how to diagnose a misfire in my Toyota Corolla E140 (2006-2013) with P0301 code

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, torque wrench, compression tester, multimeter, fuel injector cleaner.
• Safety Warnings: Disconnect the battery before working on electrical systems. Allow the engine to cool before performing any inspections.

2. Inspect and Replace Spark Plug

• Remove the engine cover if applicable.
• Disconnect the ignition coil connector and remove the ignition coil from cylinder 1.
• Use a spark plug socket to remove the spark plug from cylinder 1.
• Inspect the spark plug for fouling or excessive wear; replace if necessary.
• Install a new spark plug, ensuring it is gapped correctly (typically 0.043 inches for a Corolla).
• Reinstall the ignition coil and reconnect the connector.

3. Check and Replace Ignition Coil

• If the spark plug is fine, swap the ignition coil from cylinder 1 with another cylinder (e.g., cylinder 2).
• Clear the error codes and test drive the vehicle to see if the misfire code follows the ignition coil.
• If it does, replace the faulty ignition coil for cylinder 1 with a new one.
• Torque the coil to manufacturer's specifications (usually around 7-9 ft-lbs).

4. Compression Test

• If the misfire persists, perform a compression test on cylinder 1.
• Remove the spark plug and crank the engine to measure compression using a compression tester.
• Compare the results to the manufacturer’s specifications (typically around 180 psi).
• If compression is low, further inspection of the engine internals may be required.

5. Inspect and Clean Fuel Injector

• If the compression is satisfactory, check the fuel injector for cylinder 1.
• Use a multimeter to check for proper resistance (typically around 12-16 ohms).
• If the injector is faulty, replace it with a new one.
• Optionally, use a fuel injector cleaner to clean the injector and restore proper flow.