My Check Engine Light is On, What Do I Do?

The Check Engine Light (CEL) is never a fun thing to see come on in your dash, but it also might be something easy to fix.  You can check your own CEL codes with only a small piece of wire or paper clip (on OBD cars; for OBD II scroll down to the bottom for what the codes mean), and know right away what your problem is.  This guide was pieced together from a few articles, and should give you a good starting place to find out what is wrong with your Tercel or Paseo.

The first thing you should do is check the computer for any stored codes. Here's how to do it on your OBD Car (1994-95 or older).

EXTRACTING DIAGNOSTIC CODES (OBD):

1. Confirm initial conditions:
   1. Battery voltage is at least 11 volts.
   2. Throttle valve is fully closed.
   3. Transmission is in neutral.
   4. All accessories turned OFF.

2. Connect a service wire across terminals T and E1 of the engine check connector. You can use a small wire or paper clip for this as well.
3. Turn ignition switch ON, but DO NOT start engine.

1. Read the diagnostic code(s) as indicated by the number of flashes of the "CHECK ENGINE" light on the instrument cluster.
2. After codes are extracted, turn-off ignition switch and remove service wire.

DIAGNOSIS INDICATION - If no malfunction codes are stored in the ECU, the "CHECK ENGINE" light will flash ON and OFF two times per second repeatedly. In the event of a stored code, the light will flash every 0.5 second to indicate the first digit of the diagnostic code. This will be followed by a 1.5 second pause and 0.5 second flashes to indicate the second digit of the code.

If there are two or more codes stored in memory, there will be a 2.5 second pause between each code. After all the codes have been displayed, there will be a 4.5 second pause before the codes will be repeated in the same order. This sequence will continue as long as the check connector terminals remain shorted.

In the event that multiple codes are stored, the codes will display in numerical ascending order, with the lowest numbered code displayed first. All codes are retained in the ECU memory, until they are canceled using the proper procedure.

Once we know what the codes are, we can go from there.

Trouble Codes and Possible Causes

While a lot of the possible faults in the EFI system point to the ECU iteself, it should be noted that ECU failure is not common. The ECU should be replaced only as a last resort or if all diagnosises points to a bad ECU. Usually, troublesome EFI problems are a result of poor grounds, shorted wiring, and bad connectons at sensors.When all else fails, carefully consult your factory repair manual. It contains more comprehensive diagnostic information and great electrical wiring diagrams.

For OBD II: Autozone and other car parts stores will check your diagnostic codes for free during normal business hours.  If you own a scanner, the following are Toyota Specific Codes: