DTC 2121

^ Idle switch open, grounded, or shorted to power (circuit voltage always high or always low). The idle switch has battery power at one terminal whenever the ignition is turned on (from fuse #28). If the throttle is in the idle position, the switch is closed and conducts battery power to terminal #28 of the fuel control unit and terminal #7 of the ignition control unit. The control units measure the voltage at their respective terminals. Battery power at these terminals indicate to the ECUs that the throttle is in the idle position (idle switch closed), while no power (circuit voltage low) indicates the "off idle" state. The ECUs confirm, through other sensors, whether or not the engine is idling. If the idle switch indicates closed throttle (circuit voltage "high") or open throttle (circuit voltage low), but other sensors do not confirm that state for more than a certain length of time, then code 2121 is stored. For example, this code may set under conditions such as:

1. Idle switch circuit voltage high (indicates closed throttle).*
AND one or both of the following.
a. Full throttle switch circuit voltage high (indicates full throttle).*
b. Air flow sensor and engine speed signals indicate a load (throttle open).

OR

2. Idle switch circuit voltage low (indicates open throttle).
AND
Engine speed less than 1200 rpm for more than 2 minutes.

*When these two conditions exist simultaneously, a code 2123 will also set.

This code may be caused by:

a. Faulty idle switch.
b. Open circuit between fuse #28 and idle switch.
c. Circuit between idle switch and fuel control unit terminal #28 open, grounded, or shorted to battery voltage.
d. Circuit between terminal #28 of fuel control unit and terminal #7 of ignition control unit open, grounded, or shorted to battery voltage.

NOTE: If the idle switch circuit voltage indicates a closed switch all the time, the system may go into deceleration fuel cut-off as soon as the engine speed reaches a certain rpm. This condition can easily be confused with a low fuel pressure or restricted fuel filter problem. If the vehicle cannot be driven above a certain speed or the engine will not run above approx. 1,200 - 1,500 rpm, check the idle switch circuit.