OBD2 code lookup · Updated 2026

Check engine light on? Look up what the code actually means.

A searchable reference for the OBD2 / check engine light codes drivers hit most often — what each one means, what usually causes it, and whether it's safe to keep driving.

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1. How OBD2 codes work (and how to read yours for free)

Find the port

Every gasoline car sold in North America since 1996 (and most diesels since the early 2000s) has a 16-pin OBD2 port, almost always under the dashboard within reach of the driver's seat.

Get a reader

A basic Bluetooth or USB code reader (roughly $15–$25) paired with a free phone app (like Torque or Car Scanner) will pull the code in seconds. Many auto parts retailers — AutoZone, O'Reilly, NAPA, Canadian Tire — will also scan it for free in their parking lot.

Understand what the code actually tells you

A code names the system that tripped a threshold — it doesn't always name the exact broken part. P0420, for example, means the catalytic converter isn't working efficiently enough, but the real cause could be the converter itself, a sensor feeding it bad data, or an unrelated engine problem damaging it. Codes narrow things down; they rarely give the final answer on their own.

Clearing the code isn't fixing it

Resetting the code turns the light off but does nothing to the underlying problem. If it wasn't actually fixed, the same code (or a related one) typically returns within days of normal driving.

Got a code AND a noise? MyAutoSound's free AI diagnosis takes the sound itself and gives you a specific likely cause in under a minute.

Diagnose the noise

This page covers generic (SAE-standard) powertrain codes, which mean the same thing across all makes. Codes starting with P1 are manufacturer-specific and vary by brand — those aren't covered here.

2. Search or browse OBD2 codes

Type a code (e.g. "P0420") or a keyword (e.g. "misfire", "catalyst", "idle") to filter the list below.

All Fuel & air (P01–P02xx) Ignition & misfire (P03xx) Emissions (P04xx) Speed & idle (P05xx) Computer (P06xx) Transmission (P07xx)

Fuel & air metering codes (P0100–P0299)

P0100

Medium

Mass Air Flow (MAF) Circuit Malfunction

The MAF sensor isn't sending a usable signal. Usual causes: a dirty or contaminated sensor element, a wiring/connector fault, or a vacuum leak nearby. Can cause rough idle, hesitation, or poor fuel economy.

P0102

Medium

MAF Circuit Low Input

The MAF sensor is reporting less airflow than expected. Often a dirty sensor element (cleanable with proper MAF-safe cleaner) or an intake air leak between the sensor and engine.

P0113

Low

Intake Air Temperature (IAT) Circuit High Input

The intake air temp sensor is reading unrealistically hot (often an open circuit read as extreme heat). Usually just a wiring or sensor fault — rarely urgent on its own.

P0117

Medium

Engine Coolant Temperature (ECT) Circuit Low Input

The coolant temp sensor reads unrealistically hot (low resistance = "low input"). Can trick the computer into running a bad fuel mixture or keeping the cooling fan on constantly.

P0118

Medium

Engine Coolant Temperature (ECT) Circuit High Input

The opposite of P0117 — the sensor reads unrealistically cold. The engine may run rich and take longer to enter closed-loop fuel control, hurting economy and emissions.

P0120

High

Throttle Position Sensor (TPS) Circuit Malfunction

The computer isn't getting a reliable throttle position signal. Can cause surging, stalling, or the car defaulting to a reduced-power "limp mode" for safety.

P0128

Low

Coolant Thermostat (Below Regulating Temperature)

The engine is taking too long to reach normal operating temperature — almost always a thermostat stuck open. Hurts fuel economy and heater performance but isn't dangerous.

P0130

Medium

O2 Sensor Circuit Malfunction (Bank 1, Sensor 1)

The upstream oxygen sensor on bank 1 isn't giving a usable signal. The computer can't fine-tune the fuel mixture properly, which hurts economy and emissions over time.

P0133

Medium

O2 Sensor Slow Response (Bank 1, Sensor 1)

The sensor works but reacts too slowly to fuel mixture changes — usually just aging past its useful life (most last 60,000–100,000 miles) or contamination from oil burning or a coolant leak.

P0135

Low

O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

The internal heater that gets the sensor up to operating temperature quickly has failed. The sensor still works once the exhaust naturally warms it, so this mostly hurts cold-start emissions.

P0171

Medium

System Too Lean (Bank 1)

Too much air relative to fuel. The most common culprit by far is a vacuum leak (cracked hose, bad intake gasket); also check for a dirty MAF sensor or a weak fuel pump.

P0172

Medium

System Too Rich (Bank 1)

Too much fuel relative to air — think dark exhaust smoke or a fuel smell. Common causes: a failing MAF sensor, a leaking fuel injector, or a stuck-closed fuel pressure regulator.

P0174

Medium

System Too Lean (Bank 2)

Same as P0171 but on the engine's other cylinder bank (V6/V8 engines only). Often appears together with P0171 when the cause is shared, like a MAF sensor or a large vacuum leak.

P0175

Medium

System Too Rich (Bank 2)

The rich-condition equivalent of P0174 on bank 2. Check for the same shared causes as P0172 first before assuming two separate problems.

