P10D9: Reductant Heater 1 Control Circuit Shorted to Control Circuit

What this code means

P10D9 may mean the DEF reductant heater control circuit is seeing a possible circuit-to-circuit fault.

What the vehicle may do

The vehicle may display a Service Emission System or Service Exhaust Fluid System message.
The reductant heater can be disabled when this fault is active.
The reductant injector can also be disabled as part of the emissions system response.

Possible fault areas

Possible wiring or connector problems in the reductant heater control circuits.
Possible reductant heater issue inside the reductant pump and sensor assembly.
Possible reductant control module involvement after other circuit checks are considered.

Diagnostic path

Opening context

On this 2020 to 2025 Chevrolet Silverado 2500 with the 6.6 diesel, P10D9 is aimed at the Reductant Heater 1 control circuit. In plain terms, the controller is seeing a possible control-circuit-to-control-circuit fault in the DEF heater circuit. The truck may show a Service Emission System or Service Exhaust Fluid System message, and the reductant heater and reductant injector can be disabled when this fault sets. Broadly, think possible wiring, connector, heater element inside the A38 reductant pump and sensor assembly, or reductant control module involvement. Heater 1 is inside that A38 assembly, and heater 1 and 3 are serviced as one unit. Also keep in mind, if reductant heater control circuits are shorted together, multiple heater faults can show up and the heaters can be disabled together.

Monitor gate and first verification

Start with the basic system checks, follow a structured diagnostic approach, and make sure the diagnostic category matches the fault you are chasing. If other enabling, reductant temperature, reductant heater, or communication-related codes are present, check what they mean first before chasing P10D9 by itself. For this code’s monitor, the ignition is on or the engine is running, ignition voltage is greater than 11 V, Reductant Heater 1 is commanded on, and the monitor runs continuously after the running conditions are met for greater than 12 s. P10D9 sets when the control circuit is shorted to another control circuit for greater than 12 s. For verification, turn the ignition on with the vehicle in service mode. Command Reductant Heater 1 on with the scan tool, then look at Reductant Heater 1 Current. The expected current is 1 to 14 A. If it is outside 1 to 14 A, go into circuit testing. If it is in range, reproduce the operating conditions, or reproduce the captured conditions from when the code set, and confirm P10D9 does not reset. If it stays gone, that part of the check is all OK.

Start circuit testing at the A38 connector

For circuit testing, power the ignition and all vehicle systems off. Disconnect the A38 Reductant Pump and Sensor Assembly connector. Inspect the A38 terminals and connector for damage or corrosion, and repair or replace as necessary. Then turn the ignition on with the vehicle in service mode and check voltage between control circuit terminal 9 and ground. The expected reading is 2.5 to 4.5 V. That terminal 9 voltage decides which direction the diagnosis goes next.

Terminal 9 low or high voltage branches

If terminal 9 is less than 2.5 V, turn the ignition, vehicle, and all vehicle systems off, then disconnect the K115 Reductant Control Module connector. Test for infinite resistance between control circuit terminal 9 and ground. If resistance is less than infinite, repair the short to ground on that circuit. If resistance is infinite, test the harness path between control circuit terminal 9 at the component harness and terminal 4 at the control module harness. The expected resistance is less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the circuit. If it is less than 2 Ω, replace the K115 Reductant Control Module. If terminal 9 is greater than 4.5 V, turn the ignition, vehicle, and all systems off, disconnect K115, then turn the ignition back on in service mode. Test voltage between control circuit terminal 9 at the component harness and ground. The expected result is less than 1 V. If it is 1 V or greater, repair the short to voltage. If it is less than 1 V, replace the K115 Reductant Control Module.

Terminal 11 short-to-ground check

If terminal 9 is between 2.5 and 4.5 V, move to the Reductant Heater 1 control circuit at terminal 11. Connect a test lamp between terminal 11 and B+, then turn the ignition on in service mode. The lamp should not turn on. If the lamp turns on, turn the ignition and vehicle off, remove the test lamp, and disconnect the K115 Reductant Control Module connector. Now test for infinite resistance between control circuit terminal 11 at the component harness and ground. If resistance is less than infinite, repair the short to ground. If resistance is infinite, replace the K115 Reductant Control Module.

Terminal 11 short-to-voltage and continuity checks

If that first terminal 11 test lamp check does not turn the lamp on, connect the test lamp between Reductant Heater 1 control circuit terminal 11 and ground. Turn the ignition on in service mode again. The lamp should not turn on here either. If it does turn on, turn the ignition and vehicle off, remove the test lamp, disconnect K115, then turn the ignition back on in service mode. Test voltage between control circuit terminal 11 at the component harness and ground. The expected result is less than 1 V. If it is 1 V or greater, repair the short to voltage. For a less-than-1 V result in this specific branch, pause, recheck the earlier diagnostic path, and avoid guessing at the next action. If the test lamp does not turn on, disconnect K115 and check the harness between control circuit terminal 11 at the component harness and terminal 16 at the control module harness. The expected resistance is less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the circuit. If it is less than 2 Ω, test or replace the A38 Reductant Pump and Sensor Assembly.

Final circuit check and static component test

After the A38 testing or replacement step, operate the vehicle within the same monitor conditions, or within the captured conditions from when the fault occurred, and confirm P10D9 does not set. If the code sets again at that point, replace the K115 Reductant Control Module. If it does not set, circuit testing is all OK. The static component test is separate: with the ignition and vehicle off, disconnect the A38 Reductant Pump and Sensor Assembly connector. Measure resistance between control terminal 9 and control terminal 11. The expected value is 1 to 6 Ω. If it is not between 1 and 6 Ω, replace the A38 Reductant Pump and Sensor Assembly. Then test for infinite resistance between each terminal of the component and the component housing. If resistance is less than infinite, replace the A38 Reductant Pump and Sensor Assembly. If resistance is infinite, the component static test is all OK.

Repair verification and takeaway

After the repair, verify the repair and confirm the code stays gone. If the Service Emission System or Service Exhaust Fluid System message is displayed, run the Reductant System Tamper Warning Service Bay Test; that procedure clears the driver information message for the applicable system. The takeaway on P10D9 is simple: verify the heater current first, then use the ordered circuit checks to separate a wiring fault, an A38 heater issue, and a possible K115 control module fault without jumping ahead. For more diagnostic training, visit stepdiagnostics.com.