This document is an attempt to corral some of the alphabet soup that accompanies automotive terminology; in particular, terms that we are likely to encounter while working on fuel saving projects. If you run across an acronym that should be included here, or one that you can't find, Contact Us.


AFR: Air/Fuel Ratio. Also Air/Fuel Ratio sensor (AFS below).

AFS: Air/Fuel Ratio Sensor. This acronym is commonly shown on a sticker underneath the hood of your vehicle, for when the vehicle is equipped with Wideband-type oxygen sensors. Also shown as "A/FS" on some vehicles.

CEL: Check Engine Light. The engine trouble light, that lights up on your dash when the ECU detects an error condition. See DTC.

COP: Coil On Plug. A type of ignition system that doesn't require spark plug wires, and has a coil mounted directly to each spark plug.

CTS: Coolant Temperature Sensor.

DIS: Distributor-less Ignition Systems. Systems that utilize electronics instead of a distributor to calculate spark timing.

DTC: Diagnostic Trouble Code. The code number that the ECU gives you to help diagnose an error condition. For 1996 and newer vehicles, these can be read by standard OBD-II readers. Older vehicles may have a system for initiating a blink code, whereby the error numbers are read by the number of blinks of the Check Engine Light.

ECM: Engine Control Module. Also Electronic Control Module, or Environmental Control Unit. The computer module that manages oxygen sensors, among olthers. Commonly referred to as the ECU.

ECT: Engine Coolant Temperature sensor. Also called CTS (Coolant Temperature Sensor).

ECU: Engine Control Unit. The vehicle's computer. It manages all of the vehicle's sensor inputs, and adjusts certain aspects such as the air/fuel mixture, based on certain sensor readings. Is often called by other names, but ECU seems to be the most common.

EFI: Electronic Fuel Injection. As opposed to a carburated, or naturally-aspirated engine, the fuel delivery system is managed electronically, through the ECU.

EFIE: Electronic Fuel Injection Enhancer. A device used to modify the signal from an oxygen sensor so that the ECU will essentially lean the fuel mixture, and allow for the integration of HHO gas, or other fuel saving devices that add to the air/fuel mixture. More info on EFIE devices in our EFIE Information article.

EOP: Engine Oil Pressure sensor.

HEGO: Heated Exhaust Gas Oxygen sensor. Another name for an oxygen sensor.

HHO: Only natural that this acronym is in here, right? HHO, or Hydroxy or Oxyhydrogen, is the gas mixture of Hydrogen (H2) and Oxygen (O2) that is produced through the electrolysis process of water. This is the main source of our increased fuel economy from a HHO Generator System. Also called Brown's Gas. For more information, read our How HHO Works article.

IAT: Intake Air Temperature sensor.

MAF: Mass Air Flow. Refers to the sensor that measures the amount of air flowing into the engine.

MAP: Manifold Absolute Pressure. Refers to the sensor that measures the pressure inside the intake manifold. The ECU compares this to the barometric pressure sensor (outside air pressure) to determine the difference between the pressure in the manifold and the outside air.

MAT: Manifold Air Temperature.

MIL: Malfunction Indicator Light. Same as CEL (Check Engine Light). See above.

MMW: Milliliters per Minute per Watt. This is a unit of measure of how efficient a cell is at making hydrogen. It doesn't tell us the total output of a cell. But it does tell us how much HHO is being made per watt of energy expended to make it. It starts out with LPM or liters per minute, which is the usual way HHO output is measured. It multiplies this by 1,000, which converts Liters to Milliliters. Then it divides that number by the number of watts (watts = volts times amps). The result is MMW, or Milliliters per Minute per Watt.

O2S: Oxygen (O2) Sensor.

OBD-II: On-Board Diagnostic system. Prior to 1996 the systems varied by manufacturer. Jan 1, 1996 marked the beginning of a mandated standard for all new vehicles to conform to. This was ODB-II. Now the same reader can plug into any car and read it's information, including engine sensor readings, trouble codes, etc. For more information see this page.

PCM: Power-train Control Module. Another name for the ECU.

PCV: Positive Crankcase Ventilation. Also PCV valve. See this wiki page for more information.

PWM: Pulse Width Modulator. OK, fine. But what the heck is a pulse width modulator? For more information, please see What Is A PWM?

Stoichiometric Ratio: (pronounced stoy'-kee-o-metric) Not an acronym, we know. But it comes up a lot and this seemed like a good place for it. This is the theoretical perfect combustion ratio of 1 part fuel to 14.7 parts air.

TPS: Throttle Position Sensor.

VCM: Vehicle Control Module. Yet another name for the ECU. Aren't acronyms fun?

VSS: Vehicle Speed Sensor.

WOT: Wide Open Throttle sensor.




HHO Terms and Acronyms

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