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
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.
Compare LAUNCH Diagnostic Scanners & X431 Tools
Shop and compare LAUNCH code readers, bidirectional scanners, and professional X431 tablets. Explore real product photos, available models, and prices from our LAUNCH product catalog.
Affiliate link. Check current price and availability.
LAUNCH vs Autel: Which Diagnostic Scanner Is Better?
Compare the LAUNCH CRP919X BT and Autel MK900-BT, including bidirectional tests, full-system coverage, update costs, and professional workflows.
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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