Scan. Diagnose. Fix. Real-World OBD-II Guides
Read check engine codes, find the real cause, and fix it right — with shop-level diagnostic guides, a free AI tool, and scanner recommendations for every budget.
🚨 Flashing Check Engine Light — What to Do
The most urgent question when a CEL appears. Solid vs flashing changes everything.
Is the CEL flashing or solid?
Look at the check engine light right now. A flashing light is different from a solid light — it’s a real-time emergency signal, not a diagnostic flag.
Flashing CEL — Active misfire
Reduce load and stop driving safely. Unburned fuel is entering the exhaust right now, overheating the catalytic converter. 20 minutes at highway speed can destroy a $1,000+ converter permanently.
Solid CEL — Fault detected
You can usually drive carefully to a shop. A solid light means the PCM has logged a fault that exceeds its emissions threshold but the misfire rate isn’t high enough to flash the light.
Scan for codes — before clearing anything
Read stored, pending, and permanent codes. Note freeze frame data. Don’t clear codes before reading freeze frame — that data shows what the engine was doing when the fault occurred.
Look up your code below — then diagnose
Use the search at the top of this page or browse the code table below. Multiple codes together usually have a single root cause — fix the most fundamental one first.
Anatomy of an OBD-II Code
Every code is 5 characters. Each character tells you something specific. Using P0420 as an example:
25 Most Searched OBD-II Codes
First checks and cost estimates included. Filter by severity or search by code number.
| Code | Name | Severity | First Check | Cost Range | Guide |
|---|---|---|---|---|---|
| P0300 | Random/Multiple Cylinder Misfire | ⛔ Serious | Swap coils between cylinders to isolate — if misfire moves, coil is bad | $80–$600 | Guide → |
| P0420 | Catalyst Efficiency Below Threshold (B1) | ⚠ Moderate | Check exhaust leaks first, then O2 sensor waveform before touching cat | $60–$2,500+ | Guide → |
| P0171 | System Too Lean (Bank 1) | ⚠ Moderate | Listen for vacuum hiss at idle, check MAF sensor, inspect intake boots | $50–$400 | Guide → |
| P0301 | Cylinder 1 Misfire Detected | ⛔ Serious | Swap cyl 1 coil to another cylinder — if misfire follows, coil is bad | $80–$350 | Guide → |
| P0302 | Cylinder 2 Misfire Detected | ⛔ Serious | Swap coil from cyl 2 to confirm — inspect plug if coil is OK | $80–$350 | Guide → |
| P0303 | Cylinder 3 Misfire Detected | ⛔ Serious | Swap coil from cyl 3, check spark plug condition and gap | $80–$350 | Guide → |
| P0304 | Cylinder 4 Misfire Detected | ⛔ Serious | Swap coil from cyl 4, inspect plug gap | $80–$350 | Guide → |
| P0174 | System Too Lean (Bank 2) | ⚠ Moderate | P0171 + P0174 together = large intake or vacuum leak | $50–$400 | Guide → |
| P0172 | System Too Rich (Bank 1) | ⚠ Moderate | Check fuel pressure and upstream O2 sensor, look for injector leaks | $80–$500 | Guide → |
| P0128 | Coolant Temp Below Thermostat Temp | ✓ Low | Watch coolant gauge on cold start — barely reaching middle = bad thermostat | $80–$250 | Guide → |
| P0430 | Catalyst Efficiency Below Threshold (B2) | ⚠ Moderate | P0420 + P0430 = check oil burning before replacing converters | $60–$2,500+ | Guide → |
| P0442 | EVAP System Small Leak Detected | ✓ Low | Tighten or replace gas cap — fixes 30%+ of P0442 cases | $8–$300 | Guide → |
| P0455 | EVAP System Large Leak Detected | ✓ Low | Gas cap first, then EVAP hoses and purge valve | $8–$400 | Guide → |
| P0011 | Intake Cam Timing Over-Advanced (B1) | ⚠ Moderate | Check oil level and viscosity first — VVT systems depend on oil pressure | $100–$800 | Guide → |
| P0101 | MAF Sensor Range/Performance | ⚠ Moderate | Clean MAF with MAF cleaner spray — fixes ~40% of cases for free | $0–$350 | Guide → |
| P0131 | O2 Sensor Low Voltage (B1S1) | ⚠ Moderate | Check for exhaust leak before the 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 without replacement | $0–$400 | Guide → |
| P0456 | EVAP System Very Small Leak | ✓ Low | Gas cap first, then purge valve and vent hoses | $8–$350 | Guide → |
| P0016 | Crankshaft/Camshaft Position Correlation (B1) | ⛔ Serious | Check oil level first — rattle on startup = timing chain inspection needed | $100–$2,100 | Guide → |
| P0700 | Transmission Control System Malfunction | ⛔ Serious | P0700 is a flag — read companion codes to find the actual fault | $150–$1,500 | Guide → |
| P0118 | Engine Coolant Temp Circuit High Input | ⚠ Moderate | Check ECT connector for corrosion — test resistance cold vs warm | $80–$250 | Guide → |
| P0340 | Camshaft Position Sensor Circuit (B1) | ⛔ Serious | Check CMP sensor wiring and connector before replacing the sensor | $80–$350 | Guide → |
| P0021 | Intake Cam Timing Over-Advanced (B2) | ⚠ Moderate | Same as P0011 but bank 2 — check oil level and quality first | $100–$800 | Guide → |
| P0351 | Ignition Coil A Primary/Secondary Circuit | ⛔ Serious | Inspect coil wiring for damage before swapping the coil itself | $80–$300 | Guide → |
Showing 25 most searched codes · Search the full code library →
P · B · C · U Code Families
Click a tab to preview examples from each family — then browse the full hub page for that system.
