OBD2 (On-Board Diagnostics II) is a standardized system required on all cars sold in the US since 1996. When your car detects a problem, it stores a diagnostic trouble code (DTC) and turns on the check engine light. An OBD2 scanner lets you read these codes.
What the check engine light tells you:
The OBD2 port is a 16-pin connector, usually located under the dashboard on the driver's side. Common locations:
By law, the port must be within 2 feet of the steering wheel and accessible without tools (except possibly a fuse box cover).
Insert the scanner firmly into the port. It only goes in one way. Turn the ignition key to the ON position (you don't need to start the engine, but it's fine if you do). The scanner should power on automatically.
On most scanners, navigate to "Read Codes" or "Scan" and press enter. The scanner will communicate with your vehicle's computer and retrieve any stored codes. This takes 5-30 seconds.
The scanner will display one or more codes in the format P0301, P0420, etc. Write down ALL codes shown. Many scanners display a brief description. If multiple codes are present, address them in order - the first code is usually the primary issue.
Use your scanner's built-in code definitions, or search online for "[code number] [your car make/model]". For example: "P0420 Honda Civic 2015". Generic code definitions are standardized, but manufacturer-specific codes vary.
Advanced scanners show "freeze frame" data - a snapshot of engine conditions when the code was set (RPM, speed, temperature, etc.). This helps diagnose intermittent problems.
After fixing the problem, use the scanner to clear the codes. Select "Clear Codes" or "Erase Codes". The check engine light should turn off immediately. If the light comes back on after driving, the problem wasn't fully resolved.
Features: Read and clear codes, turn off check engine light, view basic sensor data
Pros: Cheap, simple to use, does the job for most people, pocket-sized
Cons: No advanced features, limited data, small screen
Best for: DIY car owners who just want to read codes and save on diagnostic fees
Features: Plugs into OBD2 port, connects to your smartphone app (iOS/Android), real-time data, code reading/clearing
Pros: Uses your phone's screen, free apps available, can monitor data while driving, cheapest option
Cons: Requires smartphone, app quality varies, may drain phone battery
Best for: Tech-savvy users who want advanced features at low cost
Popular apps: Torque Pro (Android), Car Scanner, OBD Fusion
Features: Larger screen, live data graphing, freeze frame data, ABS/airbag codes (some models), emissions readiness, battery test
Pros: More detailed diagnostics, standalone device (no phone needed), better screen, faster
Cons: More expensive, bulkier, features you might not need
Best for: Serious DIYers, home mechanics, anyone who works on multiple vehicles
Features: Bi-directional controls, module programming, key programming, advanced diagnostics, manufacturer-specific codes, all systems (ABS, transmission, airbag, etc.)
Pros: Professional-level diagnostics, can activate components for testing, comprehensive coverage
Cons: Expensive, complex, overkill for most DIYers, requires knowledge to use effectively
Best for: Professional mechanics, advanced hobbyists, repair shops
Price: ~$25 | Type: Basic Code Reader
Perfect for beginners. Reads and clears codes, turns off check engine light, displays I/M readiness status. Works on all 1996+ vehicles. Pocket-sized and easy to use.
Price: ~$20 | Type: Bluetooth Adapter
Connects to your phone via Bluetooth. Works with free and premium apps (Torque Pro, Car Scanner). Real-time data monitoring, code reading/clearing, fuel economy tracking.
Price: ~$90 | Type: Professional Scanner
Large color screen, live data graphing, freeze frame, I/M readiness, smog check support. Reads manufacturer-specific codes. Lifetime free updates. Great for serious DIYers.
Price: ~$100 | Type: Pro Bluetooth Scanner
Professional-grade diagnostics via smartphone app. Reads all systems (ABS, airbag, transmission). Mode 6 data, freeze frame, live data. Free app with repair reports and verified fixes from real mechanics.
Diagnostic codes follow a standardized 5-character format:
| Position | Meaning | Example |
|---|---|---|
| 1st character | System affected | P = Powertrain (engine/transmission) B = Body (airbags, seats, etc.) C = Chassis (ABS, suspension) U = Network (communication) |
| 2nd character | Code type | 0 = Generic (SAE standard) 1 = Manufacturer-specific 2/3 = Manufacturer-specific |
| 3rd character | Sub-system | 1/2 = Fuel/Air metering 3 = Ignition system 4 = Emissions control 5 = Speed/Idle control 6 = Computer/Output 7/8 = Transmission |
| 4th & 5th | Specific fault | Sequential numbers identifying exact problem |
Example: P0301
Meaning: Engine misfiring on one or more cylinders, but not consistent to one cylinder.
