Skip to content
Tahoe / Yukon Service Manual

DTC P0621

Diagnostic Instructions Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Review Strategy Based Diagnosis for an overview of the diagnostic approach. Diagnostic Procedure Instructions provides an overview of each diagnostic category. DTC Descriptor DTC P0621 Generator L-Terminal Circuit For symptom byte information refer to Symptom Byte List. Diagnostic Fault Information

Circuit Number Circuit Function Short to Ground Open/High Resistance Short to Voltage Signal Performance
2 B+ P0621 P0621 — —
25 Control P0621 P0621 P0621 —
23 Signal P0622 P0622 P0622 P0622

Circuit/System Description The K20 Engine Control Module controls the G13 Generator through the control circuit 25, also known as the generator L-terminal. The K20 Engine Control Module monitors the G13 Generator performance though the signal circuit 23, also known as the generator F-terminal. The control circuit 25 is a PWM signal of 128 Hz with a duty cycle of 0–100%. Normal duty cycle is between 5– 95%. 0–5% and 95–100% are for diagnostic purposes, with 0–5% monitoring for an open circuit and 95–100% monitoring for a short to ground. Conditions for Running the DTC Ignition ON, engine OFF (Prior to Ignition ON, the ignition must have been OFF for at least 5 s) Engine running Conditions for Setting the DTC Ignition ON, engine OFF: The K20 Engine Control Module detects greater than 3.5 V on the control circuit 25 for 5 s. Engine running: The K20 Engine Control Module detects a low voltage for 15 s on the control circuit 25 when the duty cycle voltage is commanded high. Engine running: The K20 Engine Control Module detects a high voltage for 15 s on the control circuit 25 when the duty cycle voltage is commanded low. Diagnostic Aids Single digit circuits and circuits with three or more digits, the last 2 digits of the circuit number(s) are significant in identifying its power and ground functions. Ci it 1 i P i B tt iti lt – Circuit 1 is Primary Battery positive voltage. – Circuit 2 is Battery positive voltage. – Circuit 3 is Run/Crank ignition voltage. – Circuit 4 is Accessory voltage. – Circuit 5 is Crank voltage. – Circuits ending in 42 are primary fused B+ circuits (generally the first fuse in the circuit after the battery) – Circuits ending in 40 are secondary fused circuits (generally fuses in a fuse block after the primary circuit) – Circuits ending in 39 and 41 are ignition voltage circuits. – Circuits ending in 50 are grounds. – Circuits ending in 51 are low reference (ground) circuits. Action Taken When the DTC Sets DTC P0621 is a type C DTC. Conditions for Clearing the DTC DTC P0621 is a type C DTC. Reference Information Schematic Reference Starting and Charging Schematics Connector End View Reference Master Electrical Component List Description and Operation Charging System Description and Operation Electrical Information Reference Circuit Testing Connector Repairs Testing for Intermittent Conditions and Poor Connections Wiring Repairs DTC Type Reference Powertrain Diagnostic Trouble Code (DTC) Type Definitions Scan Tool Reference Control Module References for scan tool information Circuit/System Verification

  1. Ignition OFF for greater than 5 s.
  2. Engine running.
  3. Verify DTC P0621 is not set as current. ⇒ If DTC P0621 is set as current Refer to Circuit/System Testing. ⇓ If DTC P0621 is not set or is set as history
  4. A fault is currently not present and may be an intermittent condition. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data. Circuit/System Testing
  5. Verify a test lamp illuminates between the G13 Generator B+ circuit terminal A X2 and ground. ⇒ If the test lamp does not illuminate and the circuit fuse is good 1.1. Test circuit 2 for an open/high resistance. ⇒ if an open/high resistance is found, repair the fault on the circuit. ⇓ If an open or high resistance is not found 1.2. Verify the fuse is not open and there is voltage at the fuse. ⇒ If the test lamp does not illuminate and the circuit fuse is open 1.1. Test circuit 2 for a short to ground. ⇒ If a short to ground is found, repair the fault on the circuit. ⇓ If a short to ground is not found. 1.2. Replace the G13 Generator. ⇓ If the test lamp illuminates
  6. Ignition Off, disconnect the X1 connector at the G13 Generator, engine running.
  7. Test for 1 V or greater between control circuit terminal 1 and ground. ⇒ If less than 1 V 3.1. Test circuit 25 for a short to ground. ⇒ If a short to ground is found, repair the fault on the circuit. ⇓ If a short to ground is not found 3.2. Test circuit 25 for an open/high resistance. ⇒ If an open/high resistance is found, repair the fault on the circuit. ⇓ If an open or high resistance is not found 3.3. Replace the K20 Engine Control Module. ⇓ If 1 V or greater
  8. Ignition OFF, disconnect the appropriate connector at the K20 Engine Control Module.
  9. Ignition ON.
  10. Test for less than 1 V between control circuit terminal 1 at G13 Generator connector X1 and ground. ⇒ If greater than 1 V Repair the short to voltage on circuit 25. ⇓ If 1 V or less
  11. Replace the G13 Generator. Repair Instructions Perform the Diagnostic Repair Verification after completing the repair. Generator Replacement Control Module References for control module replacement, programming, and setup