GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Epica 🢒 Engine 🢒 Engine Controls - 2.5L (LBK) 🢒 DTC P1668

Circuit Description

The engine control module (ECM) creates the engine coolant temperature gage output signal by pulsing the circuit to ground. The ECM monitors the voltage on the engine coolant temperature gage output circuit. If the ECM determines that the voltage is out of the normal operating range, a DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1668 Engine Coolant Temperature (ECT) Output Circuit

Conditions for Running the DTC
    • The engine is running.
    • The ignition 1 voltage is more than 10 volts.
    • This DTC runs continuously once the above conditions are met.

Conditions for Setting the DTC
    • The ECM detects that the commanded state of the driver and the actual state of the signal circuit do not match.
    • The above condition must be present for a minimum of 25 seconds.

Action Taken When the DTC Sets
    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC
    • 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.
    • Clear the DTC with a scan tool.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2
  1. Start the engine.
  2. Allow the engine to idle for 1 minute.

Does DTC P1668 fail this ignition?

Go to Step 4

Go to Step 3

3
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 4

Go to Intermittent Conditions

4

Are DTCs P0654 and P0656 also set?

Go to Step 7

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the engine control module (ECM).
  3. Install one lead of the J 33431-C Signal Generator and Instrument Panel Tester to the engine coolant temperature gage output signal circuit at the ECM harness connector, and the other lead to a good ground.
  4. Turn ON the ignition, with the engine OFF.
  5. Set the J 33431-C to generate a speedometer signal.

Does the engine coolant temperature gage change?

Go to Step 9

Go to Step 6

6

Test the engine coolant temperature gage output signal circuit for the following conditions:

    • An open
    • A short to ground
    • A short to voltage
    • High resistance

Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 10

7

Test the ignition 1 voltage circuit of the instrument panel cluster (IPC) for the following conditions:

    • An open
    • A short to ground
    • An open fuse
    • A short to voltage
    • High resistance

Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 8

8

Test the ground circuit of the instrument panel cluster (IPC) for an open circuit or for high resistance. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 10

9

Inspect for poor connections at harness connector of the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

10

Inspect for poor connections at the harness connector of the IPC. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

11

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

Go to Step 13

--

12

Replace the IPC. Refer to Instrument Cluster Replacement .

Did you complete the replacement?

Go to Step 13

--

13
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 14

14

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Diagnostic Trouble Code (DTC) List

System OK