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Check Engine Light (CEL) basics

Check Engine Light (CEL) basics

Postby chriscarter » Wed Oct 19, 2011 7:24 pm

Hello, boys and girls -

It seems that there are many questions posted here that concern the Check Engine Light (CEL) on our cars. There also seem to be just about as many misconceptions as to why this mysterious light comes on. First, go back to your owner's manual and read what Mr Toyota has written about the CEL and its function in your car. In addition to that, I am offering this very basic explanation of the OnBoard Diagnostic system and the CEL, which is the OBD system's early warning signal.

Due to the fact that late-model Toyotas use distributorless electronic ignition, electronically-controlled multiport fuel injection, and very sophisticated emission control systems, it was necessary to include an OnBoard Diagnostic system (OBD or OBD-II) to constantly monitor the function and performance of each of the components that make up these rather complex engine control systems. The OBD is a sub-function of the Electronic Control Unit (ECU) that is the computer "brain" which keeps all these engine management systems working happily together.

The electronic ignition system includes such parts as coils, spark plugs, camshaft position sensor, crankshaft position sensor, and knock sensor.

The fuel injection sysytem includes the fuel pump, pressure regulator, injectors, throttle position sensor, main air control valve, Mass Air Flow sensor, oxygen sensors, EGR position sensor, and more.

The emission control system includes the EGR (Exhaust Gas Recirculation) valve, catalytic converter, and evaporative emissions components.

The ECU keeps all these components in synch for maximum performance and fuel economy along with minimum exhaust emissions. The OBD function of the ECU assigns a different code to each of the components in each of these engine management systems. There are usually about 50 distinct error codes possible. The operating voltages and amperages for most of these components is very low, sometimes less than 4 volts. When the OBD senses incorrect operating voltages, current flows, or resistance/impedance in these components due to a malfunction, it fires off the CEL. So...when the CEL comes on, it indicates that at least one of these many parts is not operating properly. This is important......the CEL is the general warning of a malfunction, but does not tell which component is at fault! An OBD code reader is required to access the OBD side of the ECU to determine which component(s) is/are at fault and triggered the CEL. In such a sophisticated and complex group of synchronized and closely inter-related systems, it is impossible to determine the exact cause of an engine management problem without reading the error codes.

Some of the larger chain auto parts stores offer free OBD code reading in an effort to have you buy the offending parts there. Not a bad business plan in my opinion. You can also buy OBD code readers of various quality and at various prices if you are a DIYer.

As I said, this is a VERY basic explanation of why your car even has a Check Engine Light. For a more detailed discussion, a service manual is your best source.
Last edited by chriscarter on Thu Oct 20, 2011 7:23 am, edited 3 times in total.
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Re: Check Engine Light (CEL) basics

Postby mig0s » Wed Oct 19, 2011 8:21 pm

^^This guy knows what he's talking about!
1999 V6 5-Speed... so beautiful :' )
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