P0300 — Random/Multiple Cylinder Misfire Detected
P0300 sets when the ECM detects misfires occurring randomly across multiple cylinders based on crankshaft acceleration variations. A flashing MIL with P0300 means catalyst-damaging misfire — reduce load immediately. Power-balance isolation and component swapping before any parts replacement prevents the common 'replace all coils and plugs' approach when only one component is at fault.
Fault Overview
Worn or fouled spark plugs, failing ignition coils, vacuum leaks, low fuel pressure, dirty or failing fuel injectors, incorrect spark plug gap, EGR valve stuck open, low compression, wiring issues in coil or injector circuits, and contaminated fuel are the primary causes. On EcoBoost and direct-injection engines, carbon buildup on intake valves is a frequent contributor. Always start with spark plugs and coil swaps on individual cylinders before replacing parts.
Engine & Platform Coverage
Symptoms
- •P0300
- •random misfire
- •multiple cylinder misfire
- •engine misfire rough running
- •flashing check engine light misfire
Possible Causes
- 1.Worn or fouled spark plugs
- 2.Failing ignition coils
- 3.Vacuum leaks
- 4.Low fuel pressure
- 5.Dirty or failing fuel injectors
- 6.EGR valve stuck open
- 7.Low compression
- 8.Carbon buildup on intake valves (DI engines)
Wiring & Circuit Notes
Inspect ignition coil connectors for corrosion, oil intrusion, or poor pin tension. Check injector connectors and harness for chafing. Verify power and ground feeds to the coil and injector banks under load. Look for damaged shielding on crankshaft position sensor wiring.
DVOM Specs & Sensor Tests
Crankshaft position sensor signal integrity (scope preferred). Ignition coil primary resistance and current ramp. Injector resistance and balance. Fuel pressure and volume. Vacuum readings at idle and under load. Relative compression or power balance test results.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
7 steps locked behind Technician Mode- 1Step 1 — Record Freeze-Frame Data: Record freeze-frame data (rpm, load, temperature, fuel trims). Identify the conditions under which the misfire occurs.
- 2Step 2 — Perform Power Balance or Relative Compression Test: Perform a power balance or relative compression test to identify any cylinder contributing less than the others.
- 3Step 3 — Inspect and Replace Spark Plugs: Inspect and gap or replace spark plugs. Worn or fouled plugs are the most common cause of P0300.
- 4Step 4 — Swap Coils Between Cylinders: Swap coils between cylinders and recheck misfire counts. If the misfire moves with the coil, the coil is bad.
- 5Step 5 — Check Fuel Pressure and Injector Contribution: Check fuel pressure and perform an injector contribution test.
- 6Step 6 — Smoke-Test for Vacuum Leaks: Smoke-test the entire intake tract for vacuum leaks.
- 7Step 7 — Evaluate Cam/Crank Signals if Mechanical Timing Is Suspect: Evaluate crankshaft and camshaft sensor signals if mechanical timing is suspect.
- 8Step 8 — Address Root Cause and Verify: Address the root cause and verify misfire counters remain at zero after repair.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Replace spark plugs and coils $80–400; repair vacuum leaks $20–200; clean or replace injectors $100–600; correct fuel pressure $100–600; address carbon buildup $200–600.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.