What is SPN 5584 FMI 15 & How to Solve It?

What is SPN 5584 FMI 15:

SPN 5584 relates to the DEF controller’s heater or system circuit, and FMI 15 means a valid signal is outside of its expected operating range, indicating partial heater failure.

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Why This Happens

• The DEF heater element is degraded or overworked.
• Control unit detecting unstable current draw.
• Poor connector contact causing voltage drop.
• ECM logic flagging unexpected temperature ramp behavior.

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How to Fix It

1. Test heater resistance and voltage supply.
2. Inspect connectors and replace corroded terminals.
3. Replace the DEF heater if output is inconsistent.
4. Clear the code and confirm heater performance during cold start.
   
   

How to Avoid It Before It Happens

1. Test DEF heater function before winter operation.
2. Replace heaters with high cycle counts.
3. Keep the DEF system dry and sealed to prevent freezing.