how to fix P0300 random misfire code in Toyota Highlander Fourth Generation (2020-present)

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, torque wrench, fuel pressure gauge, vacuum gauge, compression tester.
• Safety Warnings: Disconnect the battery before working on electrical systems and allow the engine to cool before opening the hood.

2. Inspect and Replace Ignition Components

1. Remove Engine Cover: Use a socket set to remove the bolts securing the engine cover.
2. Inspect Spark Plugs:
   • Remove the spark plug wires or coils by carefully pulling them off.
   • Use a spark plug socket to remove the spark plugs and inspect them for wear or fouling.
   • Replace any damaged or worn spark plugs with OEM parts, ensuring a torque specification of around 13-15 ft-lbs.
3. Inspect Ignition Coils:
   • Check the ignition coils for cracks or damage.
   • Replace any faulty ignition coils with new OEM units.

3. Check Fuel System

1. Connect Fuel Pressure Gauge: Locate the fuel rail and connect the fuel pressure gauge to the test port.
2. Start Engine: Turn the ignition to the ON position (do not start) to prime the fuel system and check for fuel pressure. It should typically be around 50-60 psi.
3. Inspect Fuel Injectors: Examine the fuel injectors for clogs or leaks, and clean or replace as necessary.

4. Vacuum Leak Inspection

1. Visual Inspection: Check all vacuum hoses and the intake manifold for signs of wear or damage.
2. Using a Vacuum Gauge: Connect a vacuum gauge to a manifold vacuum port to check for proper vacuum levels, typically between 17-21 inHg at idle.
3. Fix Any Leaks: Replace any cracked hoses or gaskets found during inspection.

5. Compression Test

1. Remove Spark Plugs: If not already done, remove the spark plugs to facilitate the compression test.
2. Insert Compression Tester: Screw the compression tester into the spark plug hole of each cylinder.
3. Crank Engine: Have an assistant crank the engine while you observe the readings. Each cylinder should have a compression reading within the manufacturer's specified range (typically around 170-210 psi).
4. Document Results: Note any cylinders with significantly lower compression and consider further investigation.