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Free DTC code guides

Plain-English guides to 32 common OBD-II trouble codes, written around one idea: test before you replace. Each guide covers what the code means, what sets it, symptoms, whether it's likely safe to drive, common causes and a numbered diagnostic sequence, with sources.

General guidance only. Always follow the vehicle manufacturer's service information and specifications. Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information.

Misfire

• P0300 Random/Multiple Cylinder Misfire Detected
• P0301 Cylinder 1 Misfire Detected
• P0302 Cylinder 2 Misfire Detected
• P0303 Cylinder 3 Misfire Detected
• P0304 Cylinder 4 Misfire Detected

Fuel trim

• P0171 System Too Lean (Bank 1)
• P0174 System Too Lean (Bank 2)
• P0172 System Too Rich (Bank 1)
• P0175 System Too Rich (Bank 2)

Catalyst

• P0420 Catalyst System Efficiency Below Threshold (Bank 1)
• P0430 Catalyst System Efficiency Below Threshold (Bank 2)

EVAP

• P0442 Evaporative Emission System Leak Detected (Small Leak)
• P0455 Evaporative Emission System Leak Detected (Large Leak)
• P0456 Evaporative Emission System Leak Detected (Very Small Leak)
• P0446 Evaporative Emission System Vent Control Circuit

O2 sensors

• P0135 O2 Sensor Heater Circuit (Bank 1 Sensor 1)
• P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)
• P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)

Air & fuel sensors

• P0101 Mass or Volume Air Flow "A" Circuit Range/Performance
• P0113 Intake Air Temperature Sensor 1 Circuit High

Variable valve timing

• P0011 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)
• P0016 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor A)

Ignition & engine sensors

• P0335 Crankshaft Position Sensor "A" Circuit
• P0340 Camshaft Position Sensor "A" Circuit (Bank 1 or Single Sensor)
• P0325 Knock Sensor 1 Circuit (Bank 1 or Single Sensor)

EGR

• P0401 Exhaust Gas Recirculation "A" Flow Insufficient Detected

Idle control

• P0507 Idle Control System RPM Higher Than Expected

Boost

• P0299 Turbocharger/Supercharger "A" Underboost Condition

Cooling

• P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
• P0118 Engine Coolant Temperature Sensor 1 Circuit High

Transmission

• P0700 Transmission Control System (MIL Request)

Network

• U0100 Lost Communication With ECM/PCM "A"

How-to guides

• Guide How to Read Fuel Trims: A Practical Guide for Lean and Rich Codes
• Guide Using Freeze Frame Data to Diagnose Check Engine Lights
• Guide Smoke Testing for EVAP and Vacuum Leaks: Method and Safety
• Guide Voltage Drop Testing Basics for Automotive Circuits

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