SPN 4358 FMI 18: Meaning and Fix
This fault indicates the SCR differential pressure sensor reports a value below the normal operating range. In practice, this code often appears after a forced DPF regeneration when a partially clogged pressure line clears suddenly, causing a momentary low reading that latches the fault. Technicians may also see it after replacing the ECM if the sensor calibration was not transferred.
Common Symptoms
- No derate active: Engine power remains normal as the condition is moderately severe, not critical.
- MIL illumination: Amber malfunction indicator lamp turns on steady within one drive cycle.
- Intermittent code: Fault may self-clear after key cycle if pressure returns to normal range.
- No urea dosing: ECM may disable SCR injection to prevent over-dosing due to false low pressure.
Probable Causes
- Sensor drift: Internal MEMS sensor element output shifted negative due to thermal cycling or age.
- Blocked reference port: Atmospheric reference port on sensor clogged with ice or debris causing false low reading.
- Exhaust leak upstream: Cracked flex pipe or loose clamp before SCR allows pressure to bleed off.
- Wiring high resistance: Corroded 5V reference or signal return line causes voltage drop mimicking low pressure.
Advanced Technical Analysis
The ECM monitors the differential pressure sensor via a 5V reference, signal, and low reference circuit. The internal microcontroller samples the analog voltage at 100 Hz, then applies a 2-second debounce timer. If the averaged value remains below 0.10 kPa (0.4% of sensor span) for the entire debounce period, the fault is set. This logic prevents nuisance codes from transient pressure spikes during regeneration events.
Electrical analysis must include measuring sensor output at key-on, engine-off. A healthy sensor should read 0.00 ± 0.02 kPa. If the output is negative (below -0.05 kPa), the sensor has internal offset drift. Per Bosch factory manuals, the sensor uses a piezoresistive bridge with active temperature compensation; a failed compensation circuit can cause a permanent negative offset of up to -0.15 kPa.
When FMI 18 is active, the ECM does not command immediate torque derate but stores the fault and disables closed-loop SCR control. The system reverts to an open-loop urea dosing map based on engine speed and load. This can lead to under-dosing and increased NOx emissions. The fault becomes active only after the debounce timer expires, which prevents false triggers during cold starts.
Long-term prevention involves verifying that the sensor’s atmospheric reference tube is routed away from water splash areas. In MAN D26 engines, technicians frequently encounter this fault after steam cleaning the engine bay, where water enters the reference port. The recommended fix is to replace the sensor and install a protective cap per Deutz service bulletin D-2019-043.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check SCR inlet and outlet pressure lines for kinks, cracks, or loose connections.
- Sensor output test: With ignition on, engine off, measure sensor signal voltage; should be 0.50 ± 0.01V.
- Reference port check: Remove sensor and gently blow through reference port; ensure no obstruction present.
- Wiring integrity: Check 5V reference and signal wires for corrosion; resistance must be below 2 ohms.