A P0303 code on your 2018 GMC Sierra signals that the engine control module has detected consistent misfires in cylinder 3. The 2018 Sierra represents the final year of the K2 platform generation, featuring proven V8 engines with Active Fuel Management.
2018 Sierra Engine Options
The 2018 Sierra offered the 5.3L L83 V8 or 6.2L L86 V8, both featuring AFM cylinder deactivation. Cylinder 3 is on the driver's side bank. At this age, both wear-related failures and AFM-specific issues should be considered.
Common Failure Points
- AFM Lifter Collapse: Active Fuel Management lifters can fail with age
- Ignition Coil Wear: After several years of service, coils commonly fail
- Spark Plug Condition: May be due for replacement based on mileage
- Fuel Injector Deposits: Years of operation lead to injector fouling
- Valve Seal Wear: Oil consumption and valve issues increase with age
Diagnostic Approach
Connect a diagnostic scanner to review misfire patterns and AFM operation. Note if a lifter tick or noise is present. Test ignition components through swap testing. Check compression and listen for mechanical noises indicating valve train problems.
Repair Solutions
Replace aging ignition coils and spark plugs with quality parts. For AFM lifter failures, complete lifter replacement is typically required. Many owners opt for AFM delete during major repairs to prevent recurrence.