04-31677-000 | Genuine Freightliner® DEF Header Lever Sending Unit 13/23 G

1. Monitoring DEF Level

• Measures how much DEF is in the tank

• Prevents the vehicle from running out

• Sends level data to the dashboard gauge

Without accurate level data, the vehicle may trigger warnings or limit engine restart.

2. Monitoring DEF Quality

• Checks urea concentration (32.5%)

• Detects contaminated or diluted DEF

• Prevents damage to the SCR catalyst

3. Monitoring DEF Temperature

• DEF freezes at 12°F (-11°C)

• Signals the system to activate heaters

• Prevents dosing issues in cold weather

4. Supplying DEF to the System

• Contains the pickup tube that feeds DEF to the pump and dosing injector

1. DEF Gauge Reading Incorrect

• Shows empty when full

• Level stuck at one reading

• Jumps around suddenly

This is one of the most common failures.

2. “Service DEF System” Message

• Appears even after refilling DEF

• May not clear on its own

3. DEF Countdown Warning

• “No engine restart in ___ miles”

• Countdown continues even with full DEF tank

This happens because the system can’t confirm fluid level or quality.

4. Check Engine Light

Common trouble codes may include:

• DEF level sensor circuit codes

• DEF temperature sensor codes

• DEF quality sensor codes

• Communication codes

5. Incorrect DEF Quality Message

• “Incorrect DEF Quality”

• Even when using fresh, correct DEF

6. Reduced Engine Power (Limp Mode)

If the issue isn’t fixed, the vehicle may:

• Limit speed

• Prevent restart after shutdown