🚨 Diagnostic Profile
Vehicle Make: Buick
Technical Code/Description: Ignition Coil 4 Primary Feedback Circuit
This diagnostic trouble code indicates that the Powertrain Control Module (PCM) has detected an abnormal voltage signal on the primary control circuit of ignition coil #4. This typically points to an issue with the coil driver circuit, the coil itself, associated wiring/connectors, or in rare cases, an internal PCM driver failure. Left unresolved, this can cause a misfire on cylinder 4, rough idle, reduced fuel economy, and illuminated Check Engine Light (MIL).
🚨 Repair Difficulty Tier
Difficulty Rating: Intermediate
This repair requires the use of a diagnostic scan tool capable of reading live PCM data streams and ignition coil feedback voltages, along with a digital multimeter (DMM) to test circuit continuity, resistance, and voltage drops. While not as complex as ADAS calibration or module flashing, it does require a solid understanding of ignition system electrical theory and should not be attempted with only basic hand tools. A home mechanic with intermediate electrical diagnostic experience can complete this repair with the proper equipment.
🛠️ Resolution Steps
Step 1: Preliminary Inspection
Begin with a visual inspection of the ignition coil 4 assembly and its wiring harness. Check for:
– Cracked, melted, or burnt coil housing on cylinder 4
– Corroded, loose, or disconnected electrical connector pins at the coil
– Damaged, chafed, or pinched wiring between the coil and PCM
– Oil contamination around the coil boot (common on Buick V6 engines, which can cause tracking and feedback circuit faults)
– Inspect the spark plug for cylinder 4 for fouling, damage, or improper gap, as this can indirectly affect coil feedback readings
Step 2: Diagnostic Verification
Connect a scan tool capable of reading manufacturer-specific PCM data and monitor live ignition coil feedback parameters while the engine is running (if possible) or during cranking. Compare cylinder 4 coil feedback voltage against the other cylinders to identify abnormal patterns.
Next, use a digital multimeter to test the coil primary circuit:
– Verify battery reference voltage is present at the coil connector with the key on
– Check the control circuit wire back to the PCM for continuity and correct resistance values (compare to manufacturer specifications)
– Perform a voltage drop test across the ground circuit to rule out a poor ground connection
– Swap the suspected coil with a known-good coil from another cylinder to determine if the fault follows the component or stays with the circuit
Step 3: Resolution & Reset Process
If the ignition coil is found to be internally faulty, replace it with an OEM or OEM-equivalent coil, ensuring the connector is properly seated and locked. If the wiring harness is damaged, repair or replace the affected section with proper weatherproof connectors and soldered joints. If the issue is traced back to the PCM driver circuit itself, further testing or PCM replacement/reprogramming may be required.
After repairs are complete, clear the diagnostic trouble codes using a scan tool, perform a road test under varying load and RPM conditions, and recheck live data to confirm the feedback circuit is now operating within normal parameters and no codes return.
📦 Parts, Labor, and Budget Estimates
Required Components/Software: Replacement ignition coil (cylinder 4), OEM wiring harness connector/pigtail (if damaged), digital multimeter, OBD-II/manufacturer-specific scan tool for live data streaming
Average Component Price: $35 – $95 for a single OEM or quality aftermarket ignition coil; $20 – $60 for wiring harness repair connectors if needed
Estimated Labor Hours: 0.5 – 1.5 hours for diagnosis and coil replacement; up to 2.5 hours total if wiring repair or PCM-related testing is required