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OBD-II Codes
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Type or paste any engine code — P0301, P0420, U0100… — and get a clear explanation, the likely causes and the fixes. No signup, no charge.
P, B, C & U codes
Searchable dictionary
Causes & fixes included
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What is an OBD-II code?
OBD-II (On-Board Diagnostics) is the standard diagnostic system fitted to cars since 1996. When a sensor or system behaves abnormally, the ECU stores a Diagnostic Trouble Code (DTC) and usually turns on the check engine light.
Every code is a five-character string, e.g. P0301: the letter identifies the system (Powertrain, Body, Chassis, Network), the first digit tells you if it is generic or manufacturer-specific, the second points to a subsystem, and the last two identify the exact fault.
OBD-II code decoder
System
—
Type
—
Subsystem
—
Fault #
—
P0301
—
Description
—
Common causes
—
Possible fixes
—
Search by symptom
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Codes that often match
Pick a symptom to see the likely codes.
Common codes
Search codes…
| Code | Description |
|---|---|
| B0001 | Driver Frontal Stage 1 Deployment Loop |
| B0022 | Driver Airbag Circuit Malfunction |
| B1325 | Lamp Control Circuit Malfunction |
| B1342 | ECU Internal Fault |
| C0035 | Left Front Wheel Speed Sensor Circuit |
| C0040 | Right Front Wheel Speed Sensor Circuit |
| C0561 | ABS System Disabled |
| P0010 | "A" Camshaft Position Actuator Circuit (Bank 1) |
| P0011 | "A" Camshaft Position Timing Over-Advanced (Bank 1) |
| P0016 | Crankshaft/Camshaft Timing Misalignment (Bank 1) |
| P0021 | "A" Camshaft Position Timing Over-Advanced (Bank 2) |
| P0030 | HO2S Heater Control Circuit (Bank 1, Sensor 1) |
| P0031 | HO2S Heater Control Circuit Low (Bank 1, Sensor 1) |
| P0100 | Mass Air Flow (MAF) Sensor Circuit Malfunction |
| P0101 | Mass Air Flow (MAF) Sensor Range/Performance |
| P0106 | MAP Sensor Range/Performance |
| P0113 | Intake Air Temperature Sensor High Input |
| P0115 | Engine Coolant Temperature Sensor Circuit |
| P0117 | Engine Coolant Temperature Sensor Low Input |
| P0122 | Throttle Position Sensor Low Input |
| P0125 | Insufficient Coolant Temp for Closed Loop |
| P0126 | Insufficient Coolant Temperature for Stable Operation |
| P0128 | Coolant Thermostat Below Regulating Temperature |
| P0130 | O2 Sensor Circuit Malfunction (Bank 1, Sensor 1) |
| P0133 | O2 Sensor Slow Response (Bank 1, Sensor 1) |
| P0135 | O2 Sensor Heater Circuit (Bank 1, Sensor 1) |
| P0136 | O2 Sensor Circuit (Bank 1, Sensor 2) |
| P0140 | O2 Sensor No Activity (Bank 1, Sensor 2) |
| P0171 | System Too Lean (Bank 1) |
| P0172 | System Too Rich (Bank 1) |
| P0174 | System Too Lean (Bank 2) |
| P0201 | Injector Circuit Malfunction - Cylinder 1 |
| P0202 | Injector Circuit Malfunction - Cylinder 2 |
| P0203 | Injector Circuit Malfunction - Cylinder 3 |
| P0204 | Injector Circuit Malfunction - Cylinder 4 |
| P0234 | Turbocharger Overboost Condition |
| P0299 | Turbocharger Underboost Condition |
| P0300 | Random/Multiple Cylinder Misfire |
| P0301 | Cylinder 1 Misfire |
| P0302 | Cylinder 2 Misfire |
| P0303 | Cylinder 3 Misfire |
| P0304 | Cylinder 4 Misfire |
| P0325 | Knock Sensor 1 Circuit Malfunction |
| P0330 | Knock Sensor 2 Circuit Malfunction |
| P0335 | Crankshaft Position Sensor Circuit |
| P0340 | Camshaft Position Sensor Circuit (Bank 1) |
| P0350 | Ignition Coil Primary/Secondary Circuit Malfunction |
| P0351 | Ignition Coil "A" Primary Circuit |
| P0352 | Ignition Coil "B" Primary Circuit |
| P0353 | Ignition Coil "C" Primary Circuit |
| P0354 | Ignition Coil "D" Primary Circuit |
| P0401 | EGR Flow Insufficient |
| P0403 | EGR Control Circuit Malfunction |
| P0411 | Secondary Air Injection Incorrect Flow |
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) |
| P0430 | Catalyst System Efficiency Below Threshold (Bank 2) |
| P0442 | EVAP System Small Leak Detected |
| P0446 | EVAP Vent Control Circuit Malfunction |
| P0455 | EVAP System Large Leak |
| P0456 | EVAP System Very Small Leak |
| P0463 | Fuel Level Sensor High Input |
| P0480 | Cooling Fan 1 Control Circuit Malfunction |
