Theory Of Operation

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Diesel Exhaust Fluid (DEF) System

The Diesel Exhaust Fluid (DEF) System uses an injector to spray fluid into the exhaust stream. The Diesel Exhaust Fluid (DEF) Injector is mounted to the decomposition tube at the inlet of the Selective Catalytic Reduction (SCR), catalytic converter. The Powertrain Control Module (PCM) controls the DEF Injector with a Pulse Width Modulated (PWM) Signal to vary the amount of DEF sprayed into the SCR. The DEF injector operates during the three main stages: Pressure Build, Metering Control, and Purge.

Once the SCR Catalyst reaches operating temperature of 273°F (134°C), the PCM will begin the pressure build phase. The Powertrain Control Module (PCM) will activate the DEF Supply Pump to refill the line. As the line fills, the pressure builds to the set point. Then, the PCM turns on the DEF Injector to bleed air out of the Pressure Line. Each time the injector is turned off, the PCM monitors the amount of time it takes for the pressure to rise back to the set point to determine when all of the air is bled out of the line. During Metering Control, the PCM adjusts pump run time to maintain a steady pressure between 6.5 - 8.5 bar (94 - 123 psi).

When the engine is shut off, the Purge pump will activate. The purpose is to draw the DEF back into the tank to protect the line. DEF will freeze at 12°F (-11°C) and its concurrent expansion has potential to damage the pressure line. The injector is open during this time to allow the fluid to return to the tank without creating a vacuum in the line.

The P202E - Reductant Injection Valve Circuit Performance fault is a collector style fault. It will set due to an electrical or a mechanical issue with the DEF injector. The two main issues that will cause this DTC to set are the injector needle valve not moving or an injector valve coil plausibility error. The dosing needle movement faults are set during metering control. The dosing coil plausibility check is performed during cold start (before pressure build), by comparing calculated coil temperature with the upstream catalyst temperature. If the difference is greater than a calibrated threshold, the fault will set.

The P204F - Reductant System Performance fault is a collector style fault. It will set due to an electrical or pressure related issue in the system. The four main issues that will cause this DTC to set are: Pump stroke time not within expected window of operation, pressure plausibility too high, pressure plausibility too low, or supply pump coil resistance out of specification. The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled.

The P208A - Reductant Pump A Control Circuit Open fault will be set when there is an open detected during any stage of pump operation. The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled.

The P208B - Reductant Pump 1 Control Performance fault will be set when the DEF system is not able to achieve 6.5 bar (94 psi) during the pressure build phase. The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled. This is a 1 trip fault.

The P208C - Reductant Pump A Control Circuit low fault will be set when a short to ground is detected during any stage of pump operation.

The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled.

The P208D - Reductant Pump A Control Circuit High fault will be set when there is a short circuit to battery detected during any stage of pump operation. The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled.

The P20E8 - Reductant Pressure Too Low fault will be set when the pressure drops below 2.5 bar (36 psi) after 5 pressure build attempts during a single drive (key) cycle. The PCM will illuminate the MIL light immediately after the diagnostics run and fail, and at this point, the diesel exhaust fluid injection into the aftertreatment system is disabled.