P0668 — PCM/ECM/TCM Internal Temperature Sensor Circuit Low

OBD2 code P0668 flags a problem with the sensor and actuator circuits (PCM/ECM/TCM internal temperature sensor circuit low). The car is usually drivable while you plan the repair. Common culprits: corroded, loose or damaged connector, control module driver fault, open or shorted wiring in the circuit.

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0668 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the PCM/ECM/TCM internal temperature sensor for damage, corrosion, chafing or poor contact.
Subsystem check Check for supply voltage at the PCM/ECM/TCM internal temperature sensor connector first; then measure resistance from the signal wire to ground — near-zero ohms confirms a short to ground in the harness.
Subsystem check Measure reference voltage, ground and signal continuity at the component connector.
Subsystem check Wiggle-test the harness while watching the signal in live data — dropouts reveal broken wires.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0668 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0668 (parts and labor, USD, independent shop rates): Repair wiring or connectors: $80–$250; Control module repair: $300–$1,000; Replace the faulty sensor or actuator: $150–$500. Overall, fixing P0668 usually costs between $80 and $1,000. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

Will P0668 clear itself?

Sometimes P0668 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

How much does it cost to fix P0668?

Repair wiring or connectors — $80–$250; Control module repair — $300–$1,000; Replace the faulty sensor or actuator — $150–$500. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What are the symptoms of P0668?

Drivers most often notice: the affected system operates intermittently or not at all, Check Engine Light on, possible limp mode depending on the affected component, increased fuel consumption in some cases.

Is it safe to drive with P0668?

In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

What does OBD2 code P0668 mean?

When your scan tool pulls a P0668, the powertrain control module has detected that an internal temperature sensor circuit is reporting voltage that falls below the acceptable lower limit. The formal name for this code is "PCM/ECM/TCM Internal Temperature Sensor Circuit Low," and it falls under the powertrain category since it involves engine or transmission control hardware.

The phrase "circuit low" tells you the signal voltage is sitting near ground potential instead of the expected range. This happens when something pulls the circuit down — typically a short to ground somewhere in the wiring, a break in the insulation allowing a bare wire to contact metal, loss of the supply voltage the sensor needs to operate, or the sensor itself failing in a way that drags the signal low. The module does not trigger this code based on a single anomalous reading. It waits until the fault persists or recurs under the specific conditions programmed into the software, which is why the check engine light stays solid once P0668 sets.

When P0668 appears, the freeze frame data stored alongside it is worth its weight in gold. That snapshot records engine load, RPM, coolant temperature, vehicle speed, and other parameters the moment the fault was detected. Reproducing those exact conditions during your diagnosis tells you whether you're chasing an intermittent problem or something that happens consistently under load or at operating temperature.

Symptoms of P0668

Illuminated check engine light
Intermittent or complete failure of the system the affected sensor monitors
Possible limp-home mode activation depending on which control module is affected
Noticeable increase in fuel consumption on some vehicles
Reduced performance or altered shift behavior if the transmission control module is involved

Common Causes of P0668

Signal wire shorted to ground due to chafed, melted, or cut insulation
Failed PCM, ECM, or TCM internal temperature sensor pulling the circuit low
Broken or corroded supply voltage feed to the internal temperature sensor
Loose, corroded, or physically damaged connector at the sensor or control module
Poor ground connection at the control module or related component
Open circuit or additional shorts elsewhere in the affected wiring harness
Faulty sensor or actuator on the same circuit sharing the low-voltage condition

How to Diagnose P0668

Plug in an OBD2 scanner and confirm P0668 is stored in the active or pending memory. Record the freeze frame data — those engine conditions matter for recreating the failure.
Scan for any other trouble codes. P0668 sometimes accompanies other electrical faults, and those related codes often point you toward the real root cause. Also look up factory TSBs for your specific make, model, and year — the manufacturer may have released a service bulletin or software update that addresses this exact code.
Perform a thorough visual inspection of the wiring harness and connectors tied to the PCM, ECM, or TCM internal temperature sensor. Look for damaged wire insulation, corrosion buildup at the pins, bent or pushed-out terminals, and any signs of the harness rubbing against brackets or bodywork.
Test the circuit electricially. Check for proper supply voltage at the sensor connector. Then measure the resistance from the signal wire to ground — a reading near zero ohms confirms a short to ground in the harness.
Verify reference voltage, ground integrity, and signal continuity at the component itself. Swap to live data view and wiggle the harness while monitoring the sensor reading; any dropouts or erratic values indicate broken wires inside the insulation.
Clear the code, then drive the vehicle through a complete drive cycle that includes the conditions captured in the freeze frame. If P0668 does not return, the fault was likely intermittent.

Repair Options and Costs for P0668

Address the inexpensive and common culprits before spending money on major components.

Wiring harness repair or connector replacement: $80–$250
Control module internal repair or replacement: $300–$1,000
Replacement of the faulty sensor or actuator on the affected circuit: $150–$500

Total Cost Summary

Depending on which part actually failed, total repair costs for P0668 typically fall between $80 and $1,000. Wiring fixes and connector repairs sit at the low end, while control module replacement or internal repair represents the high end. Spending time on accurate diagnosis before purchasing parts almost always costs less than guessing wrong on an expensive module — the priciest scenario is rarely the first thing to check.

Can You Drive with P0668?

In most instances the vehicle remains drivable, though you may notice reduced performance or an auxiliary system that stops working. If the car stalls, fails to start, or triggers warnings related to braking or steering, treat the issue as urgent and have it diagnosed right away.