In 2020, the FIXD app detected over 18 million check engine lights. These are the 20 most common check engine codes detected and cleared by the FIXD OBD2 Sensor. Keep reading to discover symptoms associated with these OBD codes, the most common causes, and how to diagnose these common car problems at home.
#1: P0300
Of the 18 million scans completed in 2020, over 193,000 received this common engine code which indicates that one or more cylinders are experiencing misfires.
Symptoms:
- Check engine light is on or flashing
- Engine running rough
- Lack of power
- Fuel smell from engine exhaust
- Hesitation or jerking during acceleration
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0300 Meaning, Causes, Symptoms, & Fixes
#2: P0420
171,182 vehicles scanned by the FIXD OBD scanner discovered the P0420 engine code. P0420 indicates an issue with the catalytic converter not functioning properly, which increases the output of harmful pollutants by a vehicle.
Symptoms:
- Check engine light is on
- Lack of power
- Decreased fuel economy
- Rotten egg or sulfur smell
Most Common Causes:
- Faulty oxygen sensor
- Faulty air-fuel sensor
- Worn or internally failing catalytic converter (Most Common)
- A leak in the exhaust system
- Misfire (the root cause of converter failure)
- Rich or Lean air-fuel ratio (the root cause of converter failure)
- Leaded Gasoline (the root cause of converter failure)
Detailed Explanation & Diagnosis: P0420 Meaning, Causes, Symptoms, & Fixes
#3: P0171
Combustion engines work best by maintaining a fuel to air ratio of about 14.7 parts oxygen to 1 part fuel. If you get a P0171 code, it means that the ratio of fuel to oxygen is off in Bank 1 of your engine and the mixture has too much air (running lean). To try to compensate for this, the powertrain control module injects more fuel into the mixture. But when the ratios become too off balance, code P0171 is triggered.
Symptoms:
- Check engine light is on
- Lack of power
- Engine “coughing”
- Misfire
- Rough idle
Most Common Causes:
- Faulty oxygen sensor
- Dirty or faulty mass airflow sensor
- Vacuum leaks
- Weak fuel pump
- Clogged or dirty fuel injectors
- Clogged fuel filter
- Exhaust leak
- Faulty air-fuel ratio sensor
Detailed Explanation & Diagnosis: P0171 Meaning, Causes, Symptoms, & Fixes
#4: P0455
Over 122,000 drivers received the P0455 engine code in 2020, which means that the engine control module (ECM) detects a large leak from the evaporative emission control (EVAP) system. This is a problem because it means your vehicle is leaking raw fuel and harmful pollutants into the air, which is bad for the environment and your gas mileage.
Symptoms:
- Check engine light is on
- Decreased fuel economy
- Fuel smell
Most Common Causes:
- Missing, defective, damaged, or loose gas cap (most common)
- Cracked EVAP hose
Detailed Explanation & Diagnosis: P0455 Meaning, Causes, Symptoms, & Fixes
#5: P0301
Like code P0300, this engine code means a misfire has been detected. More specifically, P0301 indicates that Cylinder 1 is experiencing misfires and should be fixed as soon as possible to avoid long-term engine damage.
Symptoms:
- Check engine light is on or flashing
- Engine running rough and/or shaking
- Lack of power
- Fuel smell from the exhaust
- Hesitation or jerking during acceleration
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
- Faulty fuel injector
Detailed Explanation & Diagnosis: P0301 Meaning, Causes, Symptoms, & Fixes
#6: P0456
Over 100,000 drivers experienced this somewhat tricky to diagnose engine code. Like P0455, this error code indicates that there’s a leak in the evaporative emission control (EVAP) system. In this case, however, it’s only a small leak (less than .020” in diameter).
Symptoms:
- Check engine light is on
- Decreased fuel economy
- Increased vehicle emissions
- Fuel smell
Most Common Causes:
- Loose or damaged gas cap
- Leaking or disconnected EVAP hose
- Faulty purge volume control valve
- Faulty canister vent control valve
- Charcoal canister leak
- Leaking fuel tank
Detailed Explanation & Diagnosis: P0456 Meaning, Causes, Symptoms, & Fixes
#7: P0303
Code P0303 indicates a misfire detected in Cylinder 3. This code was experienced by 99,372 FIXD customers in 2020.
Symptoms:
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0303 Meaning, Causes, Symptoms, & Fixes
#8: P0304
Yep, you guessed it. P0304 indicates that a misfire has been detected specifically in Cylinder #4 of your engine. Like other cylinder misfires, this should be resolved as quickly as possible to avoid long-term damage to your engine.
Symptoms:
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0304 Meaning, Causes, Symptoms, & Fixes
#9: P0302
P0302 indicates that a misfire has been detected specifically in Cylinder #2 of your engine. If you get this code on your FIXD Sensor, don’t ignore it. You could risk serious engine damage.
Symptoms:
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0302 Meaning, Causes, Symptoms, & Fixes
#10: P0442
Remember the evaporative emission control (EVAP) system that keeps harmful fuel vapors from escaping into the atmosphere? Well, code P0442 indicates that you have a medium-sized leak (.020” to .040” in diameter) that’s preventing the EVAP system from doing its job. Over 94,000 drivers and FIXD customers experienced this code in 2020.
Symptoms:
- Check Engine Light is on
- Decreased fuel economy
- Increased vehicle emissions
- Fuel smell
Common Causes:
- Loose or damaged gas cap
- Leaking or disconnected EVAP hose
- Faulty purge volume control valve
- Faulty canister vent control valve
- Charcoal canister leak
- Leaking fuel tank
Detailed Explanation & Diagnosis: P0442 Meaning, Causes, Symptoms, & Fixes
#11: P0128
P0128 means your engine hasn’t reached the correct operating temperature within a specified timeframe. In other words, your engine isn’t getting (or staying) hot enough fast enough. While this isn’t a severe problem, it can hurt emissions and fuel efficiency so be sure to address it quickly.
Symptoms:
- Check Engine Light is on
- Higher than normal idle
- Decreased fuel economy
- The temperature gauge is unusually low
Most Common Causes:
- Stuck open thermostat (most common)
- Missing Thermostat
- Faulty coolant temperature sensor
- Faulty wiring for coolant temperature circuit
Detailed Explanation & Diagnosis: P0128 Meaning, Causes, Symptoms, & Fixes
#12: P0174
While not as common as P0171, more than 78,000 drivers and FIXD customers experienced this code related to air and fuel mixture control. P0174 means that Bank 2 of your engine has too much air or not enough fuel (running too lean). Driving like this for too long can result in overheating and internal engine damage.
Symptoms:
- Check Engine Light is on
- Lack of power from the engine
- Rough idle
- Engine coughing
- Engine misfiring
Most Common Causes:
- Dirty or faulty mass airflow sensor
- Vacuum leaks – PCV hoses, vacuum hoses, intake manifold gasket
- Weak fuel pump
- Clogged or dirty fuel injectors
- Clogged fuel filter
- Exhaust leak
- Faulty oxygen sensor
- Faulty air-fuel ratio sensor
Detailed Explanation & Diagnosis: P0174 Meaning, Causes, Symptoms, & Fixes
#13: P0430
P0430 means the catalytic converter in Bank 2 of your engine is not functioning efficiently. Unfortunately, this means it’s not breaking down all those pollutants your engine is producing during the combustion process, which is bad for your lungs, our atmosphere, and fuel economy.
Symptoms:
- Check Engine Light is on
- Lack of power from the engine
- Decreased fuel economy
- Rotten egg or a sulfur smell
Most Common Causes:
- Faulty oxygen sensor
- Faulty air-fuel sensor
- Worn or internally failing catalytic converter (Most Common)
- A leak in the exhaust system
- Misfire (the root cause of converter failure)
- Rich or Lean air-fuel ratio (the root cause of converter failure)
- Leaded Gasoline (the root cause of converter failure)
Detailed Explanation & Diagnosis: P0430 Meaning, Causes, Symptoms, & Fixes
#14: P0305
P0305 indicates that a misfire has been detected in Cylinder 5 of your engine. This happens when an insufficient amount of fuel is burning in the cylinder and should be addressed immediately to prevent long-term engine damage.
Symptoms:
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0305 Meaning, Causes, Symptoms, & Fixes
#15: P0306
P0306 indicates that a misfire has been detected in Cylinder 6 of your engine. This happens when an insufficient amount of fuel is burning in the cylinder and should be addressed immediately to prevent engine damage.
