OBD-II Code Library • 1996+ Vehicles • Updated Regularly
Every OBD-II Code
Explained & Searchable
Type your code below for an instant preview — or browse the expanded table. Plain-English causes, first checks, and repair costs. No sign-up.
Covers all P, B, C & U codes — type any code for an instant definition + first check
🔖 P · B · C · U codes
⚡ Instant definitions
🛠️ Causes + fixes
💰 Cost estimates
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Most drivers come here without a code. That’s fine — just describe what your car is doing and our free AI identifies the most likely codes, ranks the causes, and tells you what to check first.
Rough idle, car shakes at startup but smooths out after warming up
Check engine light + loss of power going uphill
Flashing check engine light — is it safe to drive?
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// Browse by Code Family
?
Search All
Full Code Library
Search any DTC — definitions, causes, symptoms, and fixes. Best if you already have a code.
All codes →P
Powertrain
P-Codes
Engine, transmission, emissions, fuel/air, misfires, EVAP, catalyst. Most common check engine codes.
P0000–P3999B
Body
B-Codes
Airbags (SRS), seatbelts, HVAC, power windows, locks, and interior safety electronics.
B0000–B3999C
Chassis
C-Codes
ABS, traction control, steering, suspension, wheel speed sensors, and braking systems.
C0000–C3999U
Network
U-Codes
CAN bus, module communication, wiring faults, and control module data errors. Often intermittent.
U0000–U3999// 20 Most Searched Codes — First Checks & Cost Estimates
| Code | Name | Severity | First Check | Cost Range | Guide |
|---|---|---|---|---|---|
| P0420 | Catalyst Efficiency Below Threshold (B1) | ⚠ Moderate | Check exhaust leaks first, then O2 waveform | $60–$2,500+ | Guide → |
| P0300 | Random/Multiple Cylinder Misfire | ⛔ Serious | Swap coils between cylinders to isolate | $80–$600 | Guide → |
| P0171 | System Too Lean (Bank 1) | ⚠ Moderate | Listen for vacuum leak hiss, check MAF | $50–$400 | Guide → |
| P0301 | Cylinder 1 Misfire Detected | ⛔ Serious | Swap cyl 1 coil to another — if misfire moves, coil is bad | $80–$350 | Guide → |
| P0302 | Cylinder 2 Misfire Detected | ⛔ Serious | Swap coil from cyl 2 to confirm | $80–$350 | Guide → |
| P0303 | Cylinder 3 Misfire Detected | ⛔ Serious | Swap coil from cyl 3, inspect plug | $80–$350 | Guide → |
| P0304 | Cylinder 4 Misfire Detected | ⛔ Serious | Swap coil from cyl 4, check plug gap | $80–$350 | Guide → |
| P0174 | System Too Lean (Bank 2) | ⚠ Moderate | P0171 + P0174 together = large vacuum leak | $50–$400 | Guide → |
| P0172 | System Too Rich (Bank 1) | ⚠ Moderate | Check fuel pressure, inspect O2 sensor, look for leaking injectors | $80–$500 | Guide → |
| P0128 | Coolant Temp Below Thermostat Temp | ✓ Low | Watch coolant gauge on cold start — barely reaching middle = bad thermostat | $80–$250 | Guide → |
| P0442 | EVAP Small Leak Detected | ✓ Low | Tighten or replace gas cap first | $8–$300 | Guide → |
| P0455 | EVAP Large Leak Detected | ✓ Low | Gas cap first, then EVAP hoses and purge valve | $8–$400 | Guide → |
| P0430 | Catalyst Efficiency Below Threshold (B2) | ⚠ Moderate | Both P0420 + P0430 together = check for oil burning first | $60–$2,500+ | Guide → |
| P0011 | Intake Cam Timing Over-Advanced (B1) | ⚠ Moderate | Check oil level and viscosity — VVT is oil-pressure dependent | $100–$800 | Guide → |
| P0101 | MAF Sensor Range/Performance | ⚠ Moderate | Clean MAF with MAF cleaner spray first — free fix that works ~40% of the time | $0–$350 | Guide → |
| P0131 | O2 Sensor Low Voltage (B1S1) | ⚠ Moderate | Check for exhaust leak before sensor before replacing it | $120–$350 | Guide → |
| P0507 | Idle Control System RPM High | ✓ Low | Clean throttle body — carbon buildup is the #1 cause | $0–$200 | Guide → |
| P0401 | EGR Flow Insufficient | ⚠ Moderate | Inspect EGR valve for carbon — cleaning often fixes it | $0–$400 | Guide → |
| P0456 | EVAP Very Small Leak | ✓ Low | Gas cap first, then purge valve and vent hoses | $8–$350 | Guide → |
| P0340 | Camshaft Position Sensor Circuit (B1) | ⛔ Serious | Check CMP wiring and connector before replacing sensor | $80–$350 | Guide → |
Showing 20 most searched codes. Search the full code library →
// Browse Codes by System
Most check engine codes are P-codes. They cover engine, transmission, emissions, fuel/air, EVAP, misfires, and O2 sensors. Generic P0xxx codes mean the same thing on every vehicle.
P0420Catalyst System Efficiency Below ThresholdModerate→
P0300Random/Multiple Cylinder MisfireSerious→
P0171System Too Lean (Bank 1)Moderate→
P0430Catalyst Efficiency Below Threshold (Bank 2)Moderate→
P0442EVAP System Small Leak DetectedLow→
P0128Coolant Temp Below Thermostat TempLow→
P0174System Too Lean (Bank 2)Moderate→
P0011Intake Cam Timing Over-Advanced (Bank 1)Moderate→
B-codes cover body electronics — airbags, SRS, HVAC, power windows, locks. Most basic OBD readers cannot read B-codes. You need a scanner with enhanced SRS system support.
