Ask the Mechanic: 11 reasons the check engine light comes on  

Ask the Mechanic

A warning indicator is among the most common problems you can experience in any vehicle. What are the reasons for the check engine light, and what do you need to do to get rid of it?

Read on to find out.

The check engine light is part of a vehicle’s on board diagnostics system.

Contrary to its name, it doesn’t actively monitor the engine at all. Some engine-related systems are included, like the fuel system and air intake, but for the most part it’s about emissions issues, sensors, and valves and the like.

Major mechanical parts don’t trigger the check engine light in the vast majority of situations. If yours lights up, these 11 systems are the most likely culprits.

Oxygen sensor

An oxygen sensor, or O2 sensor, monitors the amount of oxygen in a vehicle’s exhaust. An element mounted in the exhaust pipe produces voltage from exhaust oxygen levels, and the voltage is indexed to a value in the onboard diagnostic system.

If the value is either higher or lower than the reference, it triggers an error, or diagnostic trouble code. These codes may be one of the reasons the check engine light comes on.

Most cars have two or more oxygen sensors. Upstream O2 sensors or precatalytic converter oxygen sensors regulate the air-fuel mixture, while downstream O2 sensors monitor the catalytic converter’s efficiency.

Fuel cap

This seemingly unimportant part is probably the top reason people ask, “Why is my check engine light on?” Everyone knows what a fuel cap is.

The twist-on plastic cap covering the fuel filler neck. While it prevents dirt and moisture from entering the fuel system, it performs another role almost no one knows, allowing the fuel system to pressurize. Cars perform a self-check for fuel system leaks in the background.

When the system tries to pressurize while the fuel cap is loose, missing or broken, the computer sees a massive leak, triggering the check engine light.

Catalytic converter

When fuel burns in the engine, the chemical reaction creates compounds like nitrous oxides and hydrocarbons that enter the atmosphere, because these harmful chemicals contribute to smog and poor air quality, catalytic converters were implemented in the late 1960s.

Shaped like a muffler, the converter’s interior has a ceramic, honeycomb-like structure covered in reactive elements such as platinum, which triggers a response when exposed to the aforementioned chemicals.

The converter superheats the compounds and breaks them down into harmless molecules, such as carbon dioxide and water, that exit the tailpipe.

Low-quality fuel and worn engines that burn oil tend to contaminate the interior of a catalytic converter, causing it to overheat and break apart.

When the catalytic converter isn’t operating efficiently anymore, a downstream oxygen sensor detects a change and triggers the check engine light.

 Mass air flow sensor

For an efficient burn, the engine’s air-fuel mixture needs to be perfect.

Not only does the amount of fuel need to be metered, but the air taken in also needs to be calculated accurately. The mass air flow sensor performs that role.

Positioned in the air intake duct, the sensor’s element can get dirty. The connector is relatively exposed under the hood, so a slight impact could crack it or damage a wire.

If the mass air flow sensor can’t accurately read the engine’s air volume, you could risk engine damage by running lean (not burning as much fuel as it should) or rich (burning excess fuel), so it triggers a check engine light.

 Spark plugs

Spark plugs ignite the air-fuel mixture that enters the combustion chamber.

Typically screwed into the intake manifold with the electrodes exposed inside the engine, a tiny but powerful spark starts the controlled explosion that produces engine power.

Several things can go wrong with spark plugs. Over time, the electrodes can wear down or the porcelain insulator can crack.

They can be fouled by a flooded engine or oil contamination. They can also overheat if the engine is running too lean. If a spark plug fails to ignite on a consistent basis, it triggers a misfire code in the onboard diagnostic system.

 Ignition coil

The ignition coil delivers the electrical pulse to each spark plug, causing them to spark. The ignition coil has a coil of wire inside that magnifies a small electric charge into a much larger one and stores it momentarily.

When the engine computer sends a signal, the coil releases the pent-up energy to the spark plug, where it ignites the air-fuel mixture. Most cars have one coil per spark plug.

Ignition coils are prone to failure after several years. If the rubber boot breaks down, it can arc to other metal parts and degrade the charge delivered to the spark plug. Or it might not fire at all. Problems include poor fuel economy and decreased engine power, and the check engine light can come on for a misfire code.

Fuel injector

Whether you have a petrol car or a vehicle with a diesel engine, each cylinder has a fuel injector.

It’s a small, electronically activated valve that regulates how much fuel is sprayed into the cylinder during the intake cycle.

The fuel your engine burns tends to have impurities. Those impurities, as well as carbon from the combustion process, can cause the minuscule holes in the injector tip to plug or clog altogether. Occasionally, a fuel injector can be completely clogged, and an injector can stick open and continuously leak fuel into a cylinder.

A faulty fuel injector can cause the engine to run rough because of a misfire either from too little or too much fuel. This might only show up when you’re heavy on the throttle as well.

Vacuum leak

Car systems like the throttle, emissions and power brakes use engine vacuum pressure to assist them. Hoses and valves draw a vacuum from the engine’s intake manifold.

Should one of these hoses break or be dislodged, or if a valve gets stuck open, the onboard diagnostics usually see a leak.

You might notice the engine idling faster, running rough, stalling, or making a whistling sound, and it’s a safe bet the check engine light comes on.

Ignition wires

Some cars deliver the spark from the ignition coil to the spark plug through an insulated cable. Ignition cables, also known as ignition wires or spark plug wires, have a boot on either end to insulate the metal clip from arcing.

Wrapped in rubber, the ignition wires are designed to deliver the spark to the spark plug without degradation.

Potential problems come along with any kind of damage to a spark plug wire. Cracked wire insulation can cause arcing, usually accompanied by visible burns or discoloration on the spark plug wire.

A pinch or impact can sever the wire inside. Any damage causes a misfire because the spark plug can’t ignite.

Evaporative emissions purge control valve

In the emissions control system, fuel can evaporate, but vapours escaping into the atmosphere can be harmful.

An evaporative emissions system canister filters fuel vapours and condenses them back into fuel, but some vapours remain.

The evaporative purge control valve releases those vapours into the combustion chamber to be burnt off rather than letting them go into the air.

An evaporative purge control valve can become plugged, stick open or get stuck shut. The connector can also corrode and lose communication with the valve.

When that happens, the onboard computer triggers an emissions-related error, and the check engine light comes on.

Engine thermostat

A car’s engine runs best between 195 and 220º F. Lower temperatures require more fuel to run properly while higher temperatures create more harmful emissions.

The engine thermostat regulates the coolant temperature. The thermostat contains a wax pellet that controls when it opens and closes.

As the temperature reaches the right zone, the thermostat opens to allow coolant to flow to the radiator. The thermostat closes when it cools below that comfy window.

Thermostats are simple devices but they’re likely to fail at some point.

Poor maintenance can mean calcified deposits get stuck in the thermostat, keeping it open so the engine doesn’t achieve the best temperature.

The thermostat can also close, causing high temperatures or overheating. Both are reasons the check engine light come on.

 Nelson Xavier Ssenyange is the team leader at Germax Auto Spares & Garage Lukade Road, Naalya






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