What this code means
P20E8 may mean the reductant system is not building expected pressure for DEF dosing.
What the vehicle may do
- The vehicle may operate in derate mode.
- The driver may notice reduced power.
- A system warning message can remain after repair until the vehicle is driven and the message clears.
Possible fault areas
- Possible low reductant fluid level.
- Possible restriction, leak, or air in the reductant pressure side.
- Possible reductant injector dosing concern.
- Possible reductant pump pressure concern.
Diagnostic path
Set up the problem
On this 6.7 Power Stroke, P20E8 points to a reductant system that may not be building pressure the way the control module expects. The truck may be in derate, it may feel down on power, and even after the fault is repaired, the warning message can remain until the vehicle is driven long enough for it to clear. Think in broad terms first: possible low DEF level, a restriction or leak in the pressure side, a dosing injector issue, or a pump pressure problem. Also, if regeneration starts while you are testing, let it finish before you keep going, because regeneration can bias the values you are watching.
Understand when P20E8 sets
The pressure target matters here. P20E8 sets when the reductant pressure does not exceed 350 kPa (51 psi) within 45 seconds. So the diagnostic path is not just asking whether the pump runs. It is asking whether the system can build enough pressure, then deliver the correct amount through the injector.
Start with the code check
Start with the basic system checks. With the ignition ON, look at the codes that are present. If other reductant, pump, module, or communication codes are present, check what they mean first instead of chasing P20E8 in isolation. For P20E8 specifically, the path moves straight to checking reductant fluid level, but the shared pump checks are still useful context when another related fault sends you there.
Shared pump and circuit checks
When the code path calls for the shared pump checks, start simple with a visual inspection. Look over the DEF line for damage that could restrict flow, especially a pinched or kinked line. If there is a concern there, repair it as needed, clear the GDM and PCM DTCs, and repeat the self-test. If the line looks good, move into the circuit checks.
Check the pump circuits and motor
With the ignition OFF, disconnect the GDM-C connector and check the RDPC and RDPPWR circuits for an open. Measure resistance between C4863 Pin 4 and C1273C Pin B4, and between C4863 Pin 5 and C1273C Pin A4. Those resistances should be less than 5 ohms. Next, check the RDPC circuit for a short to ground by measuring resistance at C4863 Pin 4 and C4863 Pin 5; the resistance should be greater than 10000 Ω. Then turn the ignition ON and check for a short to voltage at C4863 Pin 4 and C4863 Pin 5; no voltage should be present. After that, turn the ignition OFF, disconnect the Reductant Pump Assembly connector, and check the pump motor for internal shorts by measuring between C4863 Pin 4, component side, and C4863 Pin 5, component side. The pump motor resistance should be between 3.27 - 4.31 ohms. If the circuit checks point to an open or short, repair the circuit, clear the GDM and PCM DTCs, and repeat the self-test. If the pump motor resistance is not in range, install a new Reductant Pump assembly, clear the GDM and PCM DTCs, and repeat the self-test.
Check reductant fluid level
For the P20E8 path, check the reductant fluid level. If it is low, refill the reductant tank with 2 gallons of new reductant fluid, then continue the test path. If the level is not low, keep moving; do not stop there just because the tank has fluid in it.
Measure dosing at the injector
Next, inspect the reductant injector by removing it from the SCR catalyst while leaving the electrical connector and reductant pressure line connected. Install the Tool, 5.4L Injector Test 310-190 or equivalent to the reductant injector. Turn the ignition ON and run the SCR Dosing Measurement Test with the scan tool. The expected dispensed volume is between 40-50 ml (1.35-1.69 oz). If the volume is in that range, the system is operating correctly at this time; clear the GDM and PCM DTCs and repeat the self-test. If the volume is not in that range, keep going.
Bleed air and check for leaks
Now disconnect the reductant pressure line from the reductant injector and place the line in a clean container. Be ready for fluid discharge: as much as 12 oz of fluid will be discharged, and that discharged fluid gets discarded. With the scan tool, run the SOBDMC Selective Catalytic Reduction system visual leak test for a minimum of 2 minutes. Keep running the leak test until there are no more bubbles or air in the fluid. Reconnect the pressure line to the injector, run the visual leak test again, and check the reductant system for leaks, cracks, and loose connections. Then run the SCR Dosing Measurement Test again after the visual leak check. The expected volume is still between 40-50 ml (1.35-1.69 oz). If it now passes, clear the GDM and PCM DTCs and repeat the self-test. If it still does not pass, move to the pressure check.
Recheck reductant pump pressure
With the ignition ON, access the SOBDMC and monitor the REDUCT_LINEP_CALC (kPa) PID. Use the scan tool to run the SOBDMC Selective Catalytic Reduction system visual leak test to pressurize the system, then run the visual leak test again. The pressure decision point is whether the REDUCT_LINEP_CALC (kPa) PID value is greater than 70 psi. If it is greater than 70 psi but the dosing test failed, install a new Reductant Injector, clear the GDM and PCM DTCs, and repeat the self-test. If it is not greater than 70 psi, inspect the reductant system for leaks, cracks, loose connections, crystallized urea, or punctures. Repair those concerns as necessary. If no concerns are found, install a new Reductant Pump assembly, clear the GDM and PCM DTCs, and repeat the self-test.
Confirm pump operation when the connector path leads there
When the earlier pump motor and circuit checks pass, reconnect the Reductant Pump Assembly connector and the GDM-C connector. Turn the ignition ON, access the SOBDMC, and monitor the REDUCT_LINEP_CALC (kPa) PID while running the visual leak test to pressurize the system. The PID value should increase to greater than 65 psi. If it does, the system is operating correctly at this time, and the concern may have been caused by a loose or corroded connector. If it does not, inspect the reductant system for leaks, cracks, loose connections, or punctures. Repair those concerns as needed. If no concerns are found, install a new Reductant Pump assembly, clear the GDM and PCM DTCs, and repeat the self-test.
Verify and close
Keep verification separate from the testing. After the repair path is completed, clear the GDM and PCM DTCs, repeat the self-test, and confirm the code stays gone. For P20E8, the vehicle may still show a system warning message after the repair, so drive the vehicle until that message has cleared from the IPC message center. The takeaway is simple: prove fluid level, prove dosing volume, bleed and leak-check the pressure side, then use the pressure PID to separate an injector-side problem from a pump or system leak problem. For more diagnostic training, visit stepdiagnostics.com.
Final check
P20E8 diagnosis often comes down to confirming fluid level, checking delivered dosing volume, removing air and leaks from the pressure side, and then using pump pressure to decide the next step.
For more guided automotive diagnostics, visit STEP Diagnostics.
