Car Diagnostic Codes: Troubleshooting Check Engine Light Codes Like a Pro

Troubleshooting Check

Troubleshooting Check

A blink of your car’s Check Engine Light can be alarming, but you can react more calmly if you have a good knowledge of car diagnostic codesand meanings. If you know this, you can tell for sure when you need to park your car immediately or if your car can still make the rest of the journey without any trouble.

For you to have full knowledge of diagnostic codes for cars as they relate to check engine lights, we will take you through the Engine Control Module (ECM), reasons why Check Engine Light come on, and Diagnostic Trouble Codes for Check Engine Light.

What is the function of the Engine Control Module (ECM)?

The average vehicle is admirable in its complexity and functions, not just because of its visible operations but also because of the hidden, complex activities it carries out. From the moment you start your vehicle, the Onboard Diagnostic System (OBD System), also known as Computer Control Modules (CCM), commences a self-diagnosing procedure to detect any issue your vehicle may be going through. The Engine Control Module (ECM) is one of the most significant parts of the OBD system because of the functions it carries out. It can also be referred to as the Powertrain Control Module (PCM).

The ECM diagnoses and detects any issue that may be plaguing your vehicle’s engine, stores the information, and then sends a signal to the driver via the Check Engine Light.

Check Engine Light

Common Reasons Why the Check Engine Light Comes On

Whenever your Check Engine Light comes on, it usually relates to a fault within the engine. Some of the possible issues include:

Car Diagnostic Codes and How They Function

Car Diagnostic Codes, also known as Diagnostic Trouble Codes (DTC), are five-digit alphanumeric codes that diagnose and identify irregularities and malfunctions in a vehicle. These engine fault codes do not point out the specific fault but direct you or your mechanic to which part of the engine to test. To access the code, a car diagnostic code reader is used to scan the data port.

DTCs fall into four categories, each symbolized by a letter, depending on the location of the fault. They are as follows:

P – Powertrain: This alerts regarding issues with the powertrain, such as transmission, driveshaft, axles, differential, or engine.

B – Body: This involves malfunctions relating to the vehicle body, such as airbags, recline position restraint sensors, or a seat recline back switch.

C – Chassis: This shows issues with the chassis, like the power steering control module, speed sensors, power steering sensor, or suspension systems.

U – Network Communications: This is particular to vehicles after 2006, referring to communication issues like wiring.

An ideal DTC code will be followed by certain numbers, some generic while others are enhanced. The generic ones are used by all vehicles without regard to the manufacturer.  Generic codes have “0” following the letter. On the other hand, enhanced codes are particular to only certain manufacturers and are represented with “1” as the second digit.

Common Diagnostic Trouble Codes for Check Engine Light

Car diagnostic codes for Check Engine Light usually relate to the powertrain category. There are so many codes for powertrain faults but we have selected a few common ones for you. The following codes are likely to come up after a scan:

  1. Code P0401

This code shows up whenever there’s a faulty exhaust gas recirculation (EGR) valve. Code P0401 stands for “Exhaust Gas Recirculation (EGR) Flow Insufficient Detected.” This code pops up whenever there’s a restriction causing insufficient exhaust gas recirculation and is common in vehicles like Honda or Toyota. The problem occurs whenever the EGR valve is faulty. It could also be that there are obstructed EGR passages, sensor failure, an inoperative vacuum supply to the EGR valve, circuit issues, or a bad EGR valve cooler.

It is dangerous to keep driving in this condition.

  1. Code P0402

In contrast to P0401, P0402 is triggered whenever there is excess EGR flow. It is usually caused by different faults, such as EGR valve failure, malfunctioning differential pressure sensors, clogged EGR passages caused by carbon build-up, and Powertrain Control Module (PCM) problems.

Whenever this code is detected, it is advisable to quit driving and find a solution as soon as possible.

  1. Code P0171

This car diagnostic code, meaning “System Too Lean Bank 1,”  is especially common whenever the Check Engine Light comes up and it has to do with too much oxygen gaining access to the combustion chamber, which inevitably leads to your engine receiving too much air. This means that the air-to-fuel ratio is low because the combustion chamber receives too little fuel and too much air.

Some of the causes of this include a defective fuel pressure regulator, a leaking intake manifold gasket, a faulty mass air flow sensor, an obstructed fuel filter, a vacuum leak, a bad oxygen sensor, or a weak fuel pump.

  1. Code P0172

This car diagnostic code also identifies a problem with the air-to-fuel ratio. Whereas Code P0171 relates to a lean ratio, this code indicates that the system is running too rich. This means that the combustion chamber is receiving too little air and too much fuel. This could be due to a malfunctioning mass air flow sensor, a faulty fuel injector, an inoperative coolant temperature sensor, or an engine thermostat.

  1. Code P0420

This code is named “catalyst system efficiency below threshold (Bank 1). When you see this code, just know that there is an issue with the catalytic converter in the exhaust system. It’s supposed to function to break down harmful products derived from the combustion process. Accordingly, a defective catalytic converter can trigger the code, and so can a bad oxygen sensor or mass air flow sensor.

  1. Code P0430

This code appears whenever there is a catalytic converter problem, just like P0420. The only difference is that while P0420 relates to Bank 1, this one is an alert regarding Bank 2. If you’re using a vehicle with a larger engine, such as a V6, V8, or V10 engine, then you’re more likely to receive this alert.


Car diagnostic codes are an essential aspect of every vehicle and it helps to know a thing or two about them. These codes notify you of any issues your car may have. One of the ways your car alerts you to issues after self-diagnosing is the Check Engine Light.

Notifications from the Check Engine Light should not be taken lightly since they are essential and often need urgent attention. This is because they relate to the engine of the vehicle. Some of the reasons why your Check Engine Light can come on or blink are a loose gas cap, faulty fuel injectors, catalytic converter problems, an inoperative mass air flow sensor, a faulty oxygen sensor in the exhaust system, or transmission damage.

For you to discover which one of these issues persists in your engine, you have to scan and get the codes. Some of the codes you can receive are P0420, P0430, P0171, P0172, P0401, and P0402. With a good idea of these car diagnostic codes, you can save yourself so many headaches and get right to resolving the issues.

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