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Most Common OBD-II Codes Explained: Complete Check Engine Light Guide

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Most Common OBD-II Codes

When your Check Engine Light turns on, the vehicle is trying to tell you that something has gone wrong. Unfortunately, it communicates using codes such as P0171, P0300, P0420, and P0700 instead of simply saying, “The intake hose is cracked.”

That would apparently be too convenient.

An OBD-II scanner can retrieve the stored Diagnostic Trouble Code, but reading the code is only the beginning. An OBD-II code identifies the system that detected a problem—it does not always identify the failed part.

Replacing components based only on a code description can become expensive very quickly. A code mentioning an oxygen sensor does not automatically mean the oxygen sensor is bad. A transmission code does not automatically mean the transmission needs replacement. A misfire code does not mean purchasing every ignition coil within a 20-mile radius.

This guide covers some of the most common OBD-II codes, what they mean, how serious they are, and where to find complete step-by-step diagnostic and repair instructions.

Quick OBD-II Code Reference

CodeDescriptionSeverityCommon Cause
P0011Intake camshaft timing over-advancedHighDirty oil or VVT fault
P0014Exhaust camshaft timing over-advancedHighVVT solenoid or timing wear
P0016Crankshaft/camshaft correlation faultVery HighTiming-chain stretch
P0101MAF sensor range/performanceModerateDirty MAF or intake leak
P0113Intake-air temperature circuit highModerateOpen circuit or unplugged sensor
P0117Coolant-temperature circuit lowHighShorted sensor or overheating
P0118Coolant-temperature circuit highModerateOpen circuit or failed sensor
P0128Coolant below thermostat temperatureLow–ModerateThermostat stuck open
P0171System too lean, Bank 1ModerateVacuum leak or dirty MAF
P0174System too lean, Bank 2ModerateVacuum leak or low fuel pressure
P0300Random or multiple-cylinder misfireHighIgnition, fuel or compression issue
P0301Cylinder 1 misfireHighSpark plug or ignition coil
P0304Cylinder 4 misfireHighSpark plug, coil or injector
P0420Catalyst efficiency below threshold, Bank 1ModerateWorn converter or exhaust leak
P0430Catalyst efficiency below threshold, Bank 2ModerateWorn converter or exhaust leak
P0442Small EVAP leakLowGas cap or EVAP hose
P0455Large EVAP leakLowLoose cap or disconnected hose
P0456Very small EVAP leakLowGas-cap seal or tiny hose leak
P0507Idle RPM higher than expectedModerateVacuum leak or dirty throttle body
P0700Transmission control-system faultHighAdditional TCM code stored
P0705Transmission range-sensor faultHighRange switch or linkage
P0741Torque-converter clutch stuck offHighConverter, solenoid or valve body
P2195Bank 1 upstream O2 sensor stuck leanModerateVacuum leak or sensor fault
P2197Bank 2 upstream O2 sensor stuck leanModerateIntake leak or sensor fault

What Is an OBD-II Code?

OBD-II stands for On-Board Diagnostics, Second Generation.

Vehicles sold in the United States since the 1996 model year generally use a standardized diagnostic connector and code system. When a control module detects a fault, it stores a Diagnostic Trouble Code and may illuminate the Check Engine Light.

The code typically contains one letter followed by four numbers.

Examples include:

The first letter identifies the affected system:

Most Check Engine Light complaints involve powertrain codes beginning with the letter P.

Generic Versus Manufacturer-Specific Codes

The second character helps identify whether a code is generic or manufacturer-specific.

A generic code provides a broad definition, but the exact diagnostic procedure can vary by vehicle.

For example, P0011 always relates to camshaft timing, but a Ford VCT system, BMW VANOS system, Toyota VVT-i system, and Subaru AVCS system may require very different tests.

The code is the beginning of the diagnosis—not the entire repair manual conveniently compressed into five characters.

Lean Air/Fuel Mixture Codes

Lean codes mean the Engine Control Module is adding fuel because it detects too much air, insufficient fuel, or an inaccurate sensor signal.

P0171: System Too Lean, Bank 1

P0171 indicates that Bank 1 is operating too lean.

Common causes include:

Bank 1 is the side of the engine containing Cylinder 1.

If P0171 appears alone, inspect problems affecting Bank 1. If it appears with P0174, look for a problem affecting the entire engine.

Complete guide: P0171 Code Explained: System Too Lean Bank 1

P0174: System Too Lean, Bank 2

P0174 means Bank 2 is operating too lean.

This code only applies to engines with more than one cylinder bank, including many:

Common causes include intake leaks, MAF contamination, low fuel pressure, exhaust leaks, and Bank 2 injector problems.

When P0171 and P0174 appear together, suspect:

Complete guide: P0174 Code Explained: System Too Lean Bank 2

Oxygen-Sensor Stuck-Lean Codes

Codes P2195 and P2197 indicate that an upstream oxygen or air/fuel ratio sensor remains biased toward a lean reading.

The sensor may be defective, but it may also be accurately reporting a real air/fuel mixture problem.

P2195: O2 Sensor Stuck Lean, Bank 1 Sensor 1

P2195 applies to the upstream oxygen or air/fuel ratio sensor on Bank 1.

Common causes include:

Sensor 1 is located before the catalytic converter and helps the ECM adjust fuel delivery.

Complete guide: P2195 Code Explained: O2 Sensor Stuck Lean Bank 1 Sensor 1

P2197: O2 Sensor Stuck Lean, Bank 2 Sensor 1

P2197 applies to the upstream oxygen or air/fuel ratio sensor on Bank 2.

If Bank 1 fuel trims appear normal but Bank 2 fuel trims are highly positive, investigate:

Do not replace the catalytic converter first. P2197 involves the upstream fuel-control sensor, not the downstream catalyst-monitoring sensor.

Complete guide: P2197 Code Explained: O2 Sensor Stuck Lean Bank 2 Sensor 1

Engine Misfire Codes

A misfire occurs when one or more cylinders fail to burn the air/fuel mixture correctly.

Every cylinder needs:

A failure in any of these areas may cause a misfire.

A flashing Check Engine Light indicates a severe active misfire that may damage the catalytic converter. Stop driving as soon as safely possible.

P0300: Random or Multiple-Cylinder Misfire

P0300 indicates random or multiple-cylinder misfires.

Unlike a cylinder-specific code, P0300 does not identify only one affected cylinder.

Common causes include:

Begin by checking for accompanying P0301 through P0312 codes.

Complete guide: P0300 Code Explained: Random or Multiple-Cylinder Misfire

P0301: Cylinder 1 Misfire

P0301 means the ECM detected a misfire in Cylinder 1.

The most efficient diagnostic sequence is usually:

  1. Confirm the Cylinder 1 location.
  2. Inspect its spark plug.
  3. Swap its ignition coil.
  4. Test the fuel injector.
  5. Check for a nearby intake leak.
  6. Perform compression testing.

If the misfire follows a swapped ignition coil, the coil is likely defective.

Complete guide: P0301 Code Explained: Cylinder 1 Misfire

P0304: Cylinder 4 Misfire

P0304 means Cylinder 4 is misfiring.

Common causes include:

Verify the manufacturer’s cylinder numbering before testing. Cylinder 4 is not always where an enthusiastic YouTube comment says it is.

Complete guide: P0304 Code Explained: Cylinder 4 Misfire

Catalytic Converter Efficiency Codes

The ECM monitors catalytic-converter performance by comparing upstream and downstream oxygen-sensor activity.

The upstream sensor reacts quickly to changes in the air/fuel mixture. A healthy catalytic converter stores oxygen and smooths the downstream sensor signal.

When the downstream sensor begins behaving too much like the upstream sensor, the ECM may determine that catalyst efficiency has dropped below the required threshold.

P0420: Catalyst Efficiency Below Threshold, Bank 1

P0420 indicates reduced catalytic-converter efficiency on Bank 1.

Common causes include:

Repair engine misfires and fuel-control problems before installing a new converter. Otherwise, the replacement converter may suffer the same expensive fate.

Complete guide: P0420 Code Explained: Catalyst Efficiency Below Threshold Bank 1

P0430: Catalyst Efficiency Below Threshold, Bank 2

P0430 indicates reduced catalyst efficiency on Bank 2.

P0420 and P0430 appearing together may indicate:

Inspect exhaust leaks, fuel trims, misfire counters, and sensor activity before condemning the converter.

Complete guide: P0430 Code Explained: Catalyst Efficiency Below Threshold Bank 2

EVAP Leak Codes

The Evaporative Emission Control system prevents gasoline vapors from escaping into the atmosphere.

Major components include:

EVAP codes usually do not affect how the vehicle drives, but they can illuminate the Check Engine Light and cause emissions-test failure.

P0442: Small EVAP Leak

P0442 means the ECM detected a small EVAP-system leak.

Common causes include:

A smoke test is often the fastest way to locate the leak.

Complete guide: P0442 Code Explained: Small EVAP Leak

P0455: Large EVAP Leak

P0455 means the EVAP system cannot seal or contains a relatively large leak.

Check for:

A disconnected hose frequently causes this code following recent fuel-system or underbody repairs.

Complete guide: P0455 Code Explained: Large EVAP Leak

P0456: Very Small EVAP Leak

P0456 indicates a very small EVAP-system leak.

The leak may be too small to see or smell.

Common sources include:

P0456 is rarely dangerous, but finding it without a smoke machine can become an enjoyable afternoon of staring suspiciously at rubber hoses.

Complete guide: P0456 Code Explained: Very Small EVAP Leak

Camshaft Timing and Correlation Codes

Modern engines use Variable Valve Timing to adjust camshaft position according to engine speed and load.

The system commonly relies on:

Dirty oil, low oil pressure, solenoid problems, and worn timing components can all trigger camshaft-related codes.

P0011: Intake Camshaft Timing Over-Advanced, Bank 1

P0011 means the Bank 1 intake camshaft remains more advanced than commanded.

Start by checking:

Do not immediately replace the camshaft sensor. It may simply be reporting that the mechanical and hydraulic systems have stopped cooperating.

Complete guide: P0011 Code Explained: Camshaft Timing Over-Advanced Bank 1

P0014: Exhaust Camshaft Timing Over-Advanced, Bank 1

P0014 applies to the Bank 1 exhaust camshaft.

Common causes include:

P0011 and P0014 appearing together may indicate an oil-pressure, timing-chain, or broader VVT-system problem.

Complete guide: P0014 Code Explained: Exhaust Camshaft Timing Over-Advanced

P0016: Crankshaft and Camshaft Correlation

P0016 means the crankshaft and Bank 1 intake camshaft are no longer synchronized within the expected range.

This is a serious code that may indicate:

If the engine rattles, stalls, or becomes difficult to start, stop driving and inspect the timing system immediately.

Complete guide: P0016 Code Explained: Crankshaft and Camshaft Correlation

Mass Air Flow and Intake-Air Sensor Codes

The ECM uses airflow and intake-temperature information to calculate engine load and fuel delivery.

Incorrect readings may cause:

P0101: MAF Sensor Range or Performance

P0101 means the Mass Air Flow sensor reading does not match the airflow the ECM expects.

Common causes include:

Clean the sensor only with dedicated MAF cleaner. Brake cleaner is not “basically the same thing,” regardless of how confidently the can is held.

Complete guide: P0101 Code Explained: MAF Sensor Range or Performance

P0113: Intake-Air Temperature Sensor Circuit High

P0113 means the ECM sees excessive voltage in the Intake Air Temperature sensor circuit.

A scan reading near -40 degrees commonly indicates:

Many vehicles integrate the IAT sensor into the MAF sensor assembly.

Complete guide: P0113 Code Explained: Intake-Air Temperature Circuit High

Coolant-Temperature and Thermostat Codes

The ECM uses coolant-temperature data to control:

An inaccurate signal can affect starting, fuel economy, cooling-fan operation, and engine protection.

P0117: Coolant-Temperature Sensor Circuit Low

P0117 means the ECT sensor circuit voltage is too low.

The ECM may interpret this as an extremely hot engine.

Possible causes include:

Always verify the real engine temperature before continuing to drive.

Complete guide: P0117 Code Explained: Coolant-Temperature Circuit Low

P0118: Coolant-Temperature Sensor Circuit High

P0118 means the ECT sensor circuit voltage is too high.

The ECM may display an unrealistically cold reading, frequently around -40 degrees.

Common causes include:

P0117 and P0118 are electrical circuit codes and should not be diagnosed by replacing the thermostat without testing.

Complete guide: P0118 Code Explained: Coolant-Temperature Circuit High

P0128: Coolant Below Thermostat Regulating Temperature

P0128 means the engine is not warming up as quickly as expected.

The most common cause is a thermostat stuck open.

Symptoms may include:

Unlike P0117 and P0118, P0128 usually describes a cooling-system performance problem rather than an electrical circuit fault.

Complete guide: P0128 Code Explained: Coolant Below Thermostat Temperature

High-Idle Code

P0507: Idle RPM Higher Than Expected

P0507 means actual idle speed remains higher than the ECM’s target.

Common causes include:

Some electronic throttle-body vehicles require an idle or throttle relearn after cleaning, replacement, or battery disconnection.

Complete guide: P0507 Code Explained: Idle RPM Higher Than Expected

Automatic Transmission Codes

Transmission codes should be taken seriously, but they do not automatically mean the entire transmission has failed.

Potential causes include:

P0700: Transmission Control-System Malfunction

P0700 means the Transmission Control Module has stored another fault and requested the Check Engine Light.

P0700 is a notification code, not the final diagnosis.

A scanner capable of communicating with the TCM is needed to retrieve the underlying code.

Possible companion codes include:

Complete guide: P0700 Code Explained: Transmission Control-System Malfunction

P0705: Transmission Range-Sensor Circuit Malfunction

P0705 means the ECM or TCM cannot reliably determine the selected transmission range.

The range sensor may also be called:

Common symptoms include:

Complete guide: P0705 Code Explained: Transmission Range-Sensor Malfunction

P0741: Torque-Converter Clutch Performance or Stuck Off

P0741 means the torque-converter clutch does not engage as commanded.

Common causes include:

Symptoms may include elevated highway RPM, reduced fuel economy, converter slip, and transmission overheating.

Complete guide: P0741 Code Explained: Torque-Converter Clutch Stuck Off

Which OBD-II Codes Require Immediate Attention?

Stop driving or reduce engine load when you encounter:

Codes that deserve especially prompt attention include:

EVAP codes such as P0442, P0455, and P0456 are generally less urgent, although they can cause emissions-test failure.

How to Diagnose an OBD-II Code Correctly

1. Read Every Stored and Pending Code

Do not diagnose only the first code displayed.

Related codes may identify the actual root cause.

2. Save Freeze-Frame Data

Freeze-frame data records operating conditions when the fault occurred.

Useful information includes:

3. Check Technical Basics

Inspect:

4. Use Live Data

Live sensor information can reveal whether a fault is genuine, intermittent, or caused by an inaccurate sensor.

5. Test Before Replacing Parts

Use:

6. Repair the Root Cause

A failed catalytic converter may have been damaged by a misfire. An oxygen-sensor code may be caused by a vacuum leak. A transmission notification code may be hiding a simple range-sensor problem.

Fixing only the final damaged component without correcting the original cause is an excellent way to purchase the same part twice.

Frequently Asked Questions

Can I drive with the Check Engine Light on?

It depends on the code and symptoms.

A steady Check Engine Light with normal operation may allow short-term driving. A flashing light, severe misfire, overheating, transmission slipping, or oil-pressure warning requires immediate attention.

Does an OBD-II code identify the failed part?

Not always.

The code identifies the system, circuit, or condition that failed a self-test. Additional testing is often required to identify the actual cause.

Can clearing a code fix the problem?

No.

Clearing the code erases stored information and turns off the light temporarily. If the fault remains, the code will return after the monitor runs again.

Why did my code return after replacing the sensor?

Possible reasons include:

What is the difference between Bank 1 and Bank 2?

Bank 1 contains Cylinder 1.

Bank 2 is the opposite cylinder bank. The physical side varies by manufacturer.

What is the difference between Sensor 1 and Sensor 2?

Sensor 1 primarily controls fuel delivery. Sensor 2 primarily monitors catalyst operation.

Can a cheap OBD-II scanner diagnose every code?

A basic scanner can read many generic engine codes.

Advanced diagnosis may require access to:

A $20 scanner can tell you the transmission is unhappy. It may not tell you why it has chosen violence.

Complete OBD-II Guide Directory

Air, Fuel and Oxygen-Sensor Codes

Misfire Codes

Catalytic-Converter Codes

EVAP Codes

Camshaft and Timing Codes

Cooling-System Codes

Idle-Control Code

Transmission Codes

Final Thoughts

OBD-II codes are valuable diagnostic clues, but they should never replace proper testing.

The most common Check Engine Light codes generally fall into several groups:

Start by reading all stored codes, saving freeze-frame information, inspecting basic mechanical and electrical systems, and reviewing live data.

Then use the individual Pro Street OBD-II guides above to test the system methodically before replacing parts.

The goal is not simply to turn off the Check Engine Light. The goal is to fix the problem that turned it on—preferably before the parts cannon empties your checking account.

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