Case IH 19337 (DTC 344A) - ECU – DCU (Dosing Control Unit): DEF/AdBlue dosing system backflow line pressure too high plausibility failure

The system is detecting abnormally high pressure in the DEF return (backflow) line. This is almost always caused by the DEF fluid freezing inside the line because the electrical heating element integrated into the backflow tube has failed (open circuit). When the DEF pump attempts to purge the lines and return fluid to the tank, the blockage causes a pressure spike, triggering the fault.

• MAXXUM T4E
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Steiger Series (350–600 HP)
• Magnum Series (235–340)
• Farmall Series (40-50 HP)
• Patriot Series (2240, 4430)

1. 1

   Inspect the DEF backflow line (running from the dosing pump module to the DEF tank) for physical kinks, crushing, or external damage that could cause a mechanical restriction.

2. 2

   Disconnect the electrical harness connector for the DEF backflow line heater and use a multimeter to measure resistance across the pins. An infinite resistance reading confirms an open circuit (burned out heating element).

3. 3

   Verify battery voltage to the DCU. Check if codes 19361 or 19362 are present, which indicate voltage is too high or low for proper diagnosis.

4. 4

   If operating in freezing temperatures, perform a thaw test by moving the machine into a heated shop. If the code clears after thawing, the heater circuit is definitively the point of failure.

1. 1

   Preparation

   Park the machine on a level surface, turn off the engine, and turn off the battery disconnect switch. If the system is frozen, allow the machine to thaw in a heated environment to prevent damaging the plastic fittings during removal.

2. 2

   Relieve Pressure

   Ensure the DEF system has completed its purge cycle (wait at least 2 minutes after key-off before disconnecting the battery) to relieve residual pressure.

3. 3

   Disconnect Electricals

   Locate the DEF backflow line and disconnect its dedicated electrical heater harness.

4. 4

   Disconnect Fluid Lines

   Disconnect the quick-connect fluid fittings at the DEF dosing pump and the DEF tank. Catch any spilled DEF fluid and wash the area immediately with warm water, as DEF is highly corrosive to metal and wiring.

5. 5

   Remove the Line

   Unclip the faulty DEF backflow line from its routing brackets and remove it from the machine.

6. 6

   Install the New Line

   Route the new heated DEF backflow line exactly as the old one was routed. Ensure it is not pinched, stretched, or rubbing against sharp frame components.

7. 7

   Reconnect

   Snap the fluid quick-connects into place until they click, and plug in the electrical heater harness.

8. 8

   Clear Codes & Test

   Turn on the battery and key. Use a diagnostic scan tool (like the CNH EST) to clear the 19337 / 344A fault codes from the DCU. Run a DEF dosing system override test to verify that the backflow pressure is normal and the heater circuit is functioning.