B0001Driver Frontal Stage 1 Airbag DeploymentSerious→
B0051Airbag — Driver Side Inflator CircuitSerious→
B1000ECU Internal Fault (Body System)Moderate→
B2100Door Lock Actuator CircuitLow→
C-codes cover chassis systems — ABS, traction control, ESC, steering angle sensors, wheel speed sensors. Require a scanner that supports your vehicle’s ABS module specifically.
C0031Right Front Wheel Speed Sensor CircuitSerious→
C0035Left Front Wheel Speed Sensor CircuitSerious→
C0265EBCM Relay Circuit OpenSerious→
C0561System Disabled — Stability ControlModerate→
U-codes indicate network and module communication faults. Many U-codes are caused by a different underlying fault (dead battery, bad ground, failing module) rather than a wiring problem itself.
U0001High Speed CAN Communication BusSerious→
U0100Lost Communication With ECM/PCMSerious→
U0101Lost Communication With TCMSerious→
U0121Lost Communication With ABS ModuleSerious→
// Interactive Code Decoder
Decode Any OBD-II Code — Character by Character
Type any code and see exactly what each character means in real time.
—
System
Enter a code to decode
—
Type
Generic or manufacturer
—
Subsystem
Which system is affected
——
Fault
Specific fault number
// Anatomy of an OBD-II Code
What Each Character Means
Using P0420 as an example:
P
System
0
Type
4
Subsystem
2
Fault
0
Fault
P
System Letter
P=Powertrain · B=Body · C=Chassis · U=Network
0
Code Type
0 = Generic (SAE — same on all makes). 1 = Manufacturer-specific.
4
Subsystem
0=Fuel · 1=Fuel/Air · 2=Injector · 3=Ignition · 4=Emissions · 5=Speed/Idle · 6=Computer · 7/8=Trans
20
Fault Number
The specific fault within that subsystem. 20 = Catalyst efficiency bank 1.
Pro tip: A code identifies the system with the fault — not always the failed part. P0420 triggers from the catalyst monitor failing, but the real cause could be O2 sensors, exhaust leaks, or the converter itself. Diagnose before replacing parts.
// How to Read an OBD-II Code
1
Plug In Your Scanner
OBD-II port is under the dashboard near the driver’s knee. Plug in, turn key to “on” — don’t start the engine.
2
Read All Stored Codes
Write down every code including pending codes. Don’t clear them — you’ll lose freeze frame data.
3
Note the Freeze Frame
Freeze frame captures engine conditions when the fault occurred — RPM, coolant temp, fuel trims. Diagnostic gold.
4
Look Up and Diagnose
Check companion codes, prioritise by severity, and follow the diagnostic path before replacing any parts.
// Quick Help Right Now
🚨 Flashing Check Engine Light
A flashing CEL means an active misfire right now. Reduce load immediately — it destroys catalytic converters fast.
⛽ Emissions / EVAP Codes
Failing emissions? Start with the gas cap — #1 cause and costs nothing.
💨 Lean / Rich Fuel Codes
Lean = vacuum leak or MAF. Rich = injectors or O2 sensor. Check companion codes first.
🔴 ABS / Airbag / TPMS Lights
Basic code readers can’t read these. Need a scanner with ABS/SRS support for your vehicle.
// Frequently Asked Questions
An OBD-II DTC is a five-character alphanumeric code stored by your vehicle’s ECU when it detects a fault. All US vehicles from 1996+ use OBD-II. Generic codes (0 in position 2) mean the same thing on every make. P=Powertrain, B=Body, C=Chassis, U=Network. You need an OBD2 scanner to read them from the diagnostic port under your dashboard.
A solid check engine light is often non-urgent — a loose gas cap, O2 sensor, or minor EVAP issue. A flashing check engine light means an active misfire right now — reduce load immediately, it can destroy your catalytic converter in minutes. Use our free AI Diagnostic for a specific drive/park verdict.
Generic codes have 0 as the second character (P0420) and are standardized across all manufacturers — same meaning on every car. Manufacturer-specific codes have 1 in position 2 (P1xxx) and are unique to that brand.
No. Clearing codes wipes freeze frame data and resets readiness monitors. Freeze frame captures the exact engine conditions when the fault occurred — it’s the most useful diagnostic data available. Write the codes down and let the shop read them fresh.
Multiple codes often share a root cause. A large vacuum leak can trigger P0171 (lean), P0300 (misfire), and P0420 (catalyst) simultaneously. Fix the most fundamental fault first — usually a fuel, air, or sensor code — before addressing downstream codes. Use our AI Diagnostic to identify the root cause.
Many parts stores (AutoZone, O’Reilly) read codes free — but only stored engine codes with no freeze frame or live data. A basic scanner for $25–$60 gives you everything. See our scanner picks for the best options at every budget.
// Related Guides
Diagnose & Fix GuideStep-by-step diagnostic workflow for any code
OBD2 Scanner PicksBest scanners for every budget — $30 to $350
Scanner Features ExplainedLive data, freeze frame, readiness — plain English
ABS / SRS / TPMS ToolsScanners that read beyond engine codes
Start Here — BeginnersNew to OBD2? Start with this guide
Free AI Diagnostic ToolDescribe symptoms, get instant diagnosis