OBD-II trouble code
P0666: PCM/ECM/TCM Internal Temperature Sensor Circuit
The control module has a thermometer inside it, watching its own temperature, and that circuit has reported a fault. There is no part to buy here — the sensor is built into the module — so the whole job is proving whether the module is really failing before committing to replacing it.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $1,600
- DIY difficulty
- Shop recommended
What does P0666 mean?
Almost every code in a scan tool's list describes something the module is watching out in the vehicle. This one is different: the module is reporting on itself. Inside the housing, on the circuit board, sits a small temperature sensor whose only job is to tell the processor how hot its own electronics are running. P0666 is the parent code of that circuit, set when the module decides the sensor's reading or its wiring is not behaving.
The module cares about its own temperature for two practical reasons. First, self-protection: the output stages that drive injectors, coils, solenoids and transmission valves all dissipate heat, and if the package gets too hot the module can back off those outputs rather than cook itself. Second, compensation. Semiconductor references drift with temperature, so knowing the die temperature lets the module correct measurements it makes elsewhere. A module that has lost that reading has lost both the safety net and the correction.
Now the part that decides how much this costs you. **This sensor is not a serviceable component.** It is inside a sealed module, there is no connector to unplug and no part number to order, so the only repair the code points at directly is replacing the whole module and programming it to the vehicle. That is a four-figure decision made on the strength of a code that names no external part, which is exactly why it deserves a careful diagnosis rather than a fast one.
Before accepting that verdict, three outside causes are worth eliminating, because all three are cheaper and all three can make a healthy module report this fault. Power and ground quality come first: every internal reference the module makes is measured against its own supply and ground, so a corroded ground strap, a loose case-to-body bolt or a tired feed can move readings without a sensor ever failing. Water intrusion is second, and depends on where the module physically lives — cowl-mounted and firewall-mounted units collect drainage, under-seat and inside-cabin units almost never do. Third, and most overlooked, a shorted device the module is driving. A solenoid or injector with a partially shorted winding forces one output stage to dissipate far more heat than it was designed to, and the module correctly reports that it is hot. That is not a module fault at all — it is the module telling you about a fault somewhere else, and replacing it without fixing the short will cook the replacement too.
Work those three first. If supply, ground, sealing and the driven circuits are all sound, the module is the answer and the cost is what it is.
Common causes
- Failed internal temperature sensor on the module circuit board
- Corroded, loose or high-resistance module ground
- Poor or intermittent battery feed to the module
- Water or moisture intrusion into a cowl or firewall-mounted module housing
- A shorted solenoid, injector or coil overheating one of the module's output stages
- Internal solder joint or board-level failure from years of heat cycling
- Corroded terminals in the module's main connector
- Damage from a previous jump-start, welding on the vehicle or reversed polarity
- Software calibration mismatch after a module was replaced or reprogrammed
Symptoms
- Check engine light or transmission warning lamp with P0666 stored
- No drivability complaint at all in many cases
- Other module codes stored alongside it, such as processor or memory faults
- Occasional reduced engine or transmission output where the module derates to protect itself
- Module internal temperature reading missing, frozen or implausible on a scan tool
- The code returning after a battery disconnect and relearn
- Intermittent stalling or no-start in the minority of cases where the module is genuinely failing
Diagnostic steps
- 1.Record every stored code across every module before touching anything. P0666 alongside other internal module faults tells a different story than P0666 alone.
- 2.Read the module's internal temperature value live and see whether it is missing, pegged at a rail, or simply implausible for the conditions.
- 3.Measure the module's battery feed and ground with the key on, then again with the engine running and a load applied. A voltage drop of more than a few tenths on the ground side is a finding, not background noise.
- 4.Clean and retorque the module ground point and the case-to-body fasteners, then retest before condemning anything.
- 5.Locate the module physically and inspect for water staining, corrosion or a failed seal, paying particular attention to cowl drainage on firewall-mounted units.
- 6.Check the module's main connectors for green terminals, pushed-back pins or a failed lock.
- 7.Look for a shorted output device by comparing the resistance of every solenoid, coil or injector the module drives against specification. One low reading explains a genuinely hot module and costs far less than a module.
- 8.Check for a manufacturer software update covering this code before ordering hardware. A calibration fix is not unusual on this family.
- 9.Only after supply, ground, sealing and driven circuits all test clean, quote module replacement and programming.
Repair cost
$100 – $1,600
Diagnosis is $100 to $200, and on this code the diagnosis is genuinely worth paying for because the alternative verdict is expensive. Cleaning and repairing a module ground or feed is $80 to $250. Connector or terminal repair is $110 to $350. Replacing a shorted solenoid or injector that was overheating a driver stage is $150 to $600 depending on the device. A software reflash, where one exists for this code, is $80 to $200. Module replacement with programming and any required immobiliser relearn is $600 to $1,600, and more on vehicles where the module is only available through the dealer network.
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.