how to fix P0300 random misfire code in Toyota Highlander Third Generation (2014-2019)

Solution

1. Preparation

• Gather tools and parts required.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Spark Plugs

• Tools Required: Socket set, torque wrench, gap gauge
• Remove the ignition coil by unscrewing the bolts and disconnecting the wiring harness.
• Use a socket to remove the spark plug from the cylinder head.
• Inspect the spark plug condition; if worn or fouled, replace it with a new one.
• Set the gap on the new spark plug according to manufacturer specifications, typically around 0.044 inches for Toyota vehicles.
• Install the new spark plug and torque to the manufacturer’s specification (usually around 13 lb-ft).
• Reinstall the ignition coil.

3. Inspect and Replace Ignition Coils

• Tools Required: Socket set, multimeter
• If any ignition coils were found to be faulty during inspection, remove them by disconnecting the wiring harness and bolts.
• Test each coil with a multimeter for resistance; if out of spec, replace the coil.
• Install new ignition coils and ensure they are secured properly.

4. Fuel System Inspection

• Tools Required: Fuel pressure gauge
• Attach the fuel pressure gauge to the fuel rail’s test port.
• Start the engine and check that fuel pressure meets specification (usually around 50-60 psi for this model).
• If pressure is low, check the fuel filter for clogs and replace if necessary.
• Inspect fuel injectors for proper operation; clean or replace as needed.

5. Check for Vacuum Leaks

• Tools Required: Smoke machine or carburetor cleaner
• Visually inspect all vacuum lines and hoses for cracks or disconnections.
• Use a smoke machine to detect any vacuum leaks or spray carburetor cleaner around the intake manifold and vacuum lines, watching for RPM changes.
• Replace any damaged vacuum lines or seals.

6. Compression Testing

• Tools Required: Compression tester
• Remove spark plugs to allow easy access to the cylinders.
• Insert the compression tester into each cylinder and crank the engine.
• Record the readings and compare them to the manufacturer’s specifications (typically 180-200 psi).
• If any cylinder is low, further diagnosis may be required (e.g., valve issues, head gasket failure).