GM Check Engine Light Codes: Most Common & What They Mean

The Check Engine Light (CEL) is the most misunderstood warning on any GM or Chevrolet dashboard. It can mean something as trivial as a loose gas cap — or something as serious as a failing catalytic converter. The only way to know is to read the code.

This guide covers the most common OBD-II diagnostic trouble codes (DTCs) on GM and Chevrolet vehicles, what they mean in plain English, how serious each one is, and what it typically costs to fix.

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First: How to Read GM Check Engine Light Codes

Any auto parts store (AutoZone, O'Reilly, Advance Auto Parts) will scan your codes for free. You can also buy a basic OBD-II scanner for $25–$50 that plugs into the port under your dash (driver's side, near the steering column) and pairs with a free smartphone app.

GM-specific codes may require a more advanced scanner (like a Tech2 clone, BlueDriver, or OBDLINK MX+) to read manufacturer-specific data beyond standard OBD-II codes.

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The Most Common GM Check Engine Light Codes

🔴 P0300 – Random/Multiple Cylinder Misfire

What it means: One or more cylinders are misfiring, but the PCM can't isolate it to a single cylinder. Often accompanied by rough idle, loss of power, or engine shaking.

Common causes: Worn spark plugs, bad ignition coils, dirty fuel injectors, low compression, vacuum leak

How serious: High — misfires can damage catalytic converters quickly. Don't drive long distances with this code active.

Typical fix cost: $50–$400 depending on cause (plugs and coils are cheap; injectors or compression issues cost more)

📖 Full guide: P0300 Code: Random/Multiple Cylinder Misfire Detected · Engine Misfire: Meaning, Symptoms & Repair Guide

🔴 P0301–P0308 – Cylinder-Specific Misfire

What it means: A misfire detected on a specific cylinder (P0301 = cylinder 1, P0302 = cylinder 2, etc.)

First step: Swap the ignition coil from the misfiring cylinder with an adjacent cylinder. If the misfire code follows the coil to the new cylinder, replace the coil (~$30–$60). If it stays on the original cylinder, check the spark plug and injector.

Typical fix cost: $30–$150 for coil/plug; more if injector-related

🟡 P0171 / P0174 – System Too Lean (Bank 1 / Bank 2)

What it means: The engine is running with too much air or not enough fuel. Very common on GM 5.3L and 6.0L V8 engines.

Common causes on GM vehicles: Dirty or failing Mass Air Flow (MAF) sensor, vacuum leak, weak fuel pump, dirty fuel injectors, faulty oxygen sensor

Quick DIY check: Clean the MAF sensor with MAF-safe cleaner spray ($8). This resolves P0171/P0174 surprisingly often.

Typical fix cost: $10–$300 depending on cause

📖 Full guide: P0171 Code: System Too Lean, Bank 1

🟡 P0420 / P0430 – Catalyst System Efficiency Below Threshold

What it means: The catalytic converter isn't converting exhaust gases efficiently enough. Bank 1 (P0420) or Bank 2 (P0430).

Important: Before replacing a catalytic converter ($400–$1,200), rule out a faulty oxygen sensor — a bad downstream O2 sensor mimics this code constantly. A $50 sensor can save you from an unnecessary cat replacement.

Typical fix cost: $50–$1,200

📖 Full guides: P0420 Code (Bank 1) · P0430 Code (Bank 2)

🟡 P0455 / P0442 / P0440 – EVAP System Leak

What it means: A leak has been detected in the evaporative emission control system — the system that captures fuel vapors.

P0455 = Large leak (often a loose or missing gas cap)

P0442 = Small leak (could be a cracked hose, faulty purge valve, or fuel cap seal)

First step: Tighten your gas cap fully. If the code clears after a few drive cycles, that was the culprit. GM had a TSB covering faulty OEM gas caps on many 2010–2019 models.

Typical fix cost: $0–$200

🟢 P0128 – Coolant Temperature Below Thermostat Regulating Temperature

What it means: The engine isn't reaching normal operating temperature as quickly as it should. Almost always a failing thermostat stuck in the open position.

Why it matters: A stuck-open thermostat causes reduced fuel economy, poor heater performance, and accelerated engine wear (engines are designed to run at 195–210°F).

Typical fix cost: $80–$200 (thermostats are cheap; labor is the main cost)

📖 Related guide: Thermostat Maintenance Guide for GM Vehicles

🟡 P0507 – Idle Control System RPM High

What it means: Idle speed is higher than the PCM's target. Common on GM vehicles with electronic throttle bodies.

Common causes: Dirty throttle body, vacuum leak, faulty IAC (older models), PCM issue

DIY fix: Clean the throttle body with throttle body cleaner and a rag. This is a common fix that takes 20 minutes and costs under $10.

