This thread documents a real misfire diagnosis on my 2014 GMC Sierra 1500 (5.3L L83 EcoTec3 V8) — the same truck and process from the video below. The point is to work through the ignition system in order and confirm the failed part before buying anything, instead of throwing plugs, wires and coils at it.
The symptoms
- Rough idle and a stumble under light throttle
- Check engine light flashing at times (an active, damaging misfire — not a steady light)
- Single stored code pointing at one cylinder, plus a no-spark condition on that cylinder
Step 1 — Scan the ECM and find the cylinder
Pull the code first. On a GM V8 the misfire codes are cylinder-specific: P0300 is a random/multiple misfire, while P0301–P0308 each point at a single cylinder. This truck came back with P0301 = Cylinder 1.
Cylinder 1 is the front-most cylinder on the driver side (Bank 1). Knowing the layout keeps you from testing the wrong coil.
What you need
Nothing exotic — a scan tool, an inline spark tester, a 5/8" spark plug socket, an extension and a ratchet, plus a known-good coil to swap.
Step 2 — Inspect the spark plug
Pull the plug on the dead cylinder and read it. A fouled, oil- or fuel-soaked, or worn plug can cause the miss on its own. Clean it up, check the gap, and reinstall — or replace it if it’s done.
Step 3 — Check for spark
Put an inline spark tester between the coil/plug wire and the plug and crank it. You want a strong, consistent blue spark. Weak, intermittent, or no spark tells you the problem is upstream of the plug — the wire or the coil.
Step 4 — Test the wire, then swap the coil
Check the plug wire (or boot) for cracks, carbon tracking and resistance. If the wire is good and spark is still weak/absent, swap the coil with a known-good one from another cylinder and re-scan. If the misfire follows the coil to the new cylinder, you’ve found it. That’s exactly what happened here — the coil was the failed part.
Step 5 — Don’t skip the connector and wiring
Before you call the coil bad, rule out the connection. A backed-out pin, a corroded terminal, or a broken wire under the insulation will mimic a dead coil.
Each coil connector has 12V power, a ground, and a signal (trigger) wire from the ECM. Confirm power and ground are present and the signal is reaching the connector before condemning the coil. Pinout varies — verify yours for your specific truck.
A note on AFM — take a flashing misfire seriously
These L83/L86 engines run AFM (Active Fuel Management), which deactivates cylinders 1, 4, 6 and 7 under light load. A misfire is not automatically an AFM/DOD lifter failure — this whole thread is an ignition-side fix. But if you have a flashing CEL, stop driving it. Continued driving on an active misfire risks cam and engine damage.
The order that works
Plug → spark → wire → coil → connector → wiring. Confirm the failed component before replacing it.
This same sequence applies to 2014–2018 Silverado and Sierra 1500 trucks with the 5.3L L83, and the basic plug/wire/coil/wiring checks carry over to most GM 4.8L, 5.3L, 6.0L and 6.2L truck engines.
Anyone else chased a single-cylinder misfire on a K2XX? Did yours turn out to be the coil, the plug, or a connector/wiring issue? Post your code, symptoms and what fixed it below.