What does that check engine code actually mean?

Enter your OBD-II trouble code and get a plain explanation, how serious it is, what probably broke, and what repair costs look like. No mechanic jargon. No upsell pressure.

Codes start with P, B, C, or U followed by 4 digits. Example: P0301, B0028, C0035, U0121.

Common codes:

Enter a code above to get started

Your translation will appear here with severity, likely cause, cost estimate, and DIY info.

Recent Lookups

Your last 10 code lookups are saved in this browser.

Browse Codes by Symptom

Not sure of the code? Find it by what your car is doing.

Rough idle or shaking 4 codes
  • — Random/multiple cylinder misfire
  • — Cylinder 1 misfire
  • — Cylinder 2 misfire
  • — System too lean (Bank 1)
Poor fuel economy 3 codes
  • — System too lean (Bank 1)
  • — System too lean (Bank 2)
  • — Catalyst efficiency below threshold
Slow warm-up or low temp gauge 2 codes
  • — Coolant thermostat (coolant below regulating temp)
  • — Engine coolant temperature circuit
Hissing or sulfur smell 3 codes
  • — Catalyst efficiency below threshold
  • — EVAP system small leak
  • — EVAP system gross leak
Loss of power on acceleration 3 codes
  • — Exhaust camshaft position timing
  • — Turbo/supercharger underboost
  • — Random/multiple cylinder misfire
Car won't pass emissions 3 codes
  • — Catalyst efficiency below threshold
  • — EVAP system malfunction
  • — System too lean (Bank 1)

When Multiple Codes Appear Together

Some codes are root causes. Others are symptoms. Here are the most common pairings and what they really mean.

Code Combo What's Really Happening Fix Order
P0300 + P0420 Misfires are dumping unburned fuel into the catalytic converter, damaging it. The converter code is a symptom. Fix misfire first. Clear codes. Drive 50 miles. If P0420 returns, then replace the converter.
P0171 + P0174 Both banks running lean. Usually a vacuum leak, MAF sensor, or fuel delivery problem. Not two separate issues. Check for vacuum leaks first (cracked hoses, intake gasket). Clean or replace MAF sensor. Check fuel pressure.
P0442 + P0455 Both are EVAP leak codes at different sizes. Usually one problem: loose gas cap, cracked hose, or bad purge valve. Tighten or replace gas cap first. Clear codes. If they return, smoke-test the EVAP system.
P0301 + P0302 + P0303 Multiple cylinders misfiring. Could be a bad coil pack (if coils are shared), fuel injector issue, or low compression. Swap coil packs between cylinders. If the misfire moves, it's the coil. If not, check injectors and compression.
P0014 + P0340 Camshaft timing and cam sensor codes together often mean a stretched timing chain or worn phaser, not just a bad sensor. Don't just replace the sensor. Have the mechanic check chain stretch and phaser operation.

Before You Visit the Mechanic

Print or screenshot this checklist so you walk in prepared.

What to Bring

  • The exact trouble code(s)
  • When the light came on (mileage, conditions)
  • What symptoms you've noticed
  • Any recent repairs or maintenance
  • This page's cost estimate for your code

Questions to Ask

  • "What is the root cause, not just the code?"
  • "Can you show me the failed part?"
  • "Is this covered under any recall or warranty?"
  • "What happens if I wait a week to fix this?"
  • "Can I get a written estimate before you start?"

Red Flags

  • Mechanic wants to clear codes without diagnosing
  • Quote is 2x or more above the estimate range
  • Pressure to approve expensive repairs on the spot
  • Refusal to explain what they found
  • "Your whole system needs replacing" for one code

Read Codes at Home for $15-30

A Bluetooth OBD-II scanner plugs into your car's diagnostic port and pairs with your phone. You can read and clear codes yourself, check if a repair worked, and know what's wrong before you ever set foot in a shop. It pays for itself the first time you avoid a diagnostic fee.

Look for a scanner that works with your phone's operating system. ELM327-based models are the most common and work with free apps like Torque (Android) or Car Scanner (iOS and Android).

Common Mistakes That Waste Money

Clearing codes without fixing the problem

Your car's computer needs to run self-tests after codes are cleared. If you clear them right before an emissions test, you'll fail because the tests aren't complete. Fix the issue first, then clear and drive at least 50 miles.

Replacing the most expensive part first

A P0420 code points to the catalytic converter, but the real cause is often a bad oxygen sensor or an exhaust leak. The sensor costs $50. The converter costs $1,200. Always check the cheaper causes first.

Ignoring a code because the car "feels fine"

Some problems don't change how the car drives until they cause a breakdown. A small EVAP leak won't affect performance, but it will cause you to fail emissions and can lead to a cracked fuel tank over time.

Going to the dealer for everything

Dealers charge 30-60% more than independent shops for the same work. For most repairs, a well-reviewed independent mechanic with the right diagnostic tools will do the same job for less. Save the dealer for warranty work and recalls.