OBD-II trouble code
P0668: PCM/ECM/TCM Internal Temperature Sensor Circuit Low
The module's internal temperature signal has fallen to the bottom of its range. Which module set it changes everything: on an engine controller this is a monitoring fault, but on a transmission controller sitting in fluid the same number arrives with shift complaints and a very different bill.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $3,500
- DIY difficulty
- Shop recommended
What does P0668 mean?
P0668 sets when the internal temperature signal sits at or below the low end of the range the module considers possible. Depending on how the circuit is wired that corresponds either to an implausibly cold reading or to a shorted line, but the electrical detail matters much less than a question the code name buries in a slash: which module is reporting?
The title covers the engine controller, the powertrain controller and the transmission controller, and those are not interchangeable situations. On an engine or powertrain module the internal temperature is housekeeping. It protects the output stages and trims internal references, and losing it usually costs the driver nothing they can feel.
On a transmission controller the same reading is doing real work, and on many modern vehicles it is doing it from inside the gearbox. A mechatronic unit — a control module bolted to the valve body, submerged in transmission fluid — has no meaningful distinction between its own temperature and the fluid around it. That temperature is used to schedule line pressure, decide when the torque converter clutch may lock, adapt shift timing, and protect the fluid from being worked hard while it is still cold. A module that believes it is far colder than it really is will hold the transmission in a warm-up strategy indefinitely: firm or delayed shifts that never soften, lockup that never engages, and fuel economy that drops because the converter is slipping when it should be locked.
That gives this code a symptom set nobody would predict from its name. The customer complaint is usually about the way the vehicle shifts, not about a warning light, and the transmission gets suspected long before anyone reads the module code that explains it.
It also changes the cost picture completely. An engine module is bolted somewhere accessible and replacing it is a connector, some fasteners and a programming session. A mechatronic unit means dropping the pan, draining and refilling with the correct fluid, replacing a filter and seals, adaptations relearned afterwards, and on some transmissions the valve body has to come out as an assembly. The same four-character code can mean a few hundred dollars on one vehicle and several thousand on another, and knowing which one you have before agreeing to anything is the most valuable thing on this page.
One cause worth checking before any of that: on transmission-integrated modules, the internal harness that runs from the case connector to the module lives in hot fluid for its whole life. Its sleeving hardens, its terminals corrode where fluid has been contaminated by a failing cooler or by moisture, and that harness is a far cheaper repair than the module it feeds.
Common causes
- Internal temperature sensor circuit shorted low on the module board
- Internal transmission harness with corroded or damaged terminals inside the case
- Contaminated or overheated transmission fluid attacking the internal connector
- Moisture intrusion into the module housing or case connector
- High-resistance module ground pulling internal references down
- Failed module board component after prolonged heat cycling
- Damaged case-to-module connector seal allowing fluid to wick into the harness
- Previous overheating event that damaged the module's temperature circuit
- Incorrect or incomplete programming after a module was replaced
Symptoms
- Firm, delayed or flared shifts that never soften as the vehicle warms up
- Torque converter clutch failing to lock up, with higher engine speed at cruise
- Reduced fuel economy from a converter that stays unlocked
- Transmission warning lamp or check engine light with P0668 stored
- Module internal temperature reading pegged at an implausibly low value
- The transmission staying in a cold-operation strategy on a fully warm engine
- No symptom at all where the code was set by an engine module rather than a transmission module
Diagnostic steps
- 1.Identify which module set the code before anything else. The diagnostic path, the symptoms and the cost all depend on whether this is an engine controller or a transmission controller.
- 2.Read the module internal temperature live alongside coolant temperature on a fully warm vehicle. A value pinned low while everything else is at operating temperature confirms the report.
- 3.On a transmission module, check whether shift quality and torque converter lockup match what the module believes the temperature is. Symptoms that track the false reading confirm the code is the cause rather than a bystander.
- 4.Inspect the transmission case connector and its seal for fluid wicking, corrosion or a hardened grommet before condemning anything inside the case.
- 5.Check the condition and level of the transmission fluid. Burnt or contaminated fluid is both a cause of internal connector damage and a finding that changes the repair plan.
- 6.Measure the module ground and battery feed under load. A high-resistance ground can pull internal readings toward the low rail without any sensor fault.
- 7.On an engine or powertrain module, inspect for water intrusion at the housing and check the main connector terminals for corrosion.
- 8.Check for a manufacturer software update covering this code before ordering hardware.
- 9.If the internal harness inside the transmission is accessible separately from the module, test and repair it first — it is a common failure and a fraction of the cost.
Repair cost
$100 – $3,500
The range on this code is wide for one reason: which module set it. Diagnosis is $100 to $250. Repairing a module ground or feed is $80 to $250. Replacing an internal transmission harness, where it is serviceable on its own, is typically $250 to $700 including fluid and a filter. An engine or powertrain module replaced and programmed is $600 to $1,600. A mechatronic or valve-body-mounted transmission control module is $1,200 to $3,500 once the pan, filter, seals, correct fluid and adaptation relearn are included, and more on transmissions where the valve body must come out as an assembly. Establish which vehicle you have before accepting a quote at either end of that range.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with control module replacement & programming preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.