P1112 - Intake Air Temperature (IAT) Sensor Circuit Intermittent Low Voltage

Introduction:

The P1112 diagnostic trouble code (DTC) indicates an intermittent low voltage reading in the intake air temperature (IAT) sensor circuit. This sensor plays a crucial role in informing the engine control unit (ECU) about the temperature of the air entering the engine. This information is vital for proper fuel mixture calculation, ignition timing, and overall engine performance. A faulty IAT sensor or circuit can lead to various driveability issues, including poor fuel economy, rough idling, and reduced power.

Comprehensive Table: P1112 - Intake Air Temperature (IAT) Sensor Circuit Intermittent Low Voltage

Detailed Explanations:

IAT Sensor Function:

The Intake Air Temperature (IAT) sensor is a thermistor, meaning its resistance changes with temperature. It's typically located in the intake manifold or air intake duct. The ECU sends a 5-volt reference signal to the IAT sensor. As the air temperature changes, the sensor's resistance changes, which in turn affects the voltage signal returning to the ECU. A lower air temperature results in a higher resistance and a higher voltage signal, while a higher air temperature results in a lower resistance and a lower voltage signal. This voltage reading is essential for the ECU to calculate the correct air-fuel mixture for efficient combustion.

P1112 Definition:

The P1112 code specifically indicates an intermittent low voltage reading from the IAT sensor circuit. This means the ECU has detected that the voltage signal from the IAT sensor dropped below a certain threshold, indicating a potentially incorrect (and low) temperature reading. The "intermittent" aspect signifies that this low voltage condition occurred sporadically, making diagnosis more challenging as the problem may not be present at all times.

Common Symptoms:

While the symptoms can vary depending on the vehicle and the severity of the problem, common symptoms associated with a P1112 code include poor fuel economy, rough idling, hesitation or stumbling during acceleration, and difficulty starting. The engine might also produce black smoke from the exhaust, indicating a rich fuel mixture (too much fuel, not enough air). Because the ECU relies on the IAT sensor for proper fuel mixture calculation, an incorrect reading can disrupt this balance, leading to these symptoms. The check engine light will typically illuminate to alert the driver to the problem. Due to the intermittent nature of the code, the symptoms might not always be present or easily noticeable.

Possible Causes:

The potential causes of a P1112 code are varied and include a faulty IAT sensor itself, corroded or loose electrical connectors at the IAT sensor or ECU, damaged wiring in the IAT sensor circuit (such as an open circuit or short to ground), a faulty ECU (though this is less common), or even an incorrect IAT sensor being installed (one with a resistance value outside of the specified range). An intermittent short to ground in the wiring harness or a voltage drop in the sensor's ground circuit can also trigger this code.

Diagnostic Steps:

Diagnosing a P1112 code requires a systematic approach. First, visually inspect the IAT sensor and its wiring for any obvious signs of damage or corrosion. Next, check the IAT sensor connector for a proper connection and clean it with electrical contact cleaner. Then, test the IAT sensor's resistance using a multimeter at different temperatures and compare the readings to the manufacturer's specifications. Use a multimeter to check the wiring harness for continuity and shorts to ground. Inspect the ECU connector and wiring for damage or corrosion. A crucial step is to monitor IAT sensor data using a scan tool while wiggling the wiring harness to see if the voltage fluctuates, which can help pinpoint intermittent wiring problems. Finally, perform a wiggle test on the IAT sensor connector and wiring, and check the sensor ground circuit voltage drop.

Testing the IAT Sensor:

To test the IAT sensor, disconnect the connector. Use a multimeter set to measure resistance (ohms). Measure the resistance between the two terminals of the sensor. Compare this reading to the manufacturer's specifications for the current ambient temperature. If the reading is significantly different, the sensor is likely faulty. You can also carefully heat the sensor with a hairdryer (avoid overheating) and observe if the resistance changes as expected. The resistance should decrease as the temperature increases.

