On all L-bodies (Omni/Horizon and related cars), along with all vehicles from model years before 1987 with EGR (nearly all of them), code 34 indicates that the EGR solenoid circuit is open or shorted — as indicated by voltages of 0 volts when the solenoid is on, or voltage of 12V when the solenoid is turned off. The power limited light will not be lit and the system will not enter limp mode.
Code 53 is triggered when there is a fault in the logic module. When code 53 is triggered the power limited will not be lit and the system will not enter limp mode. 25
Code 25 is thrown when the Automatic Idle Speed (AIS) motor circuit is shorted, or the target idle speed is not achieved.
The ECU (engine computer) monitors the voltage on the idle speed motor circuit, which should be near zero with the motor off and near 12 volts with the motor on. If it does not detect the appropriate voltage, it will register fault code 25. Likewise, if the computer repeatedly tries to achieve a desired idle speed and cannot, it will also register code 25. When it does this, it does not turn the Power Limited light or go into limp mode; presumably the programmers figured that the driver would know something was wrong.
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