P0190

High

Fuel Rail Pressure Sensor Circuit Malfunction

The computer can't trust the fuel rail pressure reading, which affects how well it can control the engine. Can cause hard starting, stalling, or a big loss of power.

Ignition & misfire codes (P0300–P0399)

P0300

High

Random / Multiple Cylinder Misfire Detected

More than one cylinder is misfiring, without a clear single-cylinder pattern. Common causes: worn spark plugs, a vacuum leak affecting multiple cylinders, or low fuel pressure. A flashing check engine light with this code means stop driving soon — unburned fuel can destroy the catalytic converter.

P0301

High

Cylinder 1 Misfire Detected

One specific cylinder isn't firing correctly — usually a worn spark plug, a failing ignition coil, or a leaking/clogged fuel injector on that cylinder. See the cylinder-specific table for other cylinder numbers.

P0325

Medium

Knock Sensor Circuit Malfunction (Bank 1)

The sensor that detects engine "pinging" isn't reporting properly, so the computer can't pull timing back to protect the engine if knock occurs. Get this looked at before it masks a real knocking problem.

P0335

High

Crankshaft Position Sensor Circuit Malfunction

This sensor tells the computer exactly when each cylinder fires. A failure can cause stalling, hard starting, or the engine cutting out completely without warning while driving.

P0340

Medium

Camshaft Position Sensor Circuit Malfunction

Similar role to the crank sensor but for valve timing. Can cause rough starts, reduced power, or the engine defaulting to a backup timing strategy.

P0350

Medium

Ignition Coil Primary/Secondary Circuit Malfunction

A general ignition coil circuit fault, not tied to one specific cylinder. Often paired with a misfire code — see the per-cylinder coil codes below for the specific-cylinder versions.

Emissions control codes (P0400–P0499)

P0401

Medium

EGR Flow Insufficient

Not enough exhaust is being recirculated back into the intake — usually a clogged EGR passage or a stuck-closed EGR valve. Can cause pinging/knock and higher emissions.

P0402

Medium

EGR Flow Excessive

Too much exhaust gas flowing back in, usually a stuck-open EGR valve. Can cause rough idle, stalling, or hesitation on acceleration.

P0410

Low

Secondary Air Injection System Malfunction

Part of the cold-start emissions system that pumps extra air into the exhaust. A common wear item; doesn't affect drivability, just emissions compliance.

P0420

Medium

Catalyst System Efficiency Below Threshold (Bank 1)

The single most-searched OBD2 code. Means the catalytic converter isn't cleaning exhaust efficiently enough. Could be a genuinely worn-out converter, a bad downstream O2 sensor, or an underlying misfire/oil-burning issue that's damaging the converter from the inside — worth ruling those out before replacing an expensive part.

P0430

Medium

Catalyst System Efficiency Below Threshold (Bank 2)

Same issue as P0420, on the engine's other cylinder bank (V6/V8 only).

P0440

Low

EVAP Emission Control System Malfunction

A general fault somewhere in the system that captures fuel vapor from the tank. Start by checking the gas cap — it's the single most common cause across the whole P044x/P045x family.

P0442

Low

EVAP System Leak Detected (Small Leak)

A small leak in the fuel vapor system. Check the gas cap is fully tightened first — a loose or worn cap seal causes a large share of these.

P0446

Low

EVAP Vent Control Circuit Malfunction

The valve that vents the EVAP system to atmosphere isn't responding correctly — often stuck from road grime or a wiring fault. Emissions-only, not a drivability concern.

P0455

Low

EVAP System Leak Detected (Large Leak)

A bigger leak than P0442 — most often a gas cap left loose or missing entirely. Tighten or replace the cap and the light typically clears within a few drive cycles.

P0456

Low

EVAP System Leak Detected (Very Small Leak)

The most sensitive leak threshold in the family — can be triggered by something as small as a hairline crack in a vapor hose. Purely an emissions concern.

Vehicle speed & idle control codes (P0500–P0599)

P0500

Medium

Vehicle Speed Sensor Malfunction

The computer isn't getting a reliable road-speed signal, which can affect the speedometer, cruise control, ABS, and automatic transmission shift timing all at once.

P0505

Medium

Idle Control System Malfunction

The idle air control valve (or electronic throttle idle strategy) isn't holding a steady idle speed. Often caused by a carbon-fouled throttle body or valve.

P0506

Medium

Idle Control System RPM Lower Than Expected

The engine idles rougher or stalls more than it should. Common causes: a dirty throttle body, a vacuum leak, or a failing idle control valve.

P0507

Medium

Idle Control System RPM Higher Than Expected

The engine idles too fast. A vacuum leak (unmetered air entering the engine) is the most common cause; a stuck-open idle air valve is another.

P0562

High

System Voltage Low

The electrical system is running below normal voltage — check the battery connections and alternator/charging system. Left unaddressed, this can leave you stranded with a dead battery.

P0563

Medium

System Voltage High

Usually an overcharging alternator or voltage regulator fault. Left unchecked, chronic overcharging can shorten battery life and damage electronics.

Computer & output circuit codes (P0600–P0699)

P0600

Medium

Serial Communication Link Malfunction

A module on the vehicle's internal network isn't communicating properly with the main computer. Wiring, connector corrosion, or a failing module can all cause this.

P0601

Medium

Internal Control Module Memory Checksum Error

The computer's internal memory failed a self-check. Sometimes cleared by a reflash of the module's software; occasionally means the module itself needs replacing.

P0606

High

ECM/PCM Processor Malfunction

An internal fault in the main engine computer's processor. Can cause unpredictable drivability issues since the computer may not be executing its control strategy correctly.

P0620

Medium

Generator (Alternator) Control Circuit Malfunction

A fault in the circuit the computer uses to regulate the alternator's output. Can lead to under- or over-charging the battery.

P0650

Low

Malfunction Indicator Lamp (MIL) Control Circuit

A fault in the wiring for the check engine light itself, ironically. Doesn't affect how the car drives, but means you may not get reliably warned about other issues.

Transmission codes (P0700–P0899)

P0700

Medium

Transmission Control System Malfunction

A general flag from the transmission control module telling the engine computer something is wrong. There's almost always a second, more specific transmission code alongside this one — that's the one to actually chase down.

P0705

Medium

Transmission Range Sensor Circuit Malfunction (PRNDL)

The sensor that tells the computer which gear (Park, Reverse, Neutral, Drive) the shifter is in isn't reporting correctly. Can prevent the car from starting or cause it to start in the wrong gear.

P0715

Medium

Input/Turbine Speed Sensor Circuit Malfunction

Measures how fast the transmission's input shaft is spinning. A failure can cause harsh, delayed, or incorrect gear shifts.

P0720

Medium

Output Speed Sensor Circuit Malfunction

Same idea as P0715 but on the output side of the transmission — also feeds the speedometer on many vehicles.

P0730

High

Incorrect Gear Ratio

The transmission isn't achieving the gear ratio it's being commanded to. Can point to low or degraded transmission fluid, a failing solenoid, or internal clutch wear — worth addressing before it strands you.

P0740

Medium

Torque Converter Clutch Circuit Malfunction

The lock-up clutch inside the torque converter isn't engaging as commanded. Can feel like a slight shudder around highway cruising speed, or hurt fuel economy.

P0741

Medium

Torque Converter Clutch Stuck Off

The lock-up clutch is stuck disengaged, meaning the transmission never fully "locks" at cruising speed. Mostly a fuel-economy and heat concern rather than a driveability emergency.

P0750

Medium

Shift Solenoid A Malfunction

One of the electrically-controlled valves that route fluid to change gears isn't responding correctly. Can cause harsh shifts, being stuck in one gear, or delayed engagement.

P0755

Medium

Shift Solenoid B Malfunction

Same failure mode as P0750, on a different shift solenoid. Often shows up alongside other solenoid or gear-ratio codes when the transmission fluid is old or low.

3. Cylinder-specific codes (misfire & ignition coils)

These follow a simple numbering pattern — the last two digits identify which cylinder is affected. The meaning and likely causes are the same for every cylinder; only the location changes.

Cylinder	Misfire code	Ignition coil code	Injector circuit code
1	P0301	P0351	P0201
2	P0302	P0352	P0202
3	P0303	P0353	P0203
4	P0304	P0354	P0204
5	P0305	P0355	P0205
6	P0306	P0356	P0206
7	P0307	P0357	P0207
8	P0308	P0358	P0208

Misfire (P030X): that cylinder isn't firing correctly — check spark plug, ignition coil, and injector for that cylinder first.
Ignition coil (P035X): the coil itself (or its driver circuit) has an electrical fault on that cylinder.
Injector circuit (P020X): a wiring or driver-circuit fault for that cylinder's fuel injector, separate from the injector clogging or leaking mechanically.

One cylinder misfiring often makes a distinct sound too — a rhythmic miss or stumble tied to engine speed. If you can hear it, MyAutoSound's free AI diagnosis can help confirm what's going on from the sound alone.

4. Frequently asked questions

What does a P0420 code mean? ▾

P0420 means the catalytic converter on engine bank 1 isn't cleaning exhaust as efficiently as the computer expects. It's most often caused by a failing catalytic converter, an oxygen sensor giving a bad reading, or an underlying engine issue (like a misfire or oil burning) that's damaging the converter over time.

Can I still drive with a check engine light on? ▾

A steady check engine light is usually safe to drive on for a short while, though it should be checked soon. A flashing check engine light means an active misfire that can damage the catalytic converter within minutes — in that case, reduce speed and get the car looked at as soon as possible.

How do I read my own OBD2 code for free? ▾

Every gasoline car sold in North America since 1996 has an OBD2 port, usually under the dashboard near the steering column. A cheap Bluetooth or USB code reader (around $15-25) paired with a free phone app can read it, or many auto parts retailers will scan your code for free in their parking lot.

Does clearing an OBD2 code fix the problem? ▾

No. Clearing the code only turns off the warning light — it does not repair whatever triggered it. If the underlying issue is still there, the same code (or a related one) will usually come back within a few days of driving.

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