P0300 Mazda 3

Diagnose and fix P0300 Mazda 3

Mazda has issued a service bulletin 01-020/15 to address a check engine light with possible P0300, P0301, P0302, P0304 or P0304 trouble codes on the vehicles listed below. Mazda has determined that carbon may be deposited around the intake valves, resulting in unstable engine combustion and misfiring during the accelerated warm-up system (AWS) operation.

The service bulletin details an intake valve cleaning procedure to fix the problem

P0300:00: Random misfire detected
P0301:00: Cylinder No.1 misfire detected
P0302:00: Cylinder No.2 misfire detected
P0303:00: Cylinder No.3 misfire detected
P0304:00: Cylinder No.4 misfire detected

Vehicles affected by 01-020/15

2012-2013 Mazda3 (with SKYACTIV)

Valve decarbon procedure

Use cleaner 0000-77-129 (Wurth TUNAP 129 or equivalent)
NOTE: DO NOT disconnect the negative battery cable to keep the misfire count active.
CAUTION: Take care not to break the harness clips.
CAUTION: Take care not to damage the alternator.
Remove the intake manifold and spark plugs.
Check for carbon deposit around the intake valves.
Spread shop towels under the intake ports to protect components from cleaner overflow.
Raise the vehicle and remove the right-front splash shield to access the crankshaft pulley (A).

• Rotate engine to TDC. Then add a paint mark “A” to the pulley face at TDC. Add another paint mark “B” to the pulley face 180 degrees opposite from TDC mark. When mark A is located at TDC, the intake valves are closed at cylinders #1 and #4. When mark B is located at TDC, the intake valves are closed at cylinders #2 and #3.
• Clean the intake valves of cylinders #1 and #4 with mark A at TDC by spraying the cleaner fluid onto carbon deposit around the intake valves until fully covered. Let it soak for 2 hours. Then soak up the cleaner fluid with shop towels or a temporary suction tool.
• Scrape the carbon deposits off of the stems and intake valves using shop towels and an appropriate plastic or wooden stick. CAUTION: DO NOT use a metal tool in order to avoid valve damage. Wipe out the scraped deposits with shop towels and the plastic or wooden stick.
• Clean the intake valves of cylinders #2 and #3 with mark B at TDC.
• Clean the cylinder head to intake manifold sealing surfaces.
• Reassemble the intake manifold and the other remaining parts.
• Crank the engine with the accelerator pedal fully depressed (Dechoke mode) for 10 seconds. Repeat this twice in order to clear the remaining materials in the ports. NOTE: Dechoke mode cancels fuel injection during cranking (when the accelerator pedal is fully depressed).
• Start the engine and keep the engine idling until the water temperature light shuts off. CAUTION: White smoke may occur, therefore prepare the exhaust vacuum duct or perform this step in a well ventilated area.
• Race the engine from idle to 6000 rpm by fully depressing the accelerator pedal 20 times in order to clear the remaining deposits and cleaner fluid.
• Stop the engine and replace the engine oil and filter.

You may also try using a non-invasive method of cleaning the carbon. See this post. If the non-invasive method doesn’t work, you must perform the Mazda method.