Diagnostic Aids
WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
- DTC P0128 is designed to detect a faulty thermostat.
- A skewed low ECT sensor can cause a DTC P0128 to set.
- If an intermittent condition is suspected refer to Inducing Intermittent Fault Conditions in Wiring Systems and Intermittent Conditions .
DTC P0128
| Step | Action | Values | Yes | No |
|---|---|---|---|---|
| Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) 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 | Is the cooling system coolant low? | - | Go to Draining and Filling Cooling System (with HP2)Draining and Filling Cooling System (without HP2) in Engine Cooling | Go to Step 3 |
| 3 | Perform the following actions:
Did you find and correct the condition? |
- | Go to Step 14 | Go to Step 4 |
| 4 |
|
- | Go to Step 14 | Go to Step 5 |
| 5 | Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections
, Repairing Connector Terminals
, and Connector Repairs
in Wiring Systems. Did you find and correct the condition? |
- | Go to Step 14 | Go to Step 6 |
| 6 | Measure the resistance of the ECT sensor signal circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing
in Wiring Systems. Is the resistance within the specified range? |
0-10 ohm | Go to Step 7 | Go to Step 10 |
| 7 | Measure the resistance of the ECT sensor low reference circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing
in Wiring Systems. Is the resistance within the specified range? |
0-10 ohm | Go to Step 8 | Go to Step 11 |
| 8 |
IMPORTANT:
Do not hold the ECT sensor by the probe.
|
- | Go to Step 9 | Go to Step 12 |
| 9 | Install the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement
. Is the action complete? |
- | Go to Step 13 | - |
| 10 | Repair the high resistance in the ECT sensor signal circuit. Refer to Wiring Repairs
in Wiring Systems. Did you complete the repair? |
- | Go to Step 14 | - |
| 11 | Repair the high resistance in the ECT sensor low reference circuit. Refer to Wiring Repairs
in Wiring Systems. Did you complete the repair? |
- | Go to Step 14 | - |
| 12 | Replace the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor Replacement
. Did you complete the replacement? |
- | Go to Step 14 | - |
| 13 | Replace the ECM. Refer to Engine Control Module (ECM) Replacement
. Did you complete the replacement? |
- | Go to Step 14 | - |
| 14 |
|
- | Go to Step 2 | Go to Step 15 |
| 15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? |
- | Go to Diagnostic Trouble Code (DTC) List | System OK |