Free OBD-II Code Lookup

What is an OBD-II Code?

An OBD-II code — also called a Diagnostic Trouble Code or DTC — is a 5-character code stored in your vehicle’s computer when it detects a fault. Every car sold in the United States since 1996 uses this same standard (defined in SAE J2012), which is why the same scanner works on a Honda Civic, a Ford F-150, and a BMW 3 Series.

The format is always: one letter + four digits. The letter identifies the system. The first digit (0 or 1) tells you whether the code is generic or manufacturer-specific. The remaining three digits identify the specific fault. So P0420 is a generic powertrain code, while P1399 is a manufacturer-specific powertrain code (Honda, in this case).

Code Categories Explained

Top 20 Most Common OBD-II Codes

These are the codes mechanics see most often. Click any code to decode it.

Severity Levels Explained

How to Read OBD Codes Without a Scanner

Most modern cars require a scanner to read trouble codes — but there are a few free options before you spend money:

• Use a free retail scan — AutoZone, O’Reilly, Advance Auto Parts, and most regional chains will scan your codes for free. They’ll print out the codes — bring the printout home and decode each one here.
• Buy a $20 Bluetooth dongle — An ELM327-compatible Bluetooth OBD-II dongle plus a free app like Torque, Car Scanner, or OBD Auto Doctor can read codes, clear codes, and show live data on your phone. Strongly recommended if you plan to keep older vehicles.
• Check the key-cycle method (older cars only) — Some pre-2005 GM, Chrysler, and Honda vehicles flash the codes on the dash by cycling the ignition key (typically ON-OFF-ON-OFF-ON without starting). This rarely works on modern vehicles. Search your specific year/make/model to confirm.
• Use the iPhone or Android shop apps — Some vehicles (newer Teslas, GM cars with OnStar, Ford with FordPass) display fault codes directly in the manufacturer’s app without needing a scanner.

When to Drive vs Tow

Frequently Asked Questions

What does an OBD-II code mean?

It's a 5-character code (1 letter + 4 digits) stored by your car's computer when it detects a fault. The letter identifies the affected system: P=Powertrain, B=Body, C=Chassis, U=Network.

Is it safe to drive with the check engine light on?

A solid CEL is usually OK to drive home, but a flashing CEL means an active misfire — pull over and avoid driving to prevent catalytic converter damage.

What does P0420 mean?

P0420 is 'Catalyst System Efficiency Below Threshold (Bank 1)' — your catalytic converter isn't working efficiently. Often caused by a worn cat, faulty O2 sensor, or exhaust leak. Repair: $200–$2,500.

Will clearing OBD codes pass an emissions test?

No — clearing codes resets readiness monitors. Most states require monitors to be 'set' before testing, which takes 50–200 miles of normal driving across multiple drive cycles.

Can a check engine light come back on after I clear it?

Yes, if the underlying problem isn't fixed. Many codes will return within 1–3 drive cycles. Always diagnose and repair the root cause before clearing.

What's the difference between generic and manufacturer-specific codes?

Generic codes (P0XXX) are the same across all manufacturers. Manufacturer-specific codes (P1XXX, B1XXX, C1XXX, U1XXX) are defined by the automaker and may mean different things on different brands.

How much does it cost to fix a check engine light?

Anywhere from $0 (tighten gas cap) to $4,000+ (transmission rebuild). Decode the specific code first to set the right expectation — this tool gives you a cost range for each code.

Can I read OBD-II codes without a scanner?

On most modern vehicles, no — you need a scan tool. A $20–$40 Bluetooth dongle paired with a free phone app is the cheapest option. Most auto parts stores will also scan your codes for free.