P0666

What it means

P0666 is the generic SAE J2012 code for a fault in the internal temperature sensor 'A' circuit inside a control module, usually the engine control module (ECM) or powertrain control module (PCM). The module monitors its own internal temperature to protect its electronics and to adjust some strategies. It sets the code when the sensor signal is implausible, out of range or stuck, or does not fit the expected temperature behaviour.

Symptoms

• Malfunction indicator lamp on
• Other stored codes relating to the control module or its supply
• Intermittent or no-start in some cases
• Reduced performance or limp mode on some vehicles
• Fault that comes and goes with heat or cold, or after a long drive
• No noticeable symptoms other than the warning lamp

Possible causes

• Internal fault in the control module's temperature sensing circuitlikely
• Low or unstable battery or supply voltage upsetting the module's readingspossible
• Poor module earth connections or corroded power supply connectionspossible
• Water ingress, corrosion or heat damage to the module or its connectorpossible
• Outdated or faulty module software where the manufacturer has issued a known fixpossible
• Module overheating due to mounting location, nearby heat source or damaged heat shieldingrare

How to diagnose

1. Read all stored codes and freeze frame data, noting any supply voltage, communication or other module codes, then record and clear the code to see if it returns
2. Check battery condition and charging system output, as low or erratic voltage can cause false internal faults
3. Inspect the module's connectors, pins, earths and supply fuses for corrosion, looseness, moisture or heat damage
4. Check the module casing and its mounting area for water ingress, physical damage or excess heat
5. Check the module's earth and power feeds for voltage drop under load, following the manufacturer's wiring diagram
6. Use a scan tool to view the module's internal temperature value, if the vehicle supports it, and see whether it is implausible, fixed or does not follow ambient temperature
7. Look up manufacturer service information for software updates or known issues for this model and engine
8. Repeat the road test and recheck for the code, once repairs are done, to confirm it does not return

How to fix

• Repair or replace faulty earths, power feeds, fuses or connectors found during testing
• Correct battery or charging system faults so the supply voltage is stable
• Dry out and clean the connector and module if there is minor moisture or corrosion, and fix the source of the water ingress
• Update the module software if the manufacturer lists an outdated calibration as a known cause
• Repair or replace the control module, and have it programmed or coded as the manufacturer requires, if the internal fault is confirmed

Can I keep driving?

Nearby codes

• P0665GenericIntake Manifold Tuning Valve Control Circuit High Bank 2
• P0667GenericControl Module Internal Temperature Sensor A Range/Performance