SPN 6802 FMI 0: Meaning and Fix
This diagnostic code indicates the aftertreatment 1 DEF dosing system (tank, lines, pump, doser) is frozen and cannot thaw. The ECM monitors DEF temperature and pressure; when ice prevents dosing, FMI 0 reports data valid but above normal operational range. Technicians frequently encounter this after overnight sub-zero parking without heated lines. The system will not resume dosing until thaw is verified by temperature sensors.
Common Symptoms
- Engine derate active: ECM reduces torque up to 40% to protect aftertreatment, limiting vehicle speed to 5 km/h.
- DEF warning lamp on: Amber or red DEF lamp illuminates on dash, accompanied by a 10-minute countdown timer.
- No DEF injection: Doser remains closed; NOx conversion stops, causing tailpipe emissions to rise sharply.
- Frozen tank indication: Tank level sensor shows static reading; heater circuit draws high current but ice persists.
Probable Causes
- Ambient temperature below -11°C: DEF freezes at -11°C; if heaters fail or are undersized, ice blocks the system completely.
- DEF heater circuit failure: Open circuit or blown fuse in tank or line heaters prevents thawing, per Bosch DENOX 2.2 manuals.
- Faulty DEF temperature sensor: Sensor reports -30°C even after engine warm-up, causing ECM to assume persistent ice condition.
- Blocked DEF return line: Ice plug in return line prevents pump circulation, keeping frozen fluid trapped in doser.
Advanced Technical Analysis
The ECM evaluates DEF temperature via two NTC sensors (tank and doser). When both read below -11°C for more than 30 seconds, the controller sets SPN 6802 FMI 0. The logic uses a debounce timer to avoid false triggers during transient cold starts. If the temperature rises above -5°C within 10 minutes, the fault self-clears; otherwise, a permanent derate is applied per SAE J1939-71.
Electrically, the DEF heater circuit is a PWM-controlled resistive load. The ECM monitors current draw; a drop below 2A indicates an open heater. MAN TGS workshop manuals specify checking the 30A fuse in position F14. A shorted heater (above 20A) blows the fuse instantly. The debouncing timer (5 seconds) prevents intermittent contact from setting the code prematurely.
When frozen, the ECM initiates a safety fallback: it disables DEF dosing, sets the derate to 25% torque initially, then increases to 40% after 30 minutes of driving. The DEF pump is de-energized to prevent cavitation damage. Deutz TCD 6.1 manuals confirm that the aftertreatment system enters a ‘warm-up only’ mode, allowing engine operation but prohibiting regeneration.
Long-term prevention requires verifying heater operation during pre-winter service. In practice, technicians often find this code after a truck sits idle in -20°C overnight. A common fix is to run the engine at high idle (1200 rpm) for 20 minutes to allow coolant heat to thaw the tank. If the code persists, measure resistance across the tank heater (typically 1.5 ohms) and check the relay.
Step-by-Step Troubleshooting Guide
- Scan DEF temperature: Read both tank and doser temperature PIDs; if below -11°C, run engine at high idle to thaw.
- Check heater fuses: Inspect fuses F14 and F15 (30A) in the main fuse box; replace if blown per MAN workshop manual.
- Measure heater resistance: Disconnect tank heater connector; measure 1.5-2.0 ohms across terminals. Open circuit means replacement.
- Verify pump operation: Command DEF pump on via diagnostic tool; listen for priming sound. No sound indicates frozen pump or wiring fault.