Why do I get a trouble code or that engine warning light?

Chrysler did a wonderful thing when they started to put computers into their cars back in 1984, they enabled a feature where a technician could call up a stored computer fault code easily and display it's number on the odometer of the vehicle. This eliminated the need to drag out an (at the time) big and bulky machines to read the codes. This only displayed the code, however, it did not allow the tech to actually repair it from here. DIY Mechanics quickly learned of this and have used it to avoid going to the dealer every time the check engine light came on.

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Why do I get a trouble code or that engine warning light?

Postby askdrpt » Thu May 28, 2015 7:21 pm

Your car's computer has sensors to measure air temperature, throttle position sensors, engine speed, gear position, exhaust temperatures, coolant temperature sensors, air flow through the air cleaner, vacuum in the intake manifold, pressure or vacuum in the fuel and evaporative emission system, the barometric pressure and various other sensors. It used a base line "tune" that is programmed into the computers EPROM and used the comparison of these input signal to the tune to make changes in timing, injector pulse width and other outputs to maintain the correct air/fuel ratio. It check all the sensors many times a second. If the computer sees that one of the input sensors is not sending a signal, or that the signal is out if the expected range of voltage, it trips a engine warning light or "code".

The light will typically stay on until the fault is addressed. If it is an intermittent fault (less than a couple of seconds), the light may go off after driving a while. If you get a blinking engine light, it means that the computer has detected a SERIOUS fault and should be addressed immediately! Some faults (like a bad cam sensor) will put the car into limp mode and restrict the car from driving at higher speeds.

You can clear the codes with a hand held diagnostic scanner, or by simply disconnecting the battery cable for a few minutes. Of course, this erases the codes and will prevent a technician from diagnosing your problem if you have not fixed it.

NOTE: If you clear your codes to turn off the light to go through a state inspection, be warned that their scanner will detect a "battery disconnect" and they will not pass the car. It typically takes 225 miles of driving to erase this "battery disconnect" code from the computer.

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