Common causes: Bad spark plugs/wires, faulty ignition coil, clogged fuel injector, low compression, vacuum leak
Symptoms: Rough idle, loss of power, poor fuel economy, flashing check engine light
Fix: Replace spark plugs and wires if old (60K+ miles). Check coils with multimeter. If recent, may be bad gas - try fuel system cleaner.
Meaning: Specific cylinder is misfiring (P0301 = Cylinder 1, P0302 = Cylinder 2, etc.)
Common causes: Bad spark plug or coil on that cylinder, faulty fuel injector, low compression, vacuum leak at that cylinder
Fix: Swap spark plug or coil with another cylinder. If misfire moves, you found the problem. If misfire stays, check compression and injector.
Meaning: Catalytic converter not working efficiently. This is the most common check engine code.
Common causes: Failing catalytic converter, bad oxygen sensors, exhaust leak, rich/lean fuel mixture
Symptoms: Usually no noticeable symptoms. May fail emissions test.
Fix: Check oxygen sensors first ($50-150 each) before replacing cat ($400-2000). Code often appears on high-mileage vehicles (100K+).
Meaning: Engine running with too much air, not enough fuel. P0171 = Bank 1, P0174 = Bank 2 (V6/V8 engines)
Common causes: Vacuum leak, dirty MAF sensor, weak fuel pump, clogged fuel filter, faulty oxygen sensor
Symptoms: Rough idle, hesitation on acceleration, poor fuel economy
Fix: Inspect for vacuum leaks (listen for hissing). Clean MAF sensor with MAF cleaner spray ($10). Check fuel pressure.
Meaning: Engine running with too much fuel, not enough air.
Common causes: Dirty air filter, faulty MAF sensor, leaking fuel injector, bad oxygen sensor
Symptoms: Black smoke from exhaust, strong fuel smell, poor fuel economy, rough idle
Fix: Replace air filter. Clean MAF sensor. Check for fuel injector leaks (smell gas when engine off?).
Meaning: Fuel vapor leak detected in EVAP system. P0442 = small leak, P0455 = large leak, P0456 = very small leak
Common causes: Loose or missing gas cap (80% of cases), cracked EVAP hose, faulty purge valve, leaking charcoal canister
Symptoms: Usually none. Light comes on after fueling.
Fix: Tighten or replace gas cap ($10-30). Drive for a day - if light stays off, that was it. Otherwise, inspect EVAP hoses for cracks.
Meaning: Engine not reaching normal operating temperature quickly enough.
Common causes: Stuck-open thermostat, faulty coolant temperature sensor, low coolant level
Symptoms: Slow warm-up, poor heat from heater, reduced fuel economy, common in cold weather
Fix: Replace thermostat ($20-40 part, 30 min job). Check coolant level first.
Meaning: EGR system not recirculating enough exhaust back into engine.
Common causes: Clogged EGR valve (carbon buildup), blocked EGR passages, faulty EGR sensor
Symptoms: Reduced performance, pinging/knocking under load, may fail emissions
Fix: Clean EGR valve and passages with EGR cleaner. If that doesn't work, replace valve ($100-300).
Meaning: Front oxygen sensor responding too slowly to changes in air/fuel mixture.
Common causes: Aging oxygen sensor (normal wear after 60K-100K miles), contaminated sensor, exhaust leak
Symptoms: Poor fuel economy, rough idle, may fail emissions
Fix: Replace front oxygen sensor ($50-150). High-mileage vehicles should replace all O2 sensors as preventive maintenance.
Meaning: Computer can't control engine idle speed properly.
Common causes: Dirty throttle body, faulty idle air control valve, vacuum leak, carbon buildup
Symptoms: Erratic idle (too high or too low), stalling at stops, surging idle
Fix: Clean throttle body with throttle cleaner ($8). If that doesn't help, replace idle air control valve ($50-150).