P204B: reductant pressure feedback performance

What this code means

P204B generally means the reductant pressure feedback may not be matching what the reductant control module expects.

What the vehicle may do

The vehicle may display a Service Emission System or Service Exhaust Fluid System message.
Reductant system operation may be limited.
The vehicle may continue to run, but emissions aftertreatment operation can be affected.

Possible fault areas

Possible fault areas can include the reductant pump and sensor assembly.
Possible fault areas can include the reductant control module.
Possible fault areas can include wiring, connectors, the reference circuit, the low reference circuit, or the signal circuit.

Diagnostic path

Open: what P204B is pointing us toward

Today we’re focused on P204B on the Silverado 2500 diesel. In plain terms, this code may mean the reductant pressure feedback is not lining up with what the reductant control module expects. The truck may show a Service Emission System or Service Exhaust Fluid System message, and reductant system operation may be limited. Broadly, we’re looking at the reductant pump and sensor assembly, the reductant control module, and the reference, low reference, and signal circuits between them. Don’t jump past a circuit fault and condemn a module too early. For this monitor, ignition voltage has to be Greater than 11 V, and the P204B monitor runs after ignition Off, after the running conditions are met, for greater than 33 s. The code sets when reductant pressure is not within a calibrated range for greater than 6 s. When it sets, the reductant injector, tank heaters, and pump are disabled, and the warning message is displayed.

Verification before circuit testing

Begin with ignition On and the vehicle in Service Mode. Check whether P204B is currently set. If P204B or related reductant pressure sensor circuit codes are present, check what they mean first instead of treating this as an isolated code. If the code is not set right now, reproduce the operating conditions, or reproduce the captured conditions from the vehicle data. Then verify P204B does not reset. If it does reset, go into circuit testing. If it does not reset, the result is All OK.

Low reference check

For circuit testing, turn the ignition, vehicle, and all vehicle systems Off. Keep in mind it may take up to 2 min for all vehicle systems to power down before an accurate ground or low reference continuity test can be made. Disconnect the A38 Reductant Pump and Sensor Assembly connector. Now test between Low Reference circuit terminal 3 and Ground. You want less than 20 Ω. If that reading is 20 Ω or greater, disconnect the K115 Reductant Control Module connector and test from Low Reference circuit terminal 3 at the component harness to Terminal 21 at the control module harness. That path should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in that circuit. If it is less than 2 Ω, replace the K115 Reductant Control Module.

5 V reference check

If the low reference test is less than 20 Ω, turn ignition On with the vehicle in Service Mode. Test between 5 V Reference circuit terminal 5 and Ground. The expected range is 4.6 to 5.4 V. If it is less than 4.6 V, turn the vehicle Off, disconnect the K115 Reductant Control Module, and test from 5 V Reference circuit terminal 5 at the component harness to Ground for infinite resistance. If it is less than infinite resistance, repair the short to ground on that circuit. If it is infinite resistance, test from 5 V Reference circuit terminal 5 at the component harness to Terminal 19 at the control module harness. That should be 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 the original 5 V reference reading is greater than 5.4 V, turn the vehicle Off, disconnect the K115 module, turn ignition On in Service Mode, and test 5 V Reference circuit terminal 5 at the component harness to Ground. That should be 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.

Signal circuit check

If the 5 V reference is between 4.6 and 5.4 V, move to the signal circuit. Test between Signal circuit terminal 4 and Ground. You want greater than 10 Ω. If it is 10 Ω or less, repair the short to ground on the signal circuit. If it is greater than 10 Ω, test between Signal circuit terminal 4 and Low Reference circuit terminal 3. That should be less than 1 V. If it is 1 V or greater, repair the short to voltage. If it is less than 1 V, test between Signal circuit terminal 4 and 5 V Reference circuit terminal 5. The expected range is 4.6 to 5.4 V. If it is less than 4.6 V, turn the vehicle Off, disconnect the K115 Reductant Control Module, and check continuity from Signal circuit terminal 4 at the component harness to Terminal 20 at the control module harness. The test callout here is less than 2 kΩ. The next decision point has a unit conflict, so do not guess. Pause, recheck the earlier diagnostic path, and resolve the value conflict before condemning a circuit or module. If the signal-to-5 V-reference test is between 4.6 and 5.4 V, replace the A38 Reductant Pump and Sensor Assembly.

Verify the repair and clear the message when applicable

After the circuit or component repair path is completed, keep verification separate from testing. Operate the vehicle within the monitor conditions again, or within the captured conditions from the vehicle data, and verify P204B does not reset. If the code resets at that point, replace the K115 Reductant Control Module. If the code does not reset, the result is All OK. Then 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 only for that message condition. Performing that function clears the driver information message for the applicable system. The takeaway: prove the low reference, reference, and signal circuits before replacing the pump and sensor assembly or the reductant control module. For more diagnostic training, visit stepdiagnostics.com.