Your 2021 Lexus RX starts with an unexpected roughness—a noticeable miss that may smooth out once the engine warms. A scan confirms P0301—Cylinder 1 Misfire Detected. The combination of cold-start symptoms and a specific cylinder code provides important diagnostic clues.
Why Cold Starts Matter
Misfires that appear primarily at startup often have different causes than those occurring during normal operation:
- Cold engines require richer fuel mixtures
- Ignition components work harder in cold conditions
- Fuel evaporates less readily when cold
- Oil hasn't yet thinned to optimal viscosity
- Sensors may read differently when cold
Cylinder 1 Location on the RX V6
On the 3.5L V6 in your RX, cylinder 1 is at the front of the passenger-side bank. Knowing this helps locate the specific components—ignition coil, spark plug, and fuel injector—that serve this cylinder.
Common Causes of Cold-Start Cylinder 1 Misfire
Several issues preferentially cause cold-start misfires:
- Ignition coil weakness - Coils that work hot may fail cold
- Spark plug degradation - Worn plugs struggle to ignite cold, rich mixtures
- Fuel injector drip - Injector leaking down overnight causes flooding
- Coolant temperature sensor error - Incorrect cold readings affect fuel mixture
- Low compression - Valve sealing issues more apparent when cold
- Fuel quality - Contaminated fuel ignites poorly when cold
The Coil and Plug Swap Test
To identify ignition-related problems:
- Note current code: P0301 (Cylinder 1)
- Swap cylinder 1 ignition coil with cylinder 4
- Clear fault codes
- Allow vehicle to sit overnight and cold start next morning
- If P0304 appears, the coil is faulty
Repair Costs for 2021 RX
- Ignition coil (single): $80-$180
- Spark plugs (all six): $200-$400
- Fuel injector: $300-$600
- Coolant temperature sensor: $100-$200
- Compression test: $100-$200