P0666
PCM/ECM/TCM Internal Temperature Sensor 'A' CircuitP0666 is a generic OBD-II powertrain diagnostic trouble code: PCM/ECM/TCM Internal Temperature Sensor 'A' Circuit. 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.
What P0666 means
P0666 is a generic powertrain code that refers to a temperature sensor built into the control module itself, not to any sensor on the engine or in the coolant. Modern powertrain, engine and transmission control modules contain one or more internal thermistors so the module can watch its own operating temperature, protect or derate its output drivers if it gets too hot, and temperature-compensate its internal reference circuits. Sensor 'A' is the first of these internal sensors.
The module runs a continuous self-test on this internal circuit. If the reading is missing, sits outside the electrically possible range, or is otherwise implausible for a working circuit, the module stores P0666 and lights the check engine lamp, or the transmission warning lamp when a TCM is the module that reported it. Because the sensor sits inside the sealed module housing and is not separately serviceable, the fault is usually internal: a cracked solder joint, a damaged board trace, moisture ingress, or a failed thermistor.
External conditions can still provoke it. A poor module power feed or ground, low system voltage, a noisy charging system, or the module genuinely running hot in a heat-soaked mounting location can all upset the internal reading. Because a module that cannot trust its own temperature data may limit or shut down output drivers, drivability complaints and shifting faults often accompany the code.
Common causes
Most-frequently reported root causes when P0666 is logged.
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1
Internal failure of the control module's temperature sensor or the board circuitry around it.
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2
Moisture, coolant or road salt intrusion into the module housing corroding the internal board.
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3
Corroded or loose module power supply and ground connections making internal references unstable.
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4
Low system voltage or a failing charging system feeding noisy, fluctuating voltage to the module.
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5
The module genuinely running hot because of a heat-soaked mounting position or a blocked cooling path.
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6
Collateral damage from a previous short circuit on one of the module's output drivers.
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7
A past reverse-polarity jump start, welding on the vehicle, or an overvoltage event that stressed the electronics.
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8
Outdated or corrupted module software with incorrect self-test thresholds.
Symptoms drivers notice
How to diagnose P0666
A typical diagnostic flow when this code is present.
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1
Read all stored codes and freeze-frame data, and note which module (PCM, ECM or TCM) reported the fault.
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2
Check for manufacturer service bulletins or module software updates that cover this code on the specific vehicle.
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3
Inspect the module connectors for corrosion, water tracking, bent or backed-out pins, and any sign of moisture inside the housing.
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4
Test the module's battery feed, ignition feed and ground circuits for voltage drop with the system loaded, and confirm charging system output is clean and steady.
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5
Confirm where the module is mounted and whether anything is blocking its heat path or subjecting it to unusual heat.
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6
Clear the code and record whether it returns immediately, only when hot, or only after a soak, since the pattern separates an internal fault from a supply problem.
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7
Only after power, ground and software have all been verified good should the module be considered for replacement and reprogramming.
Related powertrain codes
- P0600 — Serial Communication Link Malfunction
- P0601 — Internal Control Module Memory Check Sum Error
- P0602 — Control Module Programming Error
- P0603 — Internal Control Module Keep Alive Memory (KAM) Error
- P0604 — Internal Control Module Random Access Memory (RAM) Error
- P0605 — Internal Control Module Read Only Memory (ROM) Error
Frequently asked questions
Can I keep driving with P0666?
Short trips are usually possible if the vehicle still drives normally, but treat it as urgent. A module that cannot read its own temperature may cut back or shut off output drivers without warning, which can mean a sudden loss of power or a stall in traffic.
Can the internal temperature sensor be replaced on its own?
No. It is a component on the module's circuit board inside a sealed housing and is not sold as a service part. The repair is either a board-level repair by a module rebuilder or replacement of the module.
Will this fail an emissions test?
Yes. Any vehicle presented with the malfunction indicator lamp on fails an OBD-II emissions inspection, regardless of how well it drives.
Is this expensive to fix?
It can be. If the cause turns out to be a bad ground, a corroded connector or low charging voltage, the fix is cheap. If the module itself has failed, you are paying for the module plus programming, and on many vehicles the replacement has to be coded to the immobiliser.
Disabling P0666 in software
RaceTune can permanently disable P0666 — 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.
ECU families we can disable P0666 on
We hold the DaVinci A2L disable definitions for these families, so the exact P0666 path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.
- Bosch EDC17C60 verified
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.
Got P0666 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
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