cummins ISB 6.7 (CM2350 B101 ECM), the aftertreatment system monitors temperature across multiple sensor positions: DOC inlet, DPF inlet, DPF outlet, and SCR inlet on equipped configurations. The ECM uses these readings to manage the active regen duty cycle and to detect over-temperature conditions. Cummins INSITE can display all live aftertreatment temperature values and the freeze-frame snapshot taken at the time of fault set. The INSITE 6th injector actuation test confirms whether the hydrocarbon dosing injector opens and closes on command. If the injector does not shut off cleanly, raw fuel continues to enter the exhaust and oxidize inside the DPF, producing temperatures well above the critical threshold.

Common Symptoms

• DPF or check engine warning lamp illuminated
• Engine derate or speed limitation active
• Active regeneration halted or inhibited
• Exhaust odor of raw or partially burned fuel
• Multiple aftertreatment codes present in INSITE alongside SPN 3246 FMI 0

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

• 6th injector (hydrocarbon dosing injector) stuck open or not seating when commanded off
• Internal fuel injector leak on one or more cylinders allowing fuel past the combustion chamber
• Very high DPF soot load causing an extended regen with elevated temperature output
• DPF outlet temperature sensor failure reading higher than actual exhaust temperature
• Excessive engine oil consumption introducing additional hydrocarbon load in the exhaust

First Checks

• Connect Cummins INSITE and read the freeze-frame data for SPN 3246 FMI 0 to identify the temperature peak and any simultaneously active codes
• Monitor live aftertreatment temperature values across all sensor positions to determine whether the over-temperature is localized to the DPF outlet or spread across multiple sensors
• Use the INSITE 6th injector actuation test to verify that the hydrocarbon dosing injector opens and closes on command and does not continue to deliver fuel when commanded off
• Review cylinder balance data in INSITE to identify any cylinder with abnormal fuel contribution that may indicate internal injector leakage to the exhaust
• Inspect the DPF for signs of physical damage from thermal events — elevated differential pressure even after a regen suggests substrate damage may have already occurred

Can I Keep Driving?

The engine derate may be severe. Do not attempt a forced parked regen while this fault is active. Identify and correct the source of excess hydrocarbon input before initiating any regeneration to avoid further aftertreatment damage.

Related Lookup Pages

• FMI 0 explanation
• Cummins manufacturer hub
• Aftertreatment fault codes
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