Case IH 19337 – DCU (Dosing Control Unit): System frozen and not free in time | Defreezing Mode and Detection Errors (Back-flow line defreezing failed) | P20B9 Reagent - backflow tube heating - open circuit.

The Dosing Control Unit has detected an 'open circuit' (infinite resistance/broken connection) in the heating element specifically for the DEF backflow (return) line, causing the system to fail to thaw the line within the allotted time limit.

• Tier 4a
• Tier 4b
• Case IH Magnum Series: 235, 260, 290, 315, 340 (Series 4)
• Case IH Steiger Series: 350, 385, 400, 435, 450, 500, 540, 550, 600
• Case IH Puma & Maxxum Series

1. 1

   Visual Inspection: Inspect the DEF backflow line (returning from the dosing injector to the DEF tank) for physical damage, chafing, or melted sections.

2. 2

   Check the Connectors: Disconnect the electrical pigtail for the backflow line heater and inspect the pins for corrosion, moisture, or pushed-out terminals.

3. 3

   Ohm Test the Heater Element: Using a multimeter, measure the resistance across the heater pins on the DEF line. If the reading shows 'OL' (Open Loop) or infinite resistance, the heating wire inside the tube is broken.

4. 4

   Check Relays and Fuses: Check the DEF line heater relays and fuses in the emissions fuse block. A blown fuse or failed relay can trigger open-circuit codes.

5. 5

   Harness Continuity: If the line's heater element tests good, test the machine-side wiring harness for continuity back to the DCU to rule out a broken wire in the main harness.

1. 1

   Preparation

   Turn off the machine, remove the key, and turn off the battery disconnect switch. Allow the exhaust and DEF system to cool completely.

2. 2

   Access the Lines

   Locate the DEF backflow line routing from the dosing module/injector back to the DEF tank.

3. 3

   Disconnect Electrical

   Unplug the electrical connector for the backflow line heater.

4. 4

   Disconnect Fluid Fittings

   Release the quick-disconnect fluid fittings at both ends of the line. Note: Have a catch pan and rags ready, as residual DEF will spill. Clean any spilled DEF immediately with warm water to prevent corrosive crystal buildup.

5. 5

   Remove the Old Line

   Carefully unclip the line from its routing brackets and zip ties. Note the exact routing path.

6. 6

   Install the New Line

   Route the new heated DEF line exactly as the old one was routed. Ensure it is secured away from sharp edges, moving parts, and direct contact with hot exhaust components.

7. 7

   Reconnect

   Push the quick-disconnect fluid fittings onto the ports until they audibly 'click' and lock into place. Reconnect the electrical pigtail.

8. 8

   Clear Codes and Test

   Turn the battery disconnect back on. Use an Electronic Service Tool (EST) to clear the 19337 / P20B9 fault codes. Turn the key on to allow the DEF pump to prime the system, and inspect the new line connections for leaks.