There are hundreds of possible faults that can cause the Check Engine light
to illuminate on your dash. To find out what the problem is, you would have to have your vehicle scanned for fault codes. A fault code itself only points to a certain system or component. To find the problem, your car will need to be further diagnosed by your local dealer or a qualified mechanic.
To give you a few examples of Check Engine light problems, we have analyzed thousands of repair cases in hundreds of different cars. Although causes are often different, we found several problems that are common in many cars and produce similar symptoms. Here they are:
Vacuum leaks and mass airflow (MAF) sensor issues
Very often, issues with a mass air flow sensor or vacuum leaks can cause the engine to run lean. The term “running lean” means that the engine is not getting enough fuel, or in other words, the air-fuel mixture injected into the engine has more air and less fuel. The symptoms include RPMs fluctuating up and down when stopped, stalling, rough idle, difficulty starting when the engine is hot, lack of power, slow acceleration, etc.
The mass air flow (MAF) sensor measures the amount of air entering the engine. The engine computer adjusts the amount of fuel based on the mass air flow sensor readings. If the sensor doesn’t work properly or if there is a leak that allows the air to enter the engine bypassing the sensor (unmetered air), the engine computer cannot adjust the amount of fuel correctly. Most common Check Engine fault codes related to this problem are P0171 and P0174. Here are a few examples:
If the Mass Air Flow (MAF) sensor is found faulty it must be replaced. In some cars cleaning the sensor may help. Often the MAF sensor malfunctions when the air filter box is not closed properly.
Leaking intake manifold or plenum gaskets are common, especially in cars with plastic intake manifolds. Replacing intake gaskets could be expensive because of the amount of labor involved.
Rips or cracks in the intake boot or vacuum hoses can cause a vacuum leak. For example, some Ford and BMW vehicles are known to have this problem. .
Misfiring
An engine “misfires” when some of its cylinders don’t work. When this happens, the engine would start shaking and run “uneven” like a motorcycle motor. The Check Engine light would start flashing repeatedly or stay on solid on the dash. The car would lack power. Sometimes a misfire happens at a cold start and goes away when the engine is revved. Often the engine misfires during rainy weather or after driving through a puddle or when accelerating from a stop. In most cases, the misfiring is fairly easy to diagnose. A faulty ignition coil is the most common culprit in newer cars. Other causes include vacuum leaks, worn out spark plugs, cracked ignition wires, bad EGR valve, faulty fuel injectors, mechanical problems, etc.
A failed ignition coil often causes misfire in newer cars. To diagnose a faulty ignition coil, your mechanic may try swapping coils between two cylinders and see if the misfiring follows the suspected coil.
If the engine misfires, spark plugs are usually replaced if they are old. The recommended spark plug replacement interval ranges from 30K-50K miles for conventional plugs and 50K-120K miles for iridium or platinum plugs.
Ignition wires can deteriorate over time and can also cause misfiring, especially in wet weather. In cars with a distributor, the distributor cap and rotor are typically replaced with the wires too.
EVAP system leaks
The EVAP system prevents the gasoline vapors in your gas tank from escaping into the atmosphere. It’s a sealed system and if there is a leak, the car’s computer turns the Check Engine light on. Evap leaks rarely cause any driveability symptoms. A loose gas cap is one of the examples of an EVAP system leak. Other common sources of an EVAP system leak include a purge valve and a canister vent valve.
If you found that the gas cap was loose, close it properly and the Check Engine light will reset itself after a day or two of driving.
A sticky purge valve can also cause an EVAP leak. A purge valve can be tested fairly easily and if it sticks, it’s not very expensive to replace. Read more »
A vent valve is normally open. It closes only when the EVAP system tests itself for leaks. If the valve sticks open it can cause a leak. Read more »
EGR system problems
The EGR or Exhaust Gas Recirculation is one of the vehicle’s emission control systems. It reduces the combustion temperature by redirecting a small amount of exhaust gases back into the engine intake. The main component of the EGR system is the EGR valve. Often, carbon deposits and soot accumulated inside the valve preventing it from opening and closing freely. This will cause the Check Engine light to come on. Symptoms of a sticky EGR valve include stalling at idle, rough idle. Sometimes these symptoms happen after exiting the highway. In some cars, when the EGR system is clogged up, the Check Engine light may come on, but no other symptoms would be present. Common fault codes: P0400, P0401.
In some cars with a vacuum-operated EGR valve as the one in the photo, the leaks in one of the vacuum hoses or pipes can stop the EGR valve from working.
In some Ford vehicles, the EGR system problems are caused by a bad DPFE sensor. Replacing the DPFE sensor is not very expensive.
In many cases, to repair the problem, the EGR valve needs to be replaced. In some cars, the EGR passages need to be cleaned or even modified (Honda, Acura)
Oxygen sensor or air/fuel ratio sensor problems
An oxygen sensor is the primary sensor in the fuel injection system. Installed in the vehicle’s exhaust, it provides feedback for the engine computer on how efficiently the engine runs. A more precise air/fuel ratio sensor used in many modern cars for the same purpose is also commonly referred to as an “oxygen sensor”. Oxygen sensor problems are also common. A failed or deteriorated oxygen sensor may cause a drop in the fuel economy and the Check Engine light to come on, but the vehicle may still drive fine. Common causes include a failed oxygen (air fuel ratio) sensor and shorted out sensor wires. Common fault codes: P0130, P0133, P0134, P0135, P0136.
If your car throws a code related to the oxygen sensor, your mechanic may recommend replacing the sensor even if it appears to be working at the time.
An air/fuel ratio sensor is still called an “oxygen sensor”. It work differently, but has the same purpose. This is an air/fuel ratio sensor in Acura NSX
Sometimes oxygen sensor faults may be caused by a damaged sensor wiring. In this Mazda 3, the sensor wires were rubbing against the bracket.
Failed catalytic converter
A catalytic converter is the main emission control device installed in the vehicle’s exhaust. It has a honeycomb-like ceramic structure inside that helps to burn any unburned fuel and reduce other harmful emissions. A failing or clogged up catalytic converter often causes the Check Engine light in many cars and trucks. When a catalytic converter becomes clogged up, an engine may start and run well at idle, but bog down when accelerated. A rattling noise coming from a catalytic converter and a smell of unburned fuel from the exhaust may indicate that the ceramic structure inside the converter has disintegrated. In either case, the catalytic converter will need to be replaced. The most common Check Engine light codes: P0420 and P0430.
The catalytic converter is typically covered by 8-year 80-mile Emission Warranty. If your car is still within the warranty coverage, your dealer should be able to help you.
If your car is not under warranty and the repair quote from your dealer is too expensive, try a local muffler shop, as there are plenty of less expensive aftermarket parts available.
The most common Check Engine light code is P0420 – Catalyst System Efficiency Below Threshold (Bank 1). The code P0430 indicates the same fault for a bank 2 in a V6 or V8 engine.