P0112

What Does Code P0112 Mean?

DTC P0112 signifies an Intake Air Temperature (IAT) Circuit Low Input. This diagnostic trouble code is set by the Engine Control Module (ECM) or Powertrain Control Module (PCM) when it detects that the voltage signal from the Intake Air Temperature sensor is consistently below a calibrated threshold, typically near 0 volts (e.g., less than 0.2V or 0.5V, depending on the manufacturer specification) for a predetermined duration. The IAT sensor is a thermistor, a type of resistor whose resistance changes significantly with temperature. Specifically, it’s a Negative Temperature Coefficient (NTC) thermistor, meaning its resistance decreases as the air temperature increases. The ECM supplies a 5-volt reference voltage to the sensor, and the voltage drop across the thermistor is monitored. A low voltage signal indicates very low resistance within the sensor circuit, which the ECM interprets as an extremely high intake air temperature (often exceeding 250°F or 120°C, regardless of actual ambient conditions). This incorrect temperature reading significantly impacts the ECM’s ability to accurately calculate air density, which is crucial for determining fuel delivery, ignition timing, and transmission shift strategies, leading to potential performance and efficiency issues.

Common Symptoms

• Check Engