What Does P1120 Mean?
P1120 indicates the Engine Coolant Temperature (ECT) sensor circuit is producing an intermittent high voltage, or the secondary IAT-B circuit has a high input. This is functionally similar to P1115. The intermittent high voltage makes the ECM think the engine is colder than reality, causing excessive fuel enrichment and delayed cooling fan activation. This manufacturer-specific variant may also apply to a secondary IAT sensor circuit.
Common Causes
30%
ECT sensor connector intermittently losing contact causing open circuit
25%
ECT sensor ground wire with intermittent high resistance or open
20%
ECT sensor failing intermittently with high internal resistance
15%
IAT-B sensor circuit open intermittently (if applicable)
10%
ECM connector back-out at the ECT input pin
Diagnostic Steps
1
Step 1: Monitor ECT sensor reading during a full warm-up cycle. If it intermittently drops to -40°F or -40°C, the circuit is going open, causing high voltage on the signal line.
2
Step 2: Perform a connector wiggle test at the ECT sensor while monitoring the temperature PID. A sudden drop to very cold readings during the test identifies the fault point.
3
Step 3: Measure ECT sensor resistance at operating temperature (approximately 200°F). It should read 200–300 ohms steadily. If it jumps to infinity intermittently, the sensor is failing.
4
Step 4: Check the ECM connector for the ECT signal and ground pins. Look for backed-out pins, corrosion, or loose terminal tension.
5
Step 5: Replace the ECT sensor and verify the connector terminal tension is adequate. Apply dielectric grease to prevent future corrosion. Clear codes and confirm stable readings.
Estimated Repair Cost
$30 - $200
Parts + labor, varies by vehicle and location
P1120 indicates the Engine Coolant Temperature (ECT) sensor circuit is producing an intermittent high voltage, or the secondary IAT-B circuit has a high input. This is functionally similar to P1115. The intermittent high voltage makes the ECM think the engine is colder than reality, causing excessiv...
The most common cause of P1120 (Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input) is: ECT sensor connector intermittently losing contact causing open circuit
Typical repair costs for P1120 range from $30 to $200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A false cold reading can prevent cooling fan activation, leading to engine overheating. Excessive fuel enrichment from perceived cold conditions can also overheat catalytic converters, presenting a potential fire risk under extreme circumstances.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1120 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Management / Cooling System Monitoring
Difficulty
Type
Manufacturer
Recommended Tools
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