📖 Full guide: Throttle Body Cleaning: Meaning, Symptoms & How-To Guide

🔴 P0008 / P0009 – Engine Position System Performance (Bank 1/2)

What it means: The timing correlation between the crankshaft and camshaft is off. Very common on GM 5.3L and 6.2L engines with Active Fuel Management (AFM/DFM).

Common cause: Worn timing chain, failing VVT solenoid (cam phaser), or low oil pressure from sludge. This is a more serious code — don't ignore it.

Typical fix cost: $500–$2,000+ depending on whether it's a solenoid, timing chain, or oil system issue

📖 Full guide: Timing Chain Replacement: Meaning, Symptoms & Repair Guide

🟡 P0013 / P0014 / P0010 / P0011 – Camshaft Position / VVT Solenoid Codes

What it means: The variable valve timing system isn't responding correctly. Extremely common on GM 5.3L engines (Silverado, Tahoe, Suburban, Sierra, Yukon) as they age.

Common fix: Replace the VVT oil control solenoid ($15–$40 for the part). Often caused by dirty oil that clogs the small passages in the solenoid — another reason regular oil changes matter.

Typical fix cost: $100–$400

📖 Full guides: P0014 Code · P1011 Code

🟡 P0521 / P0523 – Engine Oil Pressure Sensor

What it means: The oil pressure sensor is reading out of range. Could be a failing sensor — or actual low oil pressure.

Critical: Don't assume it's just the sensor. Check your oil level first. If oil level is fine and you're getting this code, have the actual oil pressure tested with a mechanical gauge before replacing the sensor.

📖 Full guides: P0521 Code · P06DD Code · Oil Pressure Problems: Meaning, Symptoms & Repair Guide

🟢 B0085 / U-Codes – Body/Network Communication Codes

What it means: Module communication faults on the CAN bus network. Often caused by a low battery, recent battery replacement, or a module that lost power.

First step: Clear the codes after verifying battery health. Many U-codes resolve themselves once the battery is in good condition and the modules have cycled.

🔴 P1682 – Ignition 1 Switch Circuit 2 Malfunction

What it means: A GM-specific electrical code tied to intermittent no-start complaints — almost never the ignition switch itself.

📖 Full guide: P1682 Code: Ignition 1 Switch Circuit 2 Malfunction

🔴 P0741 – Torque Converter Clutch Circuit Performance or Stuck Off

What it means: One of the most common transmission-related codes on GM vehicles — the torque converter clutch isn't locking up correctly.

📖 Full guide: P0741 Code: Torque Converter Clutch Circuit Performance

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GM Check Engine Light: How Serious Is It?

Severity	What It Looks Like	Action
🟢 Low	CEL on, car drives fine, code is EVAP/sensor	Get it scanned soon, not urgent
🟡 Medium	CEL on, slight performance change (lean code, MAF, O2)	Address within 1–2 weeks
🔴 High	CEL flashing, misfire, rough running, oil pressure code	Stop driving — get it diagnosed immediately

Note: A flashing Check Engine Light means an active misfire is occurring severe enough to damage the catalytic converter. Pull over safely as soon as possible.

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Can You Drive With the Check Engine Light On?

It depends entirely on the code. A P0455 (loose gas cap) — yes, drive normally. A P0300 with a flashing light — no, pull over. Always scan first before making that call.

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Related GM Service Guides

Every OBD-II code covered on this site:

• P0300 – Random/Multiple Cylinder Misfire
• P0171 – System Too Lean, Bank 1
• P0420 – Catalyst Efficiency, Bank 1
• P0430 – Catalyst Efficiency, Bank 2
• P0521 – Oil Pressure Sensor Range/Performance
• P06DD – Oil Pressure Control Solenoid Stuck Off
• P0014 – Camshaft Timing Over-Advanced, Bank 1
• P1011 – Intake Camshaft Actuator Park Position
• P1682 – Ignition 1 Switch Circuit 2 Malfunction
• P0741 – Torque Converter Clutch Performance

Related engine repair guides:

• Engine Misfire: Meaning, Symptoms & Repair Guide
• 5.3L Lifter Noise: Meaning, Symptoms & Repair Guide
• 5.3L AFM Failure: Meaning, Symptoms & Repair Guide
• 6.2L Lifter Replacement: Meaning, Symptoms & Repair Guide
• GM AFM/DFM Lifter Failure: Symptoms, Causes & What To Do
• Knock Sensor Replacement: Meaning, Symptoms & Repair Guide
• Valve Cover Leaks: Meaning, Symptoms & Repair Guide
• Rear Main Seal Leaks: Meaning, Symptoms & Repair Guide
• Service StabiliTrak Light On? Here's What It Means
• GM Transmission Problems: Symptoms, Causes & What To Do
• 5 Signs Your Car Needs Immediate Service
• The Complete GM & Chevrolet Owner's Guide

— GM Service Guy

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