P0300 on Mazda
Random/Multiple Cylinder Misfire Detected
P0300 on Mazda vehicles indicates random/multiple cylinder misfire detected. Au7o has documented this code across 10 Mazda models — most commonly on 626, B-Series, CX-3. P0300 means the engine computer detected misfires occurring randomly or across multiple cylinders rather than in one specific cylinder. A misfire is when a cylinder fails to ignite its fuel-air mixture properly, which the computer senses through small fluctuations in crankshaft speed. Because it's not isolated to one cylinder, the cause is usually something that affects the whole engine — like fuel, air, or ignition system problems — rather than a single coil or plug. Persistent or heavy misfiring wastes fuel, runs rough, and can damage the catalytic converter, which is why a flashing check engine light should be taken seriously. Typical repair costs on Mazda range from $50 to $5,000, depending on the specific model and root cause.
Common Causes of P0300
- •Worn or fouled spark plugs (across cylinders)
- •Vacuum or intake air leak
- •Weak fuel pump, clogged fuel filter, or low fuel pressure
- •Failing ignition coils or worn spark plug wires
- •Dirty or faulty mass airflow (MAF) sensor
- •Clogged or dirty fuel injectors
- •Faulty crankshaft/camshaft position sensor
- •Low compression or EGR/PCV system faults
P0300 on Mazda by Model
Mazda 626(1 issue)
- Distributor Internal Coil and Hall Effect Sensor Failure1993-1997
The 626's distributor assembly contains the ignition coil and hall effect sensor, both of which fail prematurely. The distributor is mounted on the rear of the cylinder head where it is exposed to engine heat. Internal components fail between 80,000-120,000 miles causing intermittent no-start, misfires, and stalling. The failure is often intermittent, making diagnosis difficult.
Mazda B-Series(1 issue)
- Timing Chain Guide and Tensioner Failure1998-2007
Ford-sourced V6 engines develop timing chain guide wear and tensioner failure. Plastic guides break apart and can clog oil passages. Same issue as Ford Ranger with identical engines.
Mazda CX-3(1 issue)
- Skyactiv 2.0L Intake Valve Carbon Buildup2016-2021
The 2016-2021 Mazda CX-3 with the 2.0L Skyactiv-G direct injection engine develops carbon deposits on the intake valves over time. Because direct injection sprays fuel directly into the combustion chamber rather than over the intake valves, there is no fuel wash to clean the valve backsides. Oil vapors from the PCV (Positive Crankcase Ventilation) system coat the intake valves and bake into hard carbon deposits. This gradually restricts airflow, causing misfires on cold starts, loss of low-end power, and reduced fuel efficiency. Using conventional oils with high volatility worsens the problem by producing more PCV vapors.
Mazda CX-5(1 issue)
- Carbon Buildup on Intake Valves (Skyactiv)2013-2023
Direct-injection Skyactiv engines accumulate carbon deposits on intake valves due to oil vapors from the PCV system. Without port injection to clean valves, carbon can fuse to valve surfaces over time, typically appearing around 65,000+ miles.
Mazda Mazda3(1 issue)
- Carbon Buildup on Direct Injection Intake Valves2012-2025
SkyActiv direct-injection engines lack port injection to wash intake valves, causing carbon deposits to accumulate over time. Carbon buildup restricts airflow, causing rough idle, misfires, and power loss. Typically becomes noticeable around 60,000-80,000 miles.
Mazda Mazda6(1 issue)
- SkyActiv-G Intake Valve Carbon Buildup (Direct Injection)2014-2021
Because the SkyActiv-G direct-injection engine sprays fuel directly into the cylinder, no detergent fuel ever washes the back of the intake valves. Oil vapor drawn through the PCV system bakes onto the hot valve stems and accumulates as hard carbon, typically becoming noticeable on the 2.5L between 60,000 and 100,000 miles. The deposits restrict airflow and cause rough idle and cold-start misfires that are otherwise hard to diagnose.
Mazda MX-5 Miata(1 issue)
- Crankshaft Position Sensor Failure (NB)1999-2005
The NB Miata (1999-2005) crankshaft position sensor (CPS) is a known failure point. The sensor deteriorates from engine heat, causing intermittent stalling, no-start conditions, and random misfires. Failure is often heat-related - the car runs fine when cold but stalls or dies after reaching operating temperature.
Mazda MX-6(1 issue)
- Distributor Failure on V6 Models1993-1997
The MX-6 V6 (2.5L KL engine) distributor develops internal electronic failures, particularly the coil and crank angle sensor. The distributor is mounted at the rear of the engine between the heads, making it difficult to access and exposed to extreme heat. The failure causes intermittent no-start, stalling, and misfire conditions that are difficult to diagnose because they occur randomly.
Mazda RX-8(1 issue)
- Apex Seal Failure / Compression Loss2004-2012
The Renesis rotary engine in the RX-8 is infamous for apex seal wear and failure, leading to catastrophic compression loss. Apex seals maintain the combustion chamber seal between the rotor and housing. Heat, carbon deposits, and inadequate lubrication cause seals to wear, chip, or break. Compression testing below 6.5 kg/cm2 indicates rebuild is needed. This is the single most common and feared RX-8 issue.
Mazda Tribute(1 issue)
- Ignition Coil Failure and Misfires2001-2011
Ignition coil-on-plug coils fail causing misfires. Ford 3.0L Duratec V6 coils are prone to cracking and arcing. Usually fails one at a time.
Looking for P0300 on a different make?
View P0300 across all makes →Frequently Asked Questions
What does P0300 mean on Mazda?▼
P0300 stands for "Random/Multiple Cylinder Misfire Detected." P0300 means the engine computer detected misfires occurring randomly or across multiple cylinders rather than in one specific cylinder. A misfire is when a cylinder fails to ignite its fuel-air mixture properly, which the computer senses through small fluctuations in crankshaft speed. Because it's not isolated to one cylinder, the cause is usually something that affects the whole engine — like fuel, air, or ignition system problems — rather than a single coil or plug. Persistent or heavy misfiring wastes fuel, runs rough, and can damage the catalytic converter, which is why a flashing check engine light should be taken seriously. On Mazda specifically, this code is documented across 10 models.
What causes P0300 on Mazda vehicles?▼
Common causes on Mazda: Worn or fouled spark plugs (across cylinders), Vacuum or intake air leak, Weak fuel pump, clogged fuel filter, or low fuel pressure, Failing ignition coils or worn spark plug wires, Dirty or faulty mass airflow (MAF) sensor. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0300 on a Mazda?▼
Repair costs on Mazda range from $50 to $5,000, depending on the specific model and root cause.
Which Mazda models have P0300 documented?▼
Au7o has documented P0300 on 10 Mazda models: 626, B-Series, CX-3, CX-5, Mazda3, Mazda6, MX-5 Miata, MX-6, RX-8, Tribute.