Modern vehicles are sophisticated machines loaded with sensors and electronic systems designed to monitor performance, safety, and emissions. When something goes wrong, your vehicle’s onboard computer stores a Diagnostic Trouble Code (DTC), which helps pinpoint the issue
Understanding these codes can save you time, money, and stress at the repair shop. Below are 10 of the most common DTCs, what they mean, and what typically causes them.
1. P0171 – System Too Lean (Bank 1)
Meaning: The engine’s air/fuel mixture is too lean (too much air or not enough fuel).
Common Causes:
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Vacuum leaks
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Faulty or dirty Mass Air Flow (MAF) sensor
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Weak fuel pump or clogged fuel filter
Symptoms: Rough idle, poor acceleration, check engine light
2. P0300 – Random/Multiple Cylinder Misfire Detected
Meaning: The engine is misfiring, but not in a consistent pattern.
Common Causes:
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Faulty spark plugs or ignition coils
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Fuel delivery issues
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Engine vacuum leaks
Symptoms: Rough running engine, hesitation, engine shaking
3. P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
Meaning: The catalytic converter isn’t operating efficiently.
Common Causes:
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Failing catalytic converter
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Bad oxygen sensors
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Exhaust leaks near O2 sensor
Symptoms: Decreased fuel economy, failed emissions test
4. P0455 – Evaporative Emission System Leak Detected (Gross Leak)
Meaning: A large leak has been detected in the evaporative emissions system.
Common Causes:
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Loose or missing gas cap
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Cracked EVAP hose
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Faulty purge or vent valve
Symptoms: Check engine light, possible fuel smell
5. P0442 – Evaporative Emission System Leak Detected (Small Leak)
Meaning: A smaller, less obvious EVAP leak has been detected.
Common Causes:
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Loose gas cap
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Small cracks in hoses
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Faulty EVAP canister
Symptoms: Often none except the check engine light
6. P0133 – O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
Meaning: The front oxygen sensor is not responding quickly enough to changes in the exhaust gas.
Common Causes:
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Aging O2 sensor
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Exhaust leaks
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Contaminated sensor
Symptoms: Decreased fuel efficiency, check engine light
7. P0113 – Intake Air Temperature Sensor 1 Circuit High Input
Meaning: The sensor reading is too high, suggesting a possible fault.
Common Causes:
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Disconnected sensor
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Damaged wiring
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Faulty IAT sensor
Symptoms: Poor engine performance, check engine light
8. P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Meaning: The engine isn’t reaching optimal operating temperature quickly enough.
Common Causes:
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Stuck open thermostat
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Faulty coolant temperature sensor
Symptoms: Poor fuel economy, heater not working well
9. P0401 – Exhaust Gas Recirculation (EGR) Flow Insufficient
Meaning: The EGR valve isn’t allowing enough exhaust gases back into the engine.
Common Causes:
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Clogged EGR passages
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Faulty EGR valve
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Vacuum issues
Symptoms: Increased emissions, engine knocking, check engine light
10. P0700 – Transmission Control System Malfunction
Meaning: There’s a general fault in the transmission control system.
Common Causes:
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Faulty transmission control module
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Wiring issues
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Internal transmission failure
Symptoms: Transmission stuck in limp mode, harsh shifting
All in one i will just say and conclude that;
These DTCs are some of the most frequently encountered and can be triggered by a range of causes from minor issues like a loose gas cap to more serious problems like catalytic converter failure. If your check engine light is on, don’t ignore it use an OBD-II scanner to check the code, or visit a trusted mechanic. Understanding the code is the first step in preventing costly repairs and maintaining your vehicle’s performance.
