5502-02 | Genuine Paccar® DEF Coolant Flow Control Solenoid 12V

1. Prevents DEF from freezing

• DEF freezes at 12°F (-11°C)

• The solenoid allows hot engine coolant to flow to DEF components

• Thaws frozen DEF after cold starts

Why it matters:
Frozen DEF can’t be pumped or injected → emissions faults and derate.

2. Maintains proper DEF temperature

• Opens only when heating is needed

• Closes when DEF reaches operating temperature

Why it matters:
Keeps DEF within the optimal temperature range for accurate dosing.

3. Enables proper SCR operation

• SCR system relies on correctly heated DEF

• Ensures DEF is injected at the right time and amount

Why it matters:
Proper SCR operation keeps NOx emissions low and meets regulations.

4. Prevents engine derate and limp mode

• Avoids DEF heater-related fault codes

• Keeps truck from entering reduced power mode

Why it matters:
Protects drivability and prevents downtime.

5. Improves system efficiency and component life

• Avoids overheating DEF components

• Reduces unnecessary coolant flow

• Protects DEF pump, lines, and dosing module

Why it matters:
Extends component life and reduces maintenance costs.

6. Supports emissions compliance

• Required to meet EPA and emissions standards

• Works automatically under ECM control

Why it matters:
Keeps the truck legal and compliant.

1. DEF heater or SCR fault codes

• Check Engine Light on

• DEF / SCR warnings on dash
  Why: The ECM detects improper DEF temperature or heater operation.

2. Engine derate or limp mode

• Reduced power

• Speed limited after countdown
  Why: DEF system can’t meet emissions requirements.

3. DEF system not warming up

• DEF stays cold in freezing temps

• DEF tank heater ineffective
  Why: Solenoid stuck closed—coolant can’t reach DEF components.

4. DEF overheating (less common)

• DEF temperature too high

• Repeated heater-related faults
  Why: Solenoid stuck open—coolant flows constantly.

5. Frozen DEF messages in cold weather

• “DEF Frozen” or “DEF Heater Fault” warnings
  Why: Coolant heat never reaches the DEF tank or lines.

6. Poor DEF dosing performance

• Increased DEF consumption

• NOx efficiency codes
  Why: DEF temperature not in the correct range for accurate injection.

7. No clicking sound from solenoid

• No audible click when commanded on/off
  Why: Electrical failure inside the solenoid or no power/ground.

8. Coolant leaks near the solenoid

• Visible coolant residue

• Low coolant level
  Why: Internal seal failure or cracked housing.

9. Electrical fault codes

• Open circuit / short to ground codes
  Why: Failed solenoid coil or damaged wiring.

10. Intermittent issues

• Works sometimes, fails other times
  Why: Solenoid sticking internally due to contamination or corrosion.