Wiring Harness Inspection:

A thorough inspection of the wiring harness is essential. Look for any signs of damage, such as cuts, abrasions, or melted insulation. Use a multimeter to check for continuity and shorts to ground in each wire of the IAT sensor circuit. Pay close attention to the ground wire, ensuring it has low resistance. Carefully inspect areas where the wiring harness is routed near sharp edges or hot engine components, as these are common locations for damage. Also, inspect the wiring near the connectors for breaks or corrosion.

Connector Inspection:

Inspect the IAT sensor connector and the ECU connector for any signs of corrosion, bent pins, or loose connections. Clean the connectors thoroughly with electrical contact cleaner and ensure they are properly seated. Use a terminal tool to gently tighten the pins if necessary to ensure they are making good contact. A loose or corroded connector can easily cause intermittent voltage drops, triggering the P1112 code.

Scan Tool Data Monitoring:

Using a scan tool to monitor the IAT sensor data in real-time is a valuable diagnostic technique. Observe the voltage or temperature reading while the engine is running. Wiggle the wiring harness and connectors to see if the reading fluctuates. This can help identify intermittent wiring problems. Also, compare the IAT sensor reading to other temperature sensors, such as the coolant temperature sensor (CTS), to see if they are within a reasonable range of each other. A significant discrepancy could indicate a faulty IAT sensor or a wiring problem.

ECU Testing:

While ECU failures are relatively rare, they can occur. If all other possible causes have been thoroughly ruled out, it may be necessary to test the ECU. This typically involves checking the ECU's power supply and ground connections, as well as its internal circuitry. ECU testing is best left to a qualified technician with specialized equipment and expertise.

Repair Strategies:

The appropriate repair strategy will depend on the root cause of the P1112 code. If the IAT sensor is faulty, it should be replaced. If there is damaged wiring in the IAT sensor circuit, it should be repaired or replaced. Corroded electrical connectors should be cleaned or replaced. As a last resort, if the ECU is found to be faulty, it may need to be repaired or replaced. After any repairs are made, ensure the IAT sensor is properly installed and secured. Clear the DTC and retest the system to ensure the problem is resolved.

Related DTCs:

Several other DTCs can be related to the IAT sensor circuit, including P0110 (IAT Sensor Circuit Malfunction), P0111 (IAT Sensor Circuit Range/Performance), P0113 (IAT Sensor Circuit High Input), and P0114 (IAT Sensor Circuit Intermittent). These codes can provide additional information about the specific nature of the problem and can help narrow down the possible causes.

Tools Required:

Diagnosing and repairing a P1112 code typically requires a multimeter, a scan tool, electrical contact cleaner, a wiring diagram for your specific vehicle, a terminal tool, a socket set, a wrench set, a wire stripper, a crimping tool, and potentially a heat gun (optional, for testing the sensor).

Frequently Asked Questions:

• What does the P1112 code mean? The P1112 code indicates an intermittent low voltage signal from the intake air temperature (IAT) sensor circuit. The ECU has detected that the voltage signal from the IAT sensor dropped below a certain threshold sporadically.

• Can I drive with a P1112 code? It's generally not recommended to drive for extended periods with a P1112 code, as it can lead to poor fuel economy, rough idling, and potential engine damage. Address the issue as soon as possible.

• How do I fix a P1112 code? Fixing a P1112 code involves diagnosing the underlying cause, which could be a faulty IAT sensor, damaged wiring, corroded connectors, or, rarely, a faulty ECU. Follow a systematic diagnostic procedure to identify and repair the problem.

• Is the IAT sensor expensive to replace? The IAT sensor itself is typically not very expensive. However, the cost of labor can vary depending on the location of the sensor and the complexity of the repair.

• Can a bad IAT sensor cause a car to fail an emissions test? Yes, a bad IAT sensor can cause a car to fail an emissions test because it can affect the air-fuel mixture and increase emissions.

Conclusion:

The P1112 code, indicating an intermittent low voltage in the IAT sensor circuit, can lead to driveability issues and reduced fuel economy. A thorough diagnostic process, including visual inspection, component testing, and wiring checks, is essential for identifying and resolving the underlying cause of the problem.