Free Tool
OBD-II Code Lookup
Decode Your Check Engine Light.
Search a code like P0300, P0420, or P0171, or describe a symptom. Find plain-English explanations and diagnostic guidance. Reference entries that need review are clearly marked.
Decode Your Check Engine Light.
Search a code like P0300, P0420, or P0171, or describe a symptom. Find plain-English explanations and diagnostic guidance. Reference entries that need review are clearly marked.
Type a code or keyword. Press Enter to open, or click a result.
983 code entries. 441 have unreviewed severity and driving guidance; those entries are labeled.
Light is steady
If the car runs normally and no other warning lights are on, arrange diagnosis soon. A steady check-engine light alone cannot confirm it is safe to drive.
Light is flashing
Pull over safely. A flashing check-engine light can indicate a misfire that may damage the catalytic converter. If flashing or rough running continues, arrange a tow.
Either way, pull over if the car shakes hard, loses power, overheats, or makes a noise it was not making before.
Common starting points. Match the full code shown on your scanner.
Minor Worth booking Deal with it soon Urgent
Reading the code
OBD-II (On-Board Diagnostics, version 2) is a standardized system every car sold in the US since 1996 must have. When the check engine light comes on, your car has already stored a code describing what it detected. The definitions are public standards — a $20 reader pulls the same code a professional scan tool does.
ExampleP0420
System
Powertrain
Which part of the car is reporting.
Type
Generic
Same meaning on every make, not just this one.
Fault
Catalyst System Efficiency Below Threshold (Bank 1)
Generic codes share a standardized definition. Manufacturer-specific codes depend on the vehicle. The last three characters can include letters A–F, as in the hybrid code P0A80.
Look at the light itself before you look up anything. Solid and flashing mean very different things.
Solid
Check how the car is running
If it runs normally and no other warning lights are on, arrange diagnosis soon. Stop safely for severe shaking, overheating, loss of power, or oil-pressure warnings.
Flashing
Stop driving
A flashing check-engine light can mean a misfire that may damage the catalytic converter. Pull over safely; if flashing or rough running continues, arrange a tow.
A code alone cannot establish that a vehicle is safe to drive. Use the symptoms, warning lights, and your owner’s manual alongside the code guidance.
The first character narrows a fault down to one of four systems before you know anything else about it.
Powertrain
Engine and transmission
Most codes land here.
Body
Airbags, lighting, HVAC
Chassis
Brakes, ABS, suspension
Network
Communication between modules
Generic or manufacturer-specific?
Generic
Defined by SAE. Means the same thing on any car.
Manufacturer-specific
Defined by the brand. The meaning varies by make.
P3 includes both types: P3000–P3399 are manufacturer-specific; P3400–P3999 are generic. B, C, and U use different allocations. Check the full code and your vehicle’s service information.
A corded scanner starts around $20; Bluetooth dongles that pair with a phone app run $30–$50. Either one pulls the code in under a minute.
Find the port
Usually under the dashboard, near the steering column.
Ignition on, engine off
Use the ON/RUN position, not accessory. Follow your vehicle and scanner instructions.
Read the code
The scanner pulls whatever the car has stored.
Worth knowingChain parts stores — AutoZone, O'Reilly, Advance — will usually read the code for free. They will also recommend parts they think will fix it. Some of those guesses are wrong, and you pay for the wrong ones.
A code is a starting point for diagnosis, not the diagnosis. Four things it will not settle for you:
Exactly which part is bad
P0300 tells you there’s a misfire — not whether it’s spark plugs, coils, or compression.
Whether the code is the real problem
Sometimes the stored code is a symptom of something bigger.
Pending codes
Many basic OBD-II readers can show pending faults. Save whether each code is pending, confirmed, or permanent before clearing anything.
Live data
Real diagnosis often means watching sensor values in real time. That’s where shop-grade scanners earn their price.
Free, permanently. No charge, no signup, no ads, and nothing sold about you. OBD-II code explanations are provided here for reference; confirm vehicle-specific details in the service manual.
Sometimes, and it is the wrong way to do it. On older vehicles pulling the battery can wipe stored codes, but it also resets the readiness monitors an emissions test looks at, along with radio presets and learned transmission and fuel-trim values. Many modern ECUs keep codes in non-volatile memory, so the code survives the disconnect anyway and you have lost the monitors for nothing. Read the code with a scanner first so you know what it was, then clear it deliberately.
Yes, with any OBD-II scanner. A $25 Bluetooth dongle paired with a phone app can read and clear codes. Clearing without fixing the underlying issue means the light returns — sometimes within miles, sometimes after a few drive cycles once the condition reoccurs.
Usually no. After codes are cleared, the car's onboard monitors reset to "not ready." Most state inspection programs fail a vehicle with more than one or two monitors in the not-ready state. Drive at least 100 miles of mixed city and highway before re-testing so the monitors can re-run.
A pending code records a fault that has not yet met the conditions to become confirmed. A confirmed code has met the relevant criteria; some faults confirm on one trip and others require repeated detection. Many basic readers show both. A stored code does not necessarily mean the warning light is currently on. Record code status before clearing anything.
Its definition can depend on the make, model year, and engine. P1 and P3000–P3399 are manufacturer-specific; P0, P2, and P3400–P3999 are generic. B, C, and U use different allocations. Use the full code and vehicle-specific service information. Unreviewed entries in this database are labeled and do not provide a driving verdict.
Take it to a shop when: the check engine light flashes (active damage), the code relates to a safety system like ABS or airbags, you see a transmission or critical sensor code, you cannot identify the failing part after testing, or the same code returns repeatedly after clearing. Diagnostic time is cheaper than guessed repairs.
Some faults are intermittent — a loose gas cap, a single misfire, a transient sensor glitch. If the ECU doesn't see the same fault across three consecutive drive cycles, the light turns off automatically and the code moves from "stored" to "history." The underlying root cause may still be there; keep an eye on it.
More free tools