Free AI Car Diagnostic Tool
Describe your symptoms in plain English — rough idle, check engine light, weird noise — and get a shop-level diagnosis instantly. Identifies likely codes, ranks causes, gives a drive/park verdict, and estimates repair costs. No sign-up needed.
Common Codes by Vehicle Make
The same code can have different most-likely causes depending on the vehicle. Browse make-specific notes for the most common P0420, P0300, and P0171 patterns.
2AZ-FE flex pipe crack, strict PCM threshold, TSB EG013-04
B1S2 wiring damage, manifold gasket leak, CA-spec converter requirements
5.3L AFM oil consumption, cracked exhaust manifold, rear O2 sensor lazy
5.4L plug blowout, 3.5L EcoBoost turbo seal, Y-pipe fatigue crack
QR25DE oil consumption, wideband A/F sensor, PZEV warranty check
EVAP leaks from off-road flex, coil failures on 3.6L Pentastar
What’s Your Car Doing?
Search by symptom instead of code — find the most likely codes for what you’re experiencing right now.
🚨 CEL Flashing + Shaking
Active misfire in progress. Reduce load immediately — converter damage accumulates fast.
⛽ Failed Emissions Test
Failing smog? Start with the gas cap. If that’s not it, catalyst and EVAP codes are next.
💨 Poor Fuel Economy
Lean or rich running, stuck thermostat, or fouled injectors. Check fuel trims first.
🔴 ABS / Airbag / TPMS Light
Basic code readers can’t read these. You need a scanner with ABS and SRS system access.
OBD2 Scanner Picks — Every Budget
From $30 glovebox readers to full-system diagnostic tablets. Updated picks based on real-world use.
New to OBD2? Start Here
These guides walk you through how OBD-II works, what each code means, and how to use a scanner — including newer EV/hybrid codes, CAN FD, and DoIP coverage.
Introduction to On-Board Diagnostics
How modern vehicles monitor engine, emissions, and drivetrain systems — and why OBD-II is the standard every scanner speaks.
Read the OBD overview →How OBD Works — ECUs, Sensors & DTCs
ECUs, sensors, and diagnostic trouble codes explained. Understand how your car decides when to light the check engine light.
See how your ECU thinks →OBD-II Codes Explained — P, B, C, U
Break down letters and numbers: generic vs manufacturer codes, how to read code families before you even grab a wrench.
Decode OBD-II formats →Diagnose & Fix — Step-by-Step
From reading codes and freeze frame data to confirming the fix and clearing monitors. A complete diagnostic workflow in plain English.
Follow the diagnostic checklist →Live Data, Freeze Frame & Readiness Explained
What each scanner feature actually does, which ones matter for diagnosis, and when you need advanced features vs basic code reading.
Learn scanner features →Scanners for ABS, Airbag & TPMS Warning Lights
Basic scanners can’t read these systems. Find out what you actually need — and get the tools to clear the lights your code reader can’t touch.
Find ABS/SRS scanner →How to Read and Use an OBD-II Code
Four steps from blinking light to actionable diagnosis. Follow this before replacing any parts.
OBD-II Questions Answered
OBD-II Tips & Scanner Deals
Fresh code guides, tools under $50, and updated diagnostics for common codes — straight to your inbox. No spam, unsubscribe any time.
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