Common OBDII Car Diagnostic Trouble Codes Explained

On-board diagnostics (OBD) is a computer system inside your vehicle that tracks, regulates, and collects information from the array of sensors situated in your car. The OBD system constantly monitors the functioning of every component and alerts the driver if any problem occurs. Usually, these problems are displayed on the dashboard by a warning light‌ like a check engine light, ABS light, low oil pressure, etc.

When a fault occurs, a Diagnostic Trouble Code (DTC) is stored in the vehicle’s Electronic Control Unit (ECU). Then a technician connects an OBD scanner that collects all the error codes so the problem can be identified and diagnosed. The OBD system has been a great help in helping users to better understand vehicle diagnostics, making the job very convenient.

OBD scanners are diagnostic tools that are attached to the OBD port in your car so they can read the diagnostic trouble codes and the data stored in the ECU. This data is analyzed to identify the malfunctioning component. Every time you see a check engine light on your dashboard, you have to take a trip to the service station and the mechanic attaches the OBD scanner tool to find the problem. Often it's frustrating to find out, the check engine light went off due to a simple problem like a loose fuel cap that could have been fixed at home.

If you have an OBD scanner at home, you can also do some quick checks on your vehicle. However, you have to understand the error codes and what they mean to identify the problem. Read on to learn more about the most common OBD error codes and how you can resolve them.

The OBD codes were created by the Society of Automotive Engineers (SAE) to comply with OBD-2 emissions regulations in the US. These codes are divided into four categories: Powertrain (P), Body (B), Chassis (C), and Network Communications (U). These codes are further divided into two major groups: Generic or global (0 as the second digit) and enhanced or manufacturer specific (1 as the second digit). Learn more about the code format on our comprehensive list of OBD error codes.

One of the most common car problems that can cause a check engine light to come up involves a faulty oxygen sensor. The oxygen sensors allow the ECU to adjust the fueling and make the air-fuel ratio rich or lean by measuring the amount of oxygen present in the exhaust gas. The oxygen sensor trouble codes are classified into two parts: heater circuit codes and sensor codes.

Most modern oxygen sensors come with heating elements so that they can reach the ideal operating temperature quickly to provide accurate readings. When the Electronic Control Unit (ECU) detects a delay between startup and sensor activity, it assumes there is a problem with the heater circuit and triggers code P0135, which is an O2 Sensor Heater Circuit code for Bank 1 Sensor 1. This can be caused due to heater circuit failure, a damaged connector, or a bad oxygen sensor that has to be replaced. Similarly, P1041 is the Sensor Heater Circuit Malfunction code for Bank 1 sensor 2. 

Some of the most common oxygen sensor-related diagnostic trouble codes are listed below.

Apart from the heater circuit, the oxygen sensor circuit can also malfunction by throwing up diagnostic trouble codes like P01038, P1040, etc. P1038 is O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). It means the sensor fails to have a lower output due to a lack of oxygen in the exhaust gas. A low voltage (lean) oxygen sensor indicates the sensor has failed, or there's an exhaust manifold leak, or the unburned oxygen is entering the exhaust. If the oxygen sensor readings remain low (lean) or high (rich) or do not change at all, the ECU generates an error code. P1040 indicates Oxygen Sensor Circuit No Activity Detected (Bank 1, Sensor 2).

Engine misfiring is the most common issue that mechanics deal with on a daily basis. It can happen when one or more cylinders are not working properly, due to failing spark plugs, glow plugs or other issues. The OBD system tracks engine misfires by using the crankshaft position sensor to monitor the speed of the crankshaft when the engine is running.

The OBD system logs a misfire, and a related diagnostic trouble code when it detects a slight loss of speed in the crankshaft. The code helps you identify which cylinder is causing the issue. For example, a P0301 OBD code tells you that you have a misfire in cylinder one. A P0302 OBD code tells you that the problem is in cylinder two, and so on. The codes would go up to P0304 in the case of a four-cylinder engine, P0306 if you have a six-cylinder engine, and P0308 if you have an eight-cylinder engine.

However, a P0300 diagnostic trouble code means the engine has a random misfire, and it is happening in any of the cylinders. Such a problem occurs due to cracked vacuum hoses or bad intake manifold gaskets. It can also happen due to a lean mixture. If a P0300 code is accompanied by a P0171 or P0174 code, it means the engine is running too lean due to a vacuum leak or a bad EGR valve. 

Most cars are equipped with an Evaporative Emission Control (EVAP) system which prevents the fuel vapors from escaping from the fuel tank. The EVAP system consists of a charcoal canister that captures fuel vapors and a purge valve for siphoning the fuel vapors into the engine. It also features vacuum sensors that detect vapor leaks.

A P0455 error code is a large leak code, mostly caused by a loose or missing gas cap. This error code indicates that gasoline vapors are escaping the fuel tank and into the atmosphere. A P0442 is a small leak code that can be caused by a cracked or loose fuel tank hose or leaky EVAP canister. A good practice is to ensure the gas cap fits tightly and is not leaking. 

The exhaust gas recirculation (EGR) system controls engine emissions, mainly nitrous oxides (NOx). The EGR system does this by recirculating a small amount of the exhaust gas back into the intake manifold. The car’s Electronic Control Unit (ECU) opens and closes the EGR valve to regulate the exhaust gas recirculation.

If the EGR valve fails or the exhaust gasses are not passing properly due to carbon deposits under the EGR valve, it can throw up a P0401 OBD trouble code. To fix this, check if the EGR valve is functioning properly and has no carbon deposits under the valve and in the EGR passage in the intake manifold. Read our guide on how to fix the P0401 EGR error code and what parts need to be replaced.

The catalytic converter is an emissions control device that converts toxic gasses and pollutants in exhaust gas to less toxic pollutants by a redox reaction (an oxidation and a reduction reaction) in the presence of catalysts. Diagnostic trouble code P0430 and P0420 means catalyst system efficiency is below the threshold. These OBD error codes warn about a problem related to the catalytic converter of your vehicle. Vehicles are equipped with a downstream and upstream oxygen sensors that are used to monitor the efficiency of the catalytic converter by comparing the values given by both sensors. If the efficiency of the converter drops below a certain point, the ECU throws up a P0430 or P0420 code. P0420 indicates a problem with the catalytic converter mounted near engine bank 1, whereas P0430 indicates a problem with the one mounted near engine bank 2.

The catalytic converter can get clogged or damaged due to engine oil burning in the engine, leaking coolant inside the engine, physical wear, damage, etc. However, a damaged or failing oxygen sensor can also record incorrect readings, causing the ECU to show a check engine light followed by a P0430 or P0420 trouble code.

In most cases, this error code shows up because the converter has reached its maximum mileage and has to be replaced. Given the factors the error code depends on, the root cause of these diagnostic trouble codes can be several. Therefore, it is necessary to perform a proper diagnosis procedure to ensure you are replacing the correct part. Read more on how to diagnose a P0430 error code.