| P0500 | Vehicle Speed Sensor Circuit Malfunction |
| P0501 | Vehicle Speed Sensor Range/Performance |
| P0506 | Idle Control System RPM Lower Than Expected |
| P0507 | Idle Control System RPM Higher Than Expected |
| P0520 | Engine Oil Pressure Sensor Circuit |
| P0521 | Engine Oil Pressure Sensor Range/Performance |
| P0562 | System Voltage Low |
| P0601 | PCM Internal Memory Checksum Error |
| P0603 | PCM Internal Keep-Alive Memory Error |
| P0606 | PCM Processor Fault |
| P0620 | Generator Control Circuit Malfunction |
| P0627 | Fuel Pump Control Circuit |
| P0650 | Malfunction Indicator Lamp Control Circuit |
| P0700 | Transmission Control System Malfunction |
| P0703 | Torque Converter Brake Switch Circuit |
| P0705 | Transmission Range Sensor Circuit |
| P0715 | Input/Turbine Speed Sensor Circuit |
| P0717 | Input/Turbine Speed Sensor No Signal |
| P0725 | Engine Speed Input Circuit |
| P0730 | Incorrect Gear Ratio |
| P0741 | Torque Converter Clutch Circuit Stuck Off |
| P0751 | Shift Solenoid "A" Performance |
| P0841 | Transmission Fluid Pressure Sensor Performance |
| P0894 | Transmission Component Slipping |
| P0960 | Pressure Control Solenoid "A" Control Circuit |
| P2187 | System Too Lean at Idle (Bank 1) |
| U0100 | Lost Communication With ECM/PCM |
| U0101 | Lost Communication With Transmission Control Module |
| U0105 | Lost Communication With Fuel Injector Control Module |
| U0121 | Lost Communication With ABS Module |
| U0122 | Lost Communication With Vehicle Dynamics Module |
| U0140 | Lost Communication With Body Control Module |
| U0155 | Lost Communication With Instrument Panel Cluster |
No code matches your search.
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The four OBD-II families
The first letter of the code tells you which system it concerns.
P — Powertrain
- Engine & transmission
- Fuel, ignition, emissions
- Most common codes (P0300…)
- Usually triggers check engine light
B — Body
- Airbags, HVAC, lighting
- Power windows & locks
- Body control module faults
C — Chassis
- ABS & ESP
- Wheel speed sensors
- Steering & suspension
U — Network
- CAN bus communication
- Lost communication between modules
- Often battery/ground related
Quick glossary
The diagnostic terms you need to know.
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OBD-II FAQ
Are OBD-II codes the same for every car?
Generic codes (P0xxx, and most B/C/U) are standardized across brands. Manufacturer-specific codes (P1xxx, P2xxx) differ by brand — that is why we built brand-specific pages.
Do I need a professional scanner?
For most people, a cheap OBD-II reader or a phone app with an ELM327 adapter is enough to read codes. Professional scanners add live data, bi-directional tests and more depth.
Does the code tell me which part to replace?
Not always. A code describes the symptom and the circuit involved. Diagnose (test, inspect wiring, check live data) before buying parts to avoid replacing good components.
Can I clear the check engine light myself?
Yes, with a scanner, but the light will return if the problem is not fixed. Clearing a code without repairing the fault is not a solution.
Why is my check engine light on but the car drives fine?
Many codes (EVAP, O2 sensors, thermostat) do not affect drivability. Still have it checked — some faults worsen over time and can damage the catalytic converter or engine.
What is a pending code?
A pending code appears after a single failed self-test. If the test passes later, it may clear itself; if it fails repeatedly, the code becomes stored and the light turns on.
Do engine codes reset themselves?
Some do if the condition clears and enough drive cycles pass. Others stay stored until cleared with a scanner.
Which codes are the most serious?
Misfires (P0300 series), low oil pressure (P0520), overheat-related codes and network failures (U0100) deserve immediate attention to avoid engine or transmission damage.
Diagnostic info is for guidance only. Always confirm with a proper diagnosis. Clearing codes without fixing the fault is not a repair.
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Find your code in seconds
Use the decoder below — it covers P, B, C and U codes, plus 8 of the most common car brands.