SPN 3360 FMI 3: Meaning and Fix
SPN 3360 FMI 3 indicates the aftertreatment 1 DEF controller 1 has detected voltage above normal or a short-to-high condition on its internal supply or sensor circuits. This fault typically appears after a DEF pump replacement or during cold-weather operation when ice formation stresses wiring. Technicians often see this code following a failed DEF line heater short, causing the controller to lock out dosing and initiate a 25% torque derate after 50 engine hours.
Common Symptoms
- SCR Derate Active: Engine torque reduced by 25–40% after 50 hours of continuous fault, per J1939 safety strategy.
- MIL and Amber Lamp On: Dashboard malfunction indicator lamp and amber warning lamp illuminate immediately upon fault detection.
- DEF Dosing Stopped: Diesel exhaust fluid pump ceases injection; NOx conversion efficiency drops to zero.
- Frozen Display Indications: DEF level and quality readings freeze on the dash, showing last known values before the short.
Probable Causes
- Harness Chafing: DEF controller supply wire abrades against frame rail or engine bracket, causing short to battery voltage.
- Failed DEF Heater: Internal short in the DEF line heater element back-feeds 12V into the controller sense line.
- Corroded Connector: Moisture ingress at the 6-pin controller connector creates a conductive bridge between pins.
- ECM Driver Fault: Internal failure of the ECM’s 5V reference driver supplies overvoltage to the controller.
Advanced Technical Analysis
The ECM continuously monitors the DEF controller’s internal supply voltage via a dedicated analog-to-digital converter pin. If the raw ADC reading exceeds 4.85V for more than 500 ms, the diagnostic sets FMI 3. Bosch MD1 and MD1CE controllers use a 5V ±0.25V reference; any sustained reading above 5.25V triggers the fault. The timer prevents false triggers from transient load dumps.
In a short-to-high scenario, the DEF controller’s internal voltage regulator enters overvoltage shutdown, dropping the communication bus voltage below 2.5V. The ECM detects the missing CAN messages and cross-references the ADC overvoltage flag. Deutz factory manuals specify checking resistance between pin 2 (supply) and pin 5 (ground) – should be >10 kΩ; a reading below 100 Ω confirms a hard short.
Upon fault confirmation, the ECM activates a progressive torque derate: 25% after 50 hours, then 50% after 100 hours. The DEF pump is disabled, and the SCR system enters a safe state where no dosing occurs. Mercedes-Benz OM471 engines also log a freeze-frame of battery voltage at fault onset, aiding diagnosis. The derate is reversible only after the fault is cleared and a full key cycle is performed.
Long-term prevention involves inspecting the DEF harness routing during every DPF cleaning. Real-world cases show that after replacing a failed DEF pump, technicians forget to check the heater circuit, leading to a recurring short. MAN TGS trucks frequently exhibit this fault after a DEF line heater replacement if the new heater’s insulation is not properly seated. Always perform a 48-hour soak test after repair.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect DEF controller harness from frame rail to pump for chafing, cuts, or melted insulation near exhaust.
- Measure Supply Voltage: With key on, engine off, measure voltage at controller pin 1 to ground. Expect 4.75–5.25V.
- Isolate Heater Circuit: Disconnect DEF line heater connector; if fault clears, replace the heater element and check for internal short.
- Perform Pin Drag Test: Remove controller connector; check each female pin for spread or corrosion. Replace if drag force <1.5N.