P0668

PCM/ECM/TCM Internal Temperature Sensor Circuit Low

P0668 is a generic OBD-II powertrain diagnostic trouble code: PCM/ECM/TCM Internal Temperature Sensor Circuit Low. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0668
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P0668 means

P0668 is set when the PCM detects that its internal temperature sensor circuit voltage is below the minimum expected threshold. The on-chip temperature sensor typically operates on an internal reference voltage, and the PCM monitors the resulting signal voltage against calibrated minimum and maximum limits.

A signal voltage that falls below the low threshold indicates a possible short to ground in the sensor circuit, a failed sensor element, or an internal PCM hardware fault. Because this sensor is located inside the module itself, external wiring is not typically involved, making PCM internal hardware faults or software errors the primary suspects.

This fault may be intermittent if caused by thermal effects on internal components. If the PCM cannot obtain a valid internal temperature reading, it may disable certain compensation strategies or trigger thermal protection modes, potentially affecting performance and emissions control functions.

Common causes

Most-frequently reported root causes when P0668 is logged.

  • 1
    Internal PCM temperature sensor element failed low (short to internal ground).
  • 2
    PCM hardware fault causing the internal sensor reference circuit to collapse.
  • 3
    Corrupted PCM software misinterpreting the internal sensor signal.
  • 4
    Moisture or contamination inside the PCM causing sensor circuit shorting.
  • 5
    Internal PCM connector pin damage causing loss of reference voltage to the sensor.
  • 6
    Thermal stress on internal PCM components causing intermittent sensor signal loss.
  • 7
    PCM power supply fluctuations affecting internal reference voltages.

Symptoms drivers notice

MIL illuminated.
No immediate drivability impact in many cases.
Possible companion internal module fault codes stored.
PCM-dependent compensation strategies may operate on default values.
Intermittent performance issues if thermal protection is incorrectly triggered.

How to diagnose P0668

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all DTCs, paying attention to any other PCM internal fault codes.
  2. 2
    Inspect the PCM main connector for moisture, corrosion, or bent pins.
  3. 3
    Verify PCM supply and ground voltages meet specification.
  4. 4
    Check for any signs of water intrusion in the PCM housing or connector cavity.
  5. 5
    Use live data to observe the internal temperature value if the scan tool supports it, and compare to ambient conditions.
  6. 6
    Perform a PCM software update if a relevant calibration or software patch is available.
  7. 7
    If all external conditions are normal and the fault persists, internal PCM hardware failure is likely and module replacement should be evaluated.

Related powertrain codes

Frequently asked questions

Is P0668 different from P0667?

Yes. P0667 is a range or performance fault, while P0668 specifically indicates the internal temperature sensor circuit voltage is below the minimum expected level.

Can I fix P0668 without replacing the PCM?

In some cases a software reflash resolves it. If the internal sensor hardware has failed, only PCM replacement will fix it.

Does P0668 affect engine performance?

Not usually in a direct or immediate way, but if internal temperature compensation is compromised, subtle calibration shifts may occur over time.

Could a PCM programming error cause P0668?

Yes. Corrupted or incorrect calibration data can cause the PCM to misinterpret its internal sensor signal, triggering this low-circuit code.

Disabling P0668 in software

RaceTune can permanently disable P0668 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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