Troubleshooting a Toyota Engine Light

by Don Bowman


The check engine light on a Toyota is an indicator that one of the sensors or systems operated or monitored by the computer is malfunctioning. The computer manages the engine functions and emission controls as well as several other systems. All present day vehicles have the same type of regulatory system employed to operate the function of the drive train.


Since 1973 when the auto manufactures first started to employ emission controls on vehicles, computers, still in their infancy, came on the scene. Their initial duties were to monitor the exhaust emissions and make corrections in the timing and the carburetor mixture to control the emissions. A good understanding of the primary function and control the computer has on the impact of the performance of the vehicle serves to lend predictability to the issue. Computers are common place and becoming more and more advanced with increasing responsibilities in the operation of the vehicle.


The Toyota computer not only operates as an engine management center, but it controls the emissions, the security system, the air conditioning, the suspension, the new Trac systems, the transmission functions and more. It does not however, have anything to do with the internal components of the Toyota engine. The Toyota check engine light does not tell the operator if the engine is low on oil or of a particular malfunction due to wear.

The Toyota has a computer called an Engine Control Management (ECM) computer that controls engine management. There is a Automated Braking System (ABS) computer that operates the function of the brakes, a Body Ride Control that operates a multitude of accessory options such as power windows, locks, seats and so on. There is an air conditioning computer for the use of dual air conditioning, a transmission computer, and some that operate in conjunction with the main ECM computer to control the suspension.


Purchase a computer OBS scanner from a parts store. The OBS scanner is a requirement for diagnosis. It is relatively inexpensive but a necessity. Computers store malfunctions in memory and illuminate the check engine light on the dash. The computer will relinquish these codes in the form of a series of numbers similar to a bar code. To retrieve and understand these codes, you need a scanner. The scanner will come with a code sheet to correspond with the codes presented.

Attach the code scanner to the OBS connector under the driver's side dash. Turn the ignition key on until the lights illuminate on the dash. Do not start the car. The scanner at this time will be displaying read codes. Push the button to read and the scanner will begin to interrogate the computer. It only takes a few seconds and the scanner will respond with a code or codes if more than one. Write these codes down, such as P1425 and look up the codes on the sheet provided for a definition.

Just because the computer generated a code for the oxygen sensor (oxygen sensor malfunction), does not necessarily mean that it is bad and should be replaced. The oxygen sensor measures the amount of oxygen in the exhaust system by producing a very small voltage that the computer sees as an indication as to the fuel mixture being to rich or lean. A multitude of things can cause the oxygen sensor to indicate the wrong mixture. For example a bad spark plug, wire, air leak in the manifold, bad thermostat or bad gas. If the car is running well enough and you see that this code came up, know that no disaster is imminent and continue to drive until it can be repaired or you can check it out yourself. To properly diagnose a code, even with a computer, it helps to read the manual so that you know what each sensor does and what else on the vehicle may cause that sensor to generate a code.

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).