Check Engine Light: What the Common Codes Mean

A check engine light can mean anything from a loose gas cap to a serious engine problem. The good news is that the car leaves a clue: a diagnostic trouble code stored in its computer. Knowing how to read that code helps you decide how urgent the problem is and what to ask your mechanic.

How the codes work

Cars sold in the US since the 1996 model year use a standard diagnostic system called OBD-II. When the engine computer spots a problem, it stores a code and may turn on the check engine light.

Codes follow a pattern. The first letter shows the system:

• P for powertrain (engine and transmission)
• B for body
• C for chassis
• U for network communication

The next digit shows whether the code is generic (0) or manufacturer-specific (often 1). The remaining digits narrow down the problem.

Solid light vs. flashing light

Pay attention to how the light behaves.

• Solid light: A problem has been detected. It usually is not an emergency, but get it checked soon.
• Flashing light: This often means a severe misfire that can overheat and damage the catalytic converter. Reduce speed, avoid hard acceleration, and get the car checked as soon as possible.

If the light comes on along with overheating, low oil pressure or a loss of power, pull over safely and stop.

Common codes and what they point to

These generic codes come up often. A code tells you what the computer saw, not always the exact failed part.

• P0300 to P0308: Misfire. P0300 is a random or multiple misfire. P0301 through P0308 point to a specific cylinder. Causes include worn spark plugs, bad ignition coils, fuel injector problems or vacuum leaks.
• P0420: Catalyst efficiency below threshold (bank 1). The catalytic converter may not be working well. It can also be triggered by a faulty oxygen sensor, an exhaust leak or an engine problem upstream.
• P0171 and P0174: System too lean. The engine is getting too much air or too little fuel. Vacuum leaks, a dirty mass airflow sensor or weak fuel delivery are common causes.
• P0128: Coolant temperature below thermostat regulating temperature. Often a thermostat stuck open, which can hurt fuel economy and heater performance.
• P0442 and P0455: Evaporative emission system leak. P0442 is a small leak, P0455 a large one. A loose or damaged gas cap is a frequent cause, though hoses and valves can fail too.
• P0401: Exhaust gas recirculation flow insufficient. Often from carbon buildup in EGR passages or a failing EGR valve.

How to read your code

You have a few options:

1. Buy an inexpensive OBD-II scanner that plugs into the port under the dashboard.
2. Use a Bluetooth adapter with a phone app.
3. Ask an auto parts store, many of which will read codes at no charge.
4. Have a repair shop run a full diagnosis.

A basic scan gives you the code. A full diagnosis uses live data and testing to find the actual cause. Clearing the code without fixing the problem just means it will come back.

Do not ignore it

It is tempting to drive with the light on, but a few problems get more expensive if you wait. A misfire can damage the catalytic converter. A lean condition can cause engine damage over time. And while the light stays on, a new problem can show up without a new warning.

Also, if you have a service contract, a long-ignored warning light may raise questions about whether a later failure was caused by neglect.

Codes, diagnosis and coverage

Many of the parts behind these codes, like sensors, coils and fuel system components, may be covered by a factory warranty or a service contract, depending on the plan. Emissions parts sometimes have separate coverage under federal rules, so ask your dealer about that too. Diagnostic fees are handled differently from plan to plan, so check your contract. Our engine coverage page explains what many plans include.

How Warranty Wizards can help

Warranty Wizards helps you compare plans from third-party providers and explains what each covers before you buy. If you want to see options for your car, request a free quote.