PCM/ECM/TCM Internal Temperature Sensor Circuit Low
P0668 indicates the PCM/ECM/TCM internal temperature sensor circuit is reading a voltage that is too low. This sensor monitors the operating temperature of the control module itself, and a low signal suggests either a faulty sensor, wiring issue, or internal module problem that needs diagnosis to prevent shifting or starting problems.
What You Might Notice
- Engine will not start or has hard starting
- Engine stalling or hesitation during operation
- Improper or erratic transmission shifting
- Transmission warning light illuminated on dashboard
- Check engine light (MIL) illuminated
Most Common Causes
- 1
Corroded or damaged connector
Corrosion or physical damage to the PCM/ECM/TCM connector pins can interrupt the temperature sensor signal, causing low voltage readings at the control module.
- 2
Loose or defective ground strap
A loose, corroded, or broken ground connection between the control module and engine block affects sensor circuit voltage and signal integrity.
- 3
Faulty or damaged wiring
Broken, pinched, or damaged wires in the temperature sensor circuit can cause intermittent or continuous low voltage signals to the module.
- 4
Defective PCM/ECM/TCM
Internal failure of the temperature sensor component within the control module itself, requiring module replacement or reprogramming.
How to Diagnose It
- 1
Check Corroded or damaged connector — Corrosion or physical damage to the PCM/ECM/TCM connector pins can interrupt the temperature sensor signal, causing low voltage readings at the control module.
- 2
Check Loose or defective ground strap — A loose, corroded, or broken ground connection between the control module and engine block affects sensor circuit voltage and signal integrity.
- 3
Check Faulty or damaged wiring — Broken, pinched, or damaged wires in the temperature sensor circuit can cause intermittent or continuous low voltage signals to the module.
- 4
Check Defective PCM/ECM/TCM — Internal failure of the temperature sensor component within the control module itself, requiring module replacement or reprogramming.
How to Fix It
- 1
Inspect and clean connectors
Locate the PCM/ECM/TCM connector and inspect for corrosion, loose pins, or debris. Clean with electrical contact cleaner and ensure connections are tight and seated fully.
- 2
Check and repair ground connections
Inspect the main ground strap between the control module and engine block for corrosion or looseness. Clean corroded areas with a wire brush and tighten all fasteners, replacing the strap if broken.
- 3
Test and repair wiring harness
Use a multimeter to test continuity and voltage in the temperature sensor circuit wires. Repair any damaged insulation, broken conductors, or loose splices; replace the wiring harness if extensively damaged.
- 4
Replace PCM/ECM/TCM if internal failure confirmed
If all external connections test good but the code persists, the internal temperature sensor within the control module has failed and the module requires replacement or reprogramming by a specialist.
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