Look Up Chevy Check Engine Codes: A Practical Code List and How to Read It
Chevrolet check engine codes are diagnostic trouble codes (DTCs) stored by the vehicle's engine computer when it detects a fault. For Chevy models from the 1996 model year onward sold in the United States, these follow the OBD-II (On-Board Diagnostics, second generation) standard, so most powertrain codes share the same meanings used by other manufacturers. A smaller set of codes is specific to General Motors, and their definitions can vary by model year, engine and market. Most Chevrolets built before 1996 use an older GM system that reports two-digit codes instead. Which group your vehicle falls into decides how you read the code and what it means.
How a Chevy OBD-II Code Is Structured
An OBD-II code has five characters, such as P0300. Each position tells you something about the fault before you look up the full definition.
Position | Example | Meaning; 1st character (letter) | P, B, C or U | System: P = powertrain (engine and transmission), B = body, C = chassis, U = network or communication; 2nd character | 0 or 1 | 0 = generic code defined by the industry standard; 1 = manufacturer-specific code (GM-defined on a Chevrolet); 3rd character | 1 to 8 | Subsystem for powertrain codes: 1 and 2 = fuel and air metering, 3 = ignition and misfire, 4 = auxiliary emissions controls, 5 = vehicle speed and idle control, 6 = computer and output circuits, 7 and 8 = transmission; 4th and 5th characters | 00 to 99 | The specific fault within that subsystem
Common Generic Check Engine Codes on Chevrolet Vehicles
The codes below are standard OBD-II definitions, so they mean the same thing on a Chevrolet as on other OBD-II vehicles. They are widely encountered codes, but this list is not ranked by how often they appear on any specific Chevy model. The likely causes listed are typical areas to inspect, not confirmed diagnoses.
Code | Definition | Typical areas to inspect; P0101 | Mass air flow (MAF) sensor circuit range or performance | MAF sensor, air intake leaks, air filter; P0128 | Coolant temperature below thermostat regulating temperature | Thermostat, coolant temperature sensor; P0171 | System too lean, bank 1 | Vacuum leaks, MAF sensor, fuel delivery; P0174 | System too lean, bank 2 | Same as P0171 on engines with two cylinder banks; P0300 | Random or multiple cylinder misfire detected | Spark plugs, ignition coils, fuel injectors, vacuum leaks; P0301 to P0308 | Misfire detected in cylinder 1 through 8 | Ignition and fuel components for the named cylinder; P0335 | Crankshaft position sensor A circuit | Crankshaft sensor, wiring, connector; P0340 | Camshaft position sensor circuit | Camshaft sensor, wiring, connector; P0420 | Catalyst system efficiency below threshold, bank 1 | Catalytic converter, oxygen sensors, exhaust leaks; P0442 | Evaporative emission (EVAP) system small leak detected | Fuel cap, EVAP hoses, purge and vent valves; P0455 | EVAP system large leak detected | Fuel cap missing or loose, disconnected EVAP hoses; P0506 | Idle control system RPM lower than expected | Throttle body, intake restrictions; P0507 | Idle control system RPM higher than expected | Vacuum leaks, throttle body; P0700 | Transmission control system malfunction | Indicates a transmission code is stored; read the transmission module for details
GM-Specific Codes
Codes with a 1 in the second position, such as P1xxx, B1xxx, C1xxx or U1xxx, are defined by GM rather than by the industry standard. The same manufacturer code can carry different meanings across model years, engines or platforms, so a generic online list is not reliable for these. Check the definition against GM service information for your exact vehicle, or use a scan tool that decodes GM-specific codes for your model.
Pre-1996 Chevrolets: Two-Digit Flash Codes
Most Chevrolets from the early 1980s through 1995 use GM's earlier OBD-I system with a 12-pin ALDL (Assembly Line Diagnostic Link) connector, usually under the dashboard. On many of these vehicles, bridging terminals A and B with the ignition on and the engine off makes the check engine light flash stored codes as two-digit numbers. Code 12 typically indicates the diagnostic system is working. Some 1994 and 1995 models already used OBD-II, so confirm which connector your vehicle has. Two-digit code meanings vary by engine and year, so use a reference specific to your vehicle.
How to Read the Codes on a 1996-or-Newer Chevy
Find the 16-pin OBD-II port, normally under the dashboard on the driver's side. Connect an OBD-II scan tool or code reader with the ignition off, then turn the ignition on. Read and write down every stored and pending code, including any freeze-frame data the tool shows. Look up each code using the structure and list above, and use GM-specific references for any code with a 1 in the second position. Inspect and repair before clearing codes, because clearing them also erases the freeze-frame data that helps diagnosis.
Many auto parts retailers in the United States offer free code scans. Basic readers usually show generic powertrain codes. Reading body, chassis and network codes, or transmission module details, often requires a more capable scan tool.
What a Code Tells You and What It Doesn't
A trouble code identifies the fault the computer detected, not necessarily the part that failed. For example, a lean code such as P0171 can be caused by a vacuum leak, a dirty sensor or a fuel delivery problem. Treat the code as the starting point for diagnosis. A flashing check engine light, as opposed to a steady one, generally signals an active misfire severe enough to damage the catalytic converter. In that case, reduce driving and have the vehicle checked promptly.
Limits of This Guide
This guide covers standard OBD-II code structure, widely used generic definitions and the general approach for older GM systems. It does not include model-specific GM code definitions, frequency data for particular Chevrolet models or engines, technical service bulletins, or repair costs, because that information was not available for this article. For those details, consult GM service information for your exact model year, engine and market.