Symptoms:
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
Most Common Causes:
- Faulty or worn spark plugs
- Faulty spark plug wires or coils
- Distributor failure
Detailed Explanation & Diagnosis: P0306 Meaning, Causes, Symptoms, & Fixes
#16: P1000
Experienced by about 56,000 drivers and FIXD customers in 2020, P1000 is a manufacturer controlled diagnostic trouble code (DTC), used mostly by Ford, Jaguar, and Mazda. Unlike other P-codes, it can mean different things for different vehicles. However, it most likely means that your car’s engine control computer hasn’t completed its onboard diagnostic system readiness test. This could be triggered by a loss of power, such as a dead battery or running out of gas.
Symptoms:
- Check engine light is on
Most Common Causes:
- Disconnected battery
- Powertrain Control Module disconnected
- Clearing engine codes
- New vehicle
Detailed Explanation & Diagnosis:
Since P1000 is a manufacturer specific diagnostic trouble code, you’ll need an OBD2 scanner like FIXD to scan your vehicle and find out exactly what it means in your situation.
#17: P0441
Your vehicle’s EVAP system prevents raw fuel from evaporating from the fuel storage system. Ideally, it stores and then purges those vapors back into the combustion process to keep things running smoothly, but when this doesn’t happen properly, code P0441 is triggered.
Symptoms:
- Check Engine Light
- Rough Idle
- Erratic Idle
Most Common Causes:
- Faulty purge valve (solenoid)
- Damaged or cracked charcoal canister
- Loose, damaged, or missing gas cap
- Damaged or loose EVAP hoses
Detailed Explanation & Diagnosis: P0441 Meaning, Causes, Symptoms, & Fixes
#18: P0113
P0113 is defined as Intake Air Temperature (IAT) Sensor Circuit High Input Problem. In simple terms? Your engine’s computer detects a problem with how much spark and fuel is being injected into the engine which can impact air fuel ratio, ignition timing, and emissions control. Over 45,000 drivers and FIXD customers had this code in 2020.
Symptoms:
- Check Engine Light
- Trouble starting the engine
- Engine running lean
Most Common Causes:
- Faulty/Loose IAT sensor
- Faulty/Loose wiring connections
Detailed Explanation & Diagnosis: P0113 Meaning, Causes, Symptoms, & Fixes
#19: P0446
The EVAP system prevents fuel vapor from escaping and polluting the atmosphere. Detected by over 43,000 FIXD customers, code P0446 is defined as an Evaporative Emission Control System Vent Control Circuit Malfunction. In simple terms, it means there’s an electrical issue causing the EVAP vent valve to malfunction.
Symptoms:
- Check Engine Light is on
- A slight decrease in fuel economy
- Gasoline smell
Most Common Causes:
- Missing, defective, damaged, or loose gas cap (most common)
- Defective EVAP Canister Vent Control Valve
- Distorted, damaged or cracked Fuel Tank Filler Neck
- Torn or punctured Evaporative system hose(s)
- Defective Fuel Tank Sending Unit gasket or seal
- Split or damaged Carbon Canister
- Defective or damaged fuel tank
- Open or shorted electrical connections
Detailed Explanation & Diagnosis: P0446 Meaning, Causes, Symptoms, & Fixes
#20: P0102
The 20th most common engine fault code experienced by our FIXD customers in 2020 was P0102. When this code is set, it means that your vehicle’s Engine Control Module (ECM) detects a low voltage output from the Mass Air Flow Sensor. In some cases, this code won’t cause any driveability issues, but it can affect fuel mileage.
Symptoms:
- Check Engine Light
- Less power to the engine
- Low fuel consumption – causing engine damage
Most Common Causes:
- Dirty air filter
- Faulty MAF sensor
- Corroded/Damaged MAF circuit terminals
- Carbon covered MAF sensor screens
Detailed Explanation & Diagnosis: P0102 Meaning, Causes, Symptoms, & Fixes
Check Engine Light? Get FIXD
After analyzing 18 million vehicles in 2020, these were the most common car problems experienced by FIXD customers and drivers like you. Got a check engine light but not sure what it means? The FIXD Sensor and free app can help! Click here to learn more about how FIXD can help you translate warning lights into plain English and save you money on car repairs.
Wife, mom, Content Manager & Senior Copywriter at FIXD. From the garage to the gym, I love helping people learn and grow. Dream car: ‘69 Acapulco Blue Mustang.
