Detroit Diesel SPN 4353 FMI 18 – DEF Injection Quantity Low (Meaning, Causes & Fix)

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Detroit Diesel SPN 4353 FMI 18 indicates that the amount of DEF being injected into the exhaust stream is below the quantity required by the ECM to properly reduce NOx emissions. This code appears on DD13, DD15, and DD16 engines and points to a problem within the DEF delivery system that will affect SCR performance and can trigger a derate condition if not resolved.

Symptoms

  • Check engine light on
  • DEF or SCR warning light illuminated
  • Engine derate condition possible
  • Reduced SCR system efficiency
  • May appear alongside other DEF dosing or pump related codes

Cause

This code triggers when the ECM determines that the actual DEF injection quantity is less than the commanded amount. Common causes include:

  • Worn or failing DEF pump not delivering adequate flow
  • Clogged or partially blocked DEF dosing injector
  • Low DEF level in tank Restricted DEF supply lines
  • Faulty DEF metering valve Air in the DEF supply system
  • DEF filter clogged restricting flow to injector

Can You Keep Driving?

Limited driving may be possible but with insufficient DEF injection the SCR system cannot properly control NOx emissions. The engine will enter a derate condition if the delivery shortfall is not corrected. Do not ignore this code.

Recommended Action

Start by checking DEF tank level and refilling if low. Inspect the DEF filter and replace if clogged. Check DEF supply lines for restrictions or air ingestion points. If level and filter are good have the DEF pump output and dosing injector tested as wear or partial blockage in either component is a common cause of this fault.

What does Detroit Diesel SPN 4353 FMI 18 mean?

It means the DEF system is injecting less fluid into the exhaust than the ECM requires to properly reduce NOx emissions through the SCR catalyst.

What causes SPN 4353 FMI 18?

The most common causes are a worn DEF pump, a partially clogged dosing injector, low DEF level, a clogged DEF filter, or restricted supply lines.

Can I keep driving with SPN 4353 FMI 18?

Short distances may be possible but insufficient DEF injection will cause SCR efficiency to drop and a derate condition will follow if not addressed.

Will SPN 4353 FMI 18 cause a derate?

Yes. When DEF injection quantity falls below required levels the SCR system cannot meet emissions thresholds and the ECM will trigger a derate condition.

How do I clear SPN 4353 FMI 18?

Check DEF level and filter first then inspect the dosing injector and pump for wear or blockage. Once the system confirms DEF is being delivered at the correct quantity the code should clear.

Related codes

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Important Notice

The information provided above is for general informational purposes only and should not replace official OEM service procedures. Always consult the manufacturer’s service manual or a qualified diesel technician before performing repairs. Operating a vehicle with active fault codes may result in further damage.