Conditions for Clearing the DTC

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The Conditions for Setting the DTC are no longer present.
A history DTC will clear after 40 malfunction free ignition cycles.
The engine control module (ECM) receives the clear code command from the scan tool.

DTC P0621

Step	Action	Yes	No
Schematic Reference: Starting and Charging Schematics (Except HP2) or Starting and Charging Schematics (HP2) Connector End View Reference: Engine Electrical Connector End Views
1	Did you perform the Engine Electrical Diagnostic System Check?	Go to Step 2	Go to Diagnostic System Check - Engine Electrical
2	Install a scan tool. Start the engine. With a scan tool, monitor the DTC information for DTC P0621 in Engine Controls. Does the scan tool indicate that DTC P0621 has passed?	Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems	Go to Step 3
3	Test the generator turn on signal circuit for a short or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	Go to Step 6	Go to Step 4
4	Inspect for poor connections at the harness connector of the ECM. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition?	Go to Step 6	Go to Step 5
5	Replace the ECM. Refer to Engine Control Module (ECM) Replacement in Engine Controls-6.6L. Did you compete the replacement?	Go to Step 6	-
6	Review and record the scan tool Fail Records data. Clear any DTCs. Operate the vehicle within the Fail Records conditions as noted. Using a scan tool, monitor the Specific DTC info for this DTC. Does the scan tool indicate that this DTC failed this ignition?	Go to Step 2	System OK