P1120 — Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input

P1120 — Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input. A fault in the engine cooling and temperature control. Short, gentle driving to a shop is usually possible. Typical causes: faulty coolant temperature sensor, thermostat stuck open or closed, failed cooling fan or fan relay.

For Ford vehicles

throttle position sensor signal lower than min. limit

powertrain (engine/transmission)

Is it safe to drive?

Drive with caution Drive with caution and watch the temperature gauge. A thermostat stuck open mainly costs fuel; any sign of overheating means stop immediately — driving an overheating engine destroys head gaskets and worse.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P1120 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the engine cooling and temperature control for damage, corrosion, chafing or poor contact.
Subsystem check With the engine coolant temperature (ECT) sensor circuit intermittent high voltage/iat - b disconnected, measure signal voltage at the harness connector: a high reading with the component unplugged indicates a short to power in the harness; if it drops to normal, the component itself is faulty.
Subsystem check Watch coolant temperature in live data during warmup — it should climb smoothly to operating temperature.
Subsystem check Check coolant level and feel upper/lower radiator hoses to verify thermostat operation.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P1120 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P1120 (parts and labor, USD, independent shop rates): Replace the thermostat: $150–$400; Replace the coolant temperature sensor: $100–$300; Cooling system repair (pump, fan, radiator): $300–$1,200. Overall, fixing P1120 usually costs between $100 and $1,200. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What does OBD2 code P1120 mean?

Is it safe to drive with P1120?

Drive with caution and watch the temperature gauge. A thermostat stuck open mainly costs fuel; any sign of overheating means stop immediately — driving an overheating engine destroys head gaskets and worse.

How much does it cost to fix P1120?

Replace the thermostat — $150–$400; Replace the coolant temperature sensor — $100–$300; Cooling system repair (pump, fan, radiator) — $300–$1,200. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

How do I diagnose P1120 myself?

Start with a scan: confirm P1120, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the engine coolant temperature (ECT) sensor circuit intermittent high voltage/iat - b, and verify the suspect component against live data before buying parts.

What is the most common cause of P1120?

The most common causes are: faulty coolant temperature sensor, thermostat stuck open or closed, failed cooling fan or fan relay.

P1120 is a powertrain diagnostic trouble code that your vehicle's engine control module logs when it detects abnormal voltage readings from the engine coolant temperature sensor circuit. The full technical description reads "Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input." This code falls under the powertrain category, meaning it affects engine or transmission operation.

Your vehicle relies on the engine coolant temperature sensor to feed real-time data to the ECU about how hot the engine is running. The sensor works by varying its electrical resistance based on coolant temperature—when the engine is cold, resistance is high and voltage is low; as things warm up, resistance drops and voltage climbs. The ECU expects this voltage to stay within a specific window during normal operation. When the voltage spikes above expected levels—often near battery voltage—the ECU interprets this as a fault.

Three common scenarios trigger this. First, the signal wire may be shorted to a power source, forcing voltage high. Second, the ground return path for the sensor may have an open circuit, preventing proper voltage drop. Third, the sensor itself could be internally failed, pulling the circuit high when it should be low. The ECU continuously monitors this circuit while you drive, and if readings stay outside acceptable ranges for the defined number of drive cycles, it stores P1120 and illuminates the check engine light. Freeze frame data captured with the code shows the exact operating conditions when the fault occurred—engine RPM, load, coolant temperature, and vehicle speed—which gives you the best starting point for diagnosis.

Common symptoms of P1120

Check Engine Light illuminated
Temperature gauge reading lower than normal or failing to reach operating temperature
Cooling fans running continuously at high speed
Overheating warnings appearing on the dashboard in severe cases
Noticeable increase in fuel consumption
Insufficient heat output from the cabin heater

What causes P1120?

Signal wire shorted to battery positive voltage
Broken ground return wire for the ECT sensor circuit
Faulty engine coolant temperature sensor internally shorted and pulling signal high
Malfunctioning cooling fan motor or fan control relay
Damaged, corroded, or loose wiring in the sensor circuit
Thermostat stuck open causing excessive coolant circulation
Low coolant level or trapped air pockets in the cooling system

How to diagnose P1120

Connect an OBD2 scanner, verify P1120 is stored, and record the freeze frame data—this snapshot shows engine conditions when the fault triggered and guides your diagnosis.
Check for related trouble codes that often reveal the true root cause. Also search manufacturer technical service bulletins (TSB) for your specific make and model—many P0xxx codes have documented factory fixes or software updates available through the manufacturer site or nhtsa.gov.
Perform a thorough visual inspection of the engine coolant temperature sensor wiring harness and connectors, looking for damaged insulation, corrosion, broken pins, or loose connections.
With the ECT sensor disconnected, measure signal voltage at the harness connector: a high reading confirms a short to power in the wiring; if it drops to normal reference voltage, the sensor itself is failed.
Observe coolant temperature in live data mode during engine warmup—it should climb smoothly to normal operating range without stalling or jumping erratically.
Verify coolant level is adequate and test upper and lower radiator hoses for proper heat progression to confirm thermostat operation.
Clear the code, complete a full drive cycle matching the freeze frame conditions, and verify P1120 does not return.

Repair options and costs

Address the most common and affordable culprits before replacing expensive components:

Replace the thermostat: $150–$400
Replace the engine coolant temperature sensor: $100–$300
Cooling system repair (water pump, fan motor, or radiator): $300–$1,200

Total repair costs for P1120 typically range from $100 to $1,200 depending on which component has failed. An accurate diagnosis before purchasing parts is the smartest approach—the most expensive repair scenario is rarely the first one to investigate. Most shops charge one hour of labor for these jobs, with part costs varying by vehicle make.

Can you drive with P1120?

Drive with caution and keep a close eye on the temperature gauge. If the thermostat is stuck open, the main consequence is usually reduced fuel efficiency because the engine runs colder than intended. However, any sign of overheating—temperature gauge climbing into the red, steam from the hood, or warning lights—means stop immediately and let the engine cool down. Continuing to drive an overheating engine can destroy head gaskets, warp cylinder heads, and cause catastrophic engine damage. Use good judgment: short distances to a repair shop are acceptable if no overheating symptoms are present.