P1594

DTC P1594

CIRCUIT DESCRIPTION
The idle up sensor controls the idle speed during warm-up and it is installed in the driver's side instrument panel. This sensor is active only when the gear position is in the neutral position (parking or neutral position for automatic transmission vehicle). When the shift lever is moved to another position, the signal is ignored. The engine control module (ECM) receives the idle up signal from the idle up sensor and controls the fuel injection quantity. The idle up sensor has following circuits.
- 5 volts reference circuit.
- Low reference circuit.
- Idle up sensor signal circuit.

The ECM monitors the idle up sensor signals for voltages outside the normal range of the idle up sensor. If the ECM detects an excessively high idle up sensor voltage, this DTC will set.

Condition for Running the DTC
- DTCs P0642 and P0643 are not set.
- The battery voltage is between 10-16 volts.
- The ignition switch is ON.

Condition for Setting the DTC
- The ECM detects that the idle up sensor signal voltage is more than 4.8 volts for longer than 1.5 seconds.

Action Taken When the DTC Sets
12,000 lbs GVW
- The ECM illuminates the service vehicle soon (SVS) lamp when the diagnostic runs and fails.
- The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.
- The ECM disables idle up sensor control.

Except 12,000 lbs GVW
- The ECM illuminates the malfunction indicator lamp (MIL) on the second consecutive driving cycle when the diagnostic runs and fails.
- The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Freeze Frame/Failure Records on the second consecutive driving cycle when the diagnostic runs and fails.
- The ECM disables idle up sensor control.

Condition for Clearing the MIL/DTC
12,000 lbs GVW
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic after the ECM turns OFF the SVS lamp.
- Clear the SVS lamp and the DTC with a scan tool.

Except 12,000 lbs GVW
- The ECM turns OFF the MIL when the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic after the ECM turns OFF the MIL.
- Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
If an intermittent condition is suspected, the following may cause an intermittent:
- Poor connections
- Misrouted harness
- Rubbed through wire insulation
- Broken wire inside the insulation

Check for the following conditions:
- Poor connection at the ECM: Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
- Damaged harness: Inspect the wiring harness for damage. If the harness appears to be OK, observe the "Idle Up Sensor" display on the scan tool with ignition ON and engine OFF while moving connectors and the wiring harness related to the sensor. A change in the display will indicate the location of the fault.

DTC P1594
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) Connector End Views

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