Automotive Glossary


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Air Fuel Ratio – Refers to the stoichiometric balance of air and fuel combusted within the cylinder. It must be 14.7:1 for optimal power and efficiency. More fuel results in a “rich” mixture, too much air results in a “lean” mixture. Typically this ratio is controlled by the ECU in modern cars, based on information gathered by the air fuel ratio sensor, an oxygen sensor, the MAP sensor or the mass air flow sensor. Problems with the air fuel ratio can result in poor performance, misfires or bad fuel economy.

Air Fuel Ratio (A/F) Sensor – This sensor resides in the exhaust manifold before the catalytic converter and reports to the ECU information about the exhaust gas mixture. The computer then uses this information to adjust fuel delivery to compensate for excessive hydrocarbons (fuel), for example. The functionality is similar to an oxygen sensor, but an A/F sensor is not the same. These sensors typically work only at high temperatures, so they incorporate an electric heating element to expedite the warm up process. Failure of the sensor or the heater circuit will set the check engine light and may experience poor fuel economy, and in worst case scenarios, poor performance.


Catalytic Converter – This is the main component in reducing vehicle emissions on modern vehicles. Most catalytic converters are of the “Three-way” style, which means they are capable of eliminating HCs, CO, and NOx gasses. The converter uses precious metals like palladium and platinum to catalyze chemical reactions in the gasses emitted from your engine during the combustion process. The three-way converter (TWC) operates in such a way as to store excessive oxygen when the vehicle is running lean, then utilizes the oxygen to convert unburned fuel into CO2. It also stores excessive hydrocarbons to increase temperatures and catalyze the NOx emissions. The converter resides in the exhaust and tends to be very expensive to replace due to the precious metal content. They are often stolen from large trucks for resale to salvage.

Clutch – In order to connect the engine to the transmission, a driver uses the clutch. The clutch is a friction disc that grips the spinning mass of the flywheel, which is attached to the engine. Click here to learn more about the clutch.


Electronic Control Unit (ECU) – The main vehicle computer responsible for making calculations for fuel trim and interpreting other signals required for your car’s operation. Also sometimes referred to as the Powertrain Control Module (PCM), Engine Control Module (ECM) or simply “the computer.” Typically speaking, the ECU is one of the last components to fail on a vehicle. Many other shops will condemn the ECU before fully understanding the interactions among all system components. ECUs also have software like your home computer (more accurately, they have firmware like your home computer’s processor), and will periodically require recalibration in the form of an ECU reflash. This procedure is often necessary to correct calculation errors for OBD II emissions monitors.

Engine Air Filter – This filter resides underneath the hood and prevents particulate matter from the ambient air from entering the engine or fouling engine controls like the mass air flow sensor. If it becomes too dirty, it fails to filter properly and the engine internals may suffer premature wear, and the MAF sensor may report incorrect information causing poor driveability.


Fuel Economy – A vehicle’s efficiency is determined by a great number of factors. This extensive article will give you information necessary about fuel economy.


Hydrocarbons – This is a term that refers generally to a family of explosive or combustible gasses and/or liquids consisting of carbon and water molecules. Gasoline, kerosene, propane, etc. are all classified as hydrocarbons. In the automotive world, presence of hydrocarbons in the cooling system indicates a breach between the cylinder head and the engine block. The seal between these two components is called the head gasket and is a major undertaking to repair. Hydrocarbons are naturally present in the exhaust prior to the catalytic converter which then catalyzes the HCs into water vapor and carbon dioxide.


Manifold Absolute Pressure (MAP) Sensor – The sensor responsible for reporting to the ECU the current intake vacuum measurement. The computer can infer engine behavior from this reading. For example, high intake vacuum indicates idle or deceleration, and the computer may choose this time to active the EGR system. Problems with the MAP sensor can result in poor performance, misfires or bad fuel economy.

Mass Air Flow (MAF) Sensor – This sensor is a hot wire situated within the intake boot between the engine air filter and the throttle body. Its job is to report the amount of oxygen entering the engine to the ECU for use in fuel control. Neglected MAF sensors tend to get dirty, and they fail to accurately report the amount of air. As a result, MORE air enters the engine than the computer is adding fuel to anticipate. Typically the fuel trim will be too lean, and the car may have a lack of power. Problems with the MAF sensor may or may not set the check engine light.

Cylinder Misfire

Emissions Monitors –


Oil Change – What exactly is changing the oil? It’s a standard procedure of replacing the engine oil filter, draining the engine oil, and refilling. This is necessary because engine oil naturally becomes contaminated as combustion chamber gasses blow passed the piston rings. These particulates enter the oil, eventually breaking down the lubricating properties. Likewise, oil leaks passed the control rings, burning up and reducing the volume. Read more about why you need to change your oil every 3000 miles or so.

On-board Diagnostics version 2 (OBD II) –

Oxygen Sensor – This device is located in the exhaust: before and/or after the catalytic converter. Oxygen sensors are designed, as their name implies, to sense the amount of oxygen present in the exhaust gas. This information is transmitted to the ECU, which can make decisions about fuel control and the health of the catalytic converter. Vehicles can be equipped with multiple oxygen sensors; it is not unlikely for a modern V6 motor to have four (4) oxygen sensors! This sometimes confuses people who think they “just had one replaced” when in reality, it was the other bank sensor. Bad oxygen sensors can cause poor fuel economy if they’re upstream of the catalytic converter(s), and they can prevent the catalyst monitor from completing, causing a smog failure.


ECU Reflash


Throttle body –