Need Car Diagnostic Codes? Here’s A Helpful Guide

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Jane Taylor in Technology

Last updated: 21 January 2020, 06:54 GMT

If you are confused by car diagnostic codes, you are in good company. Most people, even mechanics, can be puzzled by the alphanumeric codes spit out by car code readers. The overly-vague "check engine light" is of little help without a diagnostic tool that can decipher the codes that car computers produce.

What is even more confusing is that the code provided by a code reader does not necessarily indicate that you need to replace something. The code refers to a systemic issue that requires a further diagnosis to determine what the problem is. There is a method to the apparent madness, and we will go over the details of what all the different terms mean.

If you get a car code reader, learn how to use it, and write down the code provided to you by the machine. You will be able to reference our helpful car diagnostic codes list and find out what those strange codes actually mean. There are hundreds of possible codes that your car can give you, so having a helpful chart will save you the headache of figuring out what the codes mean.

How Car Codes Work


Before we get into what the specific codes mean, we should go over some basic information when it comes to how the codes are formatted. Understanding the format will help you memorize what the main diagnostic codes are. You can save time when you get something like a check engine light flashing on your dash.

There are two main types of codes coming from the On-Board Diagnostic System, or OBD-II. There are generic codes that have the P0xxx format. These have explanations that are easy to find. Then there are also “manufacturer codes” which are a little more confusing. To decipher those codes, you will need to find a good repair manual. For the generic codes, you will have an easier time figuring them out because they follow the following format.

  • The first digit in the code sequence is actually a letter that stands for different things. You have B for body, C for chassis, P for powertrain, and U for the network.
  • The following digit in the code sequence is either 0 for a generic code or 1 for a manufacturer code.
  • The third digit will have a number between one and eight.
    • One is for fuel and air metering.
    • Two is for fuel and air metering injector circuit.
    • Three is for the ignition system, which also includes misfires.
    • Four are for axillary emissions controls.
    • Five is for vehicle speed controls and the idle control system.
    • Six is for the computer output circuit.
    • Seven and eight are both for the transmission.
  • The fourth and fifth digits are for fault codes between 0 and 99.

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Car Diagnostic Codes List


Now that we have laid the groundwork, it is time to give you the car diagnostic codes list of the most common ones you’ll see. These codes will sometimes be combined since there are multiple codes for specific issues.

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  • P0133, P0135, and P0141 – These codes involve the oxygen, or O2, sensors. These sensors monitor how much oxygen is in your car's exhaust. The engine's computer will be able to appropriately modify the fuel mixture so that emissions are reduced and fuel economy is maximized.
  • P0171, P0175 – These codes involve the O2 sensor. You will not be passing an emissions test if you get one of these. The oxygen sensor monitors the amount of oxygen that is in your car's exhaust, allowing the on-board engine computer to modify the fuel mixture. If you get a code within this range, your car is likely to randomly stall when you least expect it.
  • P0300, P0305 – These codes involve engine misfires. This could indicate that there are malfunctioning cylinders in your car's engine. If you get a code within this range, your vehicle is likely to experience vibrations and shaking while idle. Your vehicle will also have a terrible fuel economy. 
  • P0325 – This code involves the knock sensor, a sensor that generates a signal when it identifies vibrations coming from the engine. These vibrations are created by detonations during strong acceleration or lugging of the engine. The problem here can also be a faulty sensor. You can also get vibrations if you are driving on very rough roads and off-road terrain.
  • P0411, P0440, P0442, P0446, P0455 – These codes involve the evaporative system. This is a system that gets rid of the vapors created after fuel is burned. These codes usually indicate that you did not close the gas cap or you did not twist it tight enough. It is probably the easiest fix out of anything else a diagnostic code could indicate.
  • P0401 – This code involves exhaust gas recirculation, also known as EGR. The EGR controls the car's emission of pollutants such as nitrous oxides. EGR is the process of recirculating some exhaust back into the engine cylinders. If you get this code, it’s like that you haven’t changed your oil when you were supposed to. You also may be taking too many short car trips.
  • P0420 and P0430 – These two codes refer to an issue with the catalytic converter. There is a downstream O2 sensor that keeps tabs on the catalytic converter. If it finds that the converter has become contaminated, then you will get one of these codes. You will have more tailpipe emissions, which can even mean that your car's engine is burning oil or leaking coolant.

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Wrapping Up


You now hopefully have a clearer understanding of how the car diagnostic codes work. Our car diagnostic codes list should be a helpful go-to guide. You can use it whenever your car decides to let you know that there is something that needs attention. All you will need is a diagnostic code reader, and you’ll be able to get the diagnosis you have been looking for to fix your car.




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