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P1120 Manufacturer-specific Basic entry

Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input

Updated 2026-10-02 · Manufacturer-specific powertrain

Severity
High
Typical repair cost
40 – 350 EUR
System
Powertrain

This is a manufacturer-specific code. Its exact meaning can differ between makes; the definition below follows the most common usage.

What it means

P1120 is stored when the engine control module measures a voltage in the Engine Coolant Temperature (ECT) /IAT - B circuit above the expected maximum, typically because the signal wire is shorted to battery voltage or the circuit is open and pulled up internally.

This entry is compiled automatically from the standard definition and typical diagnostic practice for this type of fault. A detailed, reviewed article is in preparation.

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Symptoms

  • Check engine light on
  • Rough or unstable idle
  • Hesitation or poor acceleration
  • Increased fuel consumption
  • Possible limp mode with reduced power

Common causes

  1. Damaged, chafed or corroded wiring in the Engine Coolant Temperature (ECT) /IAT - B circuit
  2. Corroded, loose or water-damaged connector terminals
  3. Failed Engine Coolant Temperature (ECT) /IAT - B component (sensor, solenoid or actuator)
  4. Poor ground connection for the module or component
  5. Faulty control module driver (rare, confirm all else first)

How to diagnose

  1. Read freeze frame data and note when the code set (cold start, under load, after rain)
  2. Inspect the Engine Coolant Temperature (ECT) /IAT - B connector for bent pins, corrosion and water; wiggle the harness with the engine running and watch live data
  3. Measure supply voltage and ground at the connector with the key on
  4. Check the signal wire for shorts to ground or battery and for continuity to the control module
  5. Test or substitute the Engine Coolant Temperature (ECT) /IAT - B only after wiring and connectors are confirmed good
  6. Clear the code and complete a drive cycle to confirm the repair

Frequently asked questions

Can I keep driving with P1120?

Not for long. This code can lead to expensive follow-up damage or affects safety systems; have it checked as soon as possible.

Will clearing P1120 fix it?

No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.

Related codes

P1119

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