Technical Description

System Too Lean (Bank 2)

What does that mean?

Basically this means that an oxygen sensor in bank 2 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 2 is generally the side of the engine that doesn't have cylinder #1.

Note: This DTC is very similar to P0171, and in fact your vehicle may show both codes at the same time.

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Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration.
Causes

A code P0174 may mean that one or more of the following has happened:

    * The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
    * There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions

Possible solutions include:

    * In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
    * Inspect all vacuum and PCV hoses, replace if necessary
    * Check for a dirty fuel filter and proper fuel pressure

Technical Description

Insufficient EGR Flow

What does that mean?

EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emmissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor EGR (DPFE). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0401 code means that OBD detected an insufficient amount of EGR.

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Symptoms

You may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. There may also be other symptoms.
Causes

A code P0401 most likely means one or more of the following has happened:

    * The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
    * There is a blockage in the EGR (tube), most likely carbon buildup
    * The EGR valve is faulty
    * The EGR valve may not be opening due to a lack of vaccuum

Possible Solutions

In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.

    * Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open
    * Clean out the EGR valve and/or tubing to remove deposits
    * Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
    * Replace the DPFE sensor (with a good quality / OEM one)
    * Replace the EGR valve

Technical Description

Random/Multiple Cylinder Misfire Detected

What does that mean?

Basically this means that the the car's computer has detected that not all of the engine's cylinders are firing properly.

A P0300 OBD code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

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Symptoms
Symptoms may include:

    * the engine may be harder to start
    * the engine may stumble / stumble, and/or hesitate
    * other symptoms may also be present

Causes

A code P0300 may mean that one or more of the following has happened:

    * Faulty spark plugs or wires
    * Faulty coil (pack)
    * Faulty oxygen sensor(s)
    * Faulty fuel injector(s)
    * Burned exhaust valve
    * Faulty catalytic converter(s)
    * Stuck/blocked EGR valve / passages
    * Faulty camshaft position sensor
    * Defective computer

Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

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Symptoms

You will likely not notice any drivability problems, although there may be symptoms.
Causes

A code P0420 may mean that one or more of the following has happened:

    * Leaded fuel was used where unleaded was called for
    * An oxygen sensor is not reading (functioning) properly
    * The engine coolant temperature sensor is not working properly
    * Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
    * Retarded spark timing
    * The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions

Some suggested steps for troubleshooting a P0420 code include:

    * Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
    * Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
    * Inspect the downstream heated oxygen sensor (HO2), replace if necessary
    * Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

Technical Description

System Too Lean (Bank 1)

What does that mean?

Basically this means that an oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.

Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.

Symptoms

You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration.

Causes

A code P0171 may mean that one or more of the following has happened:

    * The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
    * There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions

Possible solutions include:

    * In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
    * Inspect all vacuum and PCV hoses, replace if necessary

    * Check for a dirty fuel filter and proper fuel pressure

FAQ: OBD2 Codes Explained.

OBD-II codes consist of a number of parts. Here is a sample OBD2 code:

P0171

Here is a breakdown of what each digit of the code means:

First Character - System

The first character identifies identifies the system related to the trouble code.

• P = Powertrain
• B = Body
• C = Chassis
• U = Undefined

Second Digit - Code Type

The second digit identifies whether the code is a generic code (same on all OBD-II equpped vehicles), or a manufacturer specific code.

• 0 = Generic (this is the digit zero -- not the letter "O")
• 1 = Enhanced (manufacturer specific)

Third Digit - Sub-System

The third digit denotes the type of sub-system that pertains to the code

• 1 = Emission Management (Fuel or Air)
• 2 = Injector Circuit (Fuel or Air)
• 3 = Ignition or Misfire
• 4 = Emission Control
• 5 = Vehicle Speed & Idle Control
• 6 = Computer & Output Circuit
• 7 = Transmission
• 8 = Transmission
• 9 = SAE Reserved
• 0 = SAE Reserved

Fourth and Fifth Digits

These digits, along with the others, are variable, and relate to a particular problem. For example,a P0171 code means P0171 - System Too Lean (Bank 1). To lookup your particular code, visit our OBD Trouble Codes section.