How to read OBD2 codes and what each character means

Step 1: pull the codes from the car

Every gasoline car and light truck sold in the US since the 1996 model year has a 16-pin OBD2 port, usually under the dash on the driver side. Plug a scan tool into that port, turn the key to on (or start the engine), and let the tool connect. If you have never done this, the step by step scanner guide and the OBD2 port location guide cover the details.

1. 1

   Find the OBD2 port under the dash and plug in the scanner firmly.

2. 2

   Turn the ignition to on. Many tools also work with the engine running.

3. 3

   Let the tool read the vehicle ID or choose the make and model if asked.

4. 4

   Open the read codes or DTC menu and write down every code, including pending ones.

5. 5

   Save or photograph the freeze frame data before you clear anything.

Note.Do not clear codes before you write them down. Clearing erases the freeze frame snapshot and resets readiness monitors, which you may need later.

Step 2: read the five characters

OBD2 trouble codes follow the SAE J2012 format. Each position has a job, so you can tell a lot about a code before you ever look it up.

The first character is a letter for the system. P is powertrain (engine, transmission, emissions). B is body (airbags, seats, lighting). C is chassis (ABS, steering, suspension). U is network communication between modules.

The second character is a digit. 0 means a generic code defined by SAE that means the same thing on every make. 1 means a manufacturer specific code, so the same number can mean different things on a Ford and a Toyota. For P codes, 2 is mostly generic and 3 is a mix of generic and manufacturer ranges.

The third character names the subsystem. For powertrain codes, the common groups are listed in the table below. The last two characters identify the specific fault inside that subsystem.

Step 3: look up the exact meaning

Once you know the structure, look up the full definition. Generic P0 codes have the same meaning on every vehicle, so a general code list works. Manufacturer codes (P1, B1, C1, U1 and so on) need a make specific source. A good scan tool shows the definition on screen, and our code decoder breaks any code into its parts and links to a full write up when one exists.

Read the whole definition, not just the short label. P0171 says system too lean on bank 1. That tells you the engine computer added fuel to correct a lean mixture and hit its limit. It does not say a part is bad. It says what the computer observed. Most codes work this way: they describe a symptom the computer measured, and you diagnose why.

Our most common code pages walk through causes in order: P0420 catalyst efficiency, P0171 system too lean, P0300 random misfire and P0455 EVAP large leak.

Pending, stored and permanent codes

Scanners often show codes in separate lists. Knowing the difference helps you decide what to do next.

• Pending codes are faults the computer saw once but has not confirmed. Many two trip faults set a pending code first. If the fault shows up again on the next qualifying drive, it becomes stored and the light comes on.
• Stored or confirmed codes are the ones that turned on the check engine light. These are what most people mean by a code.
• Permanent codes were added for 2010 and newer vehicles. They cannot be erased with a scan tool. The car clears them itself after it confirms the repair by running the related monitor and passing.

Note.A pending code with no light is an early warning. It is worth reading and watching, especially before an inspection.

Use freeze frame data to add context

When a code sets, the computer stores a freeze frame: a snapshot of engine speed, load, coolant temperature, vehicle speed, fuel trims and more at the moment of the fault. This tells you whether the problem happened cold or warm, at idle or on the highway. A lean code that only sets at idle points toward a vacuum leak. One that sets under heavy load points more toward fuel delivery. The freeze frame guide explains each field.

Live data adds even more. Watching fuel trims, oxygen sensor readings and misfire counters while the engine runs is how you go from a code to a confirmed cause.

Turn the code into a repair plan

A code points to a system, not a part. The standard approach is to check the cheapest and most likely causes first, confirm with data, and only then replace parts.

1. 1

   Write down all codes and freeze frame data.

2. 2

   Fix the earliest or most basic code first. A misfire or lean code can cause a catalyst code later, so treat the root cause.

3. 3

   Inspect the obvious: loose gas cap, cracked vacuum hoses, unplugged connectors, damaged wiring.

4. 4

   Check for technical service bulletins for your vehicle and code.

5. 5

   Use live data to confirm the fault before buying parts.

6. 6

   After the repair, clear the code and complete a drive cycle so the monitors can rerun.

See scanners that show live data and freeze frame →

Which scanner you need to read codes

A basic code reader pulls generic engine codes and clears them. That covers the check engine light. To read ABS, airbag, transmission or body codes you need a tool that talks to those modules, which most basic readers cannot do. The scanner vs code reader comparison explains the difference.

If you work on your own cars often, a scanner with live data, freeze frame, and access to all modules saves time. Our guide to the best OBD2 scanners for DIY mechanics covers good options, and the ABS and SRS scanner guide covers tools that read beyond the engine.

Frequently asked questions

Is there a full OBD2 codes list? Generic codes are defined in SAE J2012 and are the same on every vehicle. Manufacturer specific codes vary by make, so no single list covers them all. Use a scanner with built in definitions or our code decoder for any code.

What does a code mean if the first digit is 1? It is a manufacturer specific code. The same number can mean different things on different makes, so look it up for your exact vehicle.

Can I read codes without a scanner? Some older vehicles flash codes through the dash, but on OBD2 cars you need a scan tool. Many auto parts stores in the US will read codes for free.

Does reading codes erase them? No. Reading only displays them. Codes and freeze frame are erased only when you choose clear codes, disconnect the battery on many cars, or the computer clears them itself after enough good drives.