SPN 5392 FMI 31: Meaning and Fix
SPN 5392 FMI 31 indicates the aftertreatment DEF dosing unit 1 has lost prime, meaning the pump cannot maintain fluid pressure. This fault commonly appears after a forced DPF regeneration when the DEF tank runs low, or after replacing the ECM without properly repriming the system. Air ingress or a failed dosing pump are typical root causes.
Common Symptoms
- MIL Illuminated: Malfunction indicator lamp and aftertreatment warning lamp turn on steady within 10 seconds of key-on.
- Derated Power: Engine torque reduced to 40% after 30 minutes of continuous fault active operation.
- No DEF Dosing: SCR system stops injecting DEF, causing NOx levels to rise above 5 g/kWh.
- Audible Pump Noise: Dosing unit emits a high-pitched whine or rapid clicking as it attempts to reprime.
Probable Causes
- Air Ingress: Air drawn into suction line due to loose fittings, cracked hose, or low DEF level below 10%.
- Clogged Pickup Screen: DEF tank pickup strainer blocked by crystallized urea, restricting flow to the pump.
- Faulty Dosing Pump: Internal pump wear or check valve failure prevents maintaining prime after shutdown.
- ECM Calibration Error: Incorrect pump prime sequence timing in ECM software after module replacement or flash update.
Advanced Technical Analysis
The ECM monitors dosing unit pressure via a feedback signal from the pump’s internal pressure sensor. When pressure drops below 3.5 bar for more than 5 seconds during active dosing, the ECM sets SPN 5392 FMI 31. The controller then enters a reprime cycle, attempting to build pressure three times before locking out dosing.
Electrical analysis shows the pump receives a PWM signal at 125 Hz. A duty cycle below 15% or above 85% for 2 seconds triggers a debouncing timer. If the condition persists for 10 consecutive seconds, the fault becomes active. Technicians should verify supply voltage at the pump connector is 24V ± 2V with no ripple exceeding 0.5V.
Once the fault is active, the ECM disables DEF injection and commands a 25% torque derate after 1 hour of operation. If the fault remains for 3 consecutive engine hours, a 50% derate is applied. The aftertreatment system also suspends passive regeneration, increasing soot load risk.
Long-term prevention includes replacing the DEF tank pickup filter every 2000 hours per Bosch guidelines. In a real workshop case, a MAN TGS repeatedly set this code after a DPF regeneration. The root cause was a partially clogged vent line creating vacuum, preventing reprime. Cleaning the vent and performing a manual prime using diagnostic software resolved the issue permanently.
Step-by-Step Troubleshooting Guide
- Check DEF Level: Ensure DEF tank level is above 15%. Top up if low and perform a manual prime via diagnostic tool.
- Inspect Suction Line: Look for cracks, loose clamps, or kinks in the hose from tank to dosing unit. Tighten or replace as needed.
- Test Pump Pressure: Connect a pressure gauge at the pump outlet. Prime manually; pressure should reach 4.5-5.5 bar within 10 seconds.
- Verify ECM Calibration: Check ECM software version against manufacturer bulletin. Reflash if prime sequence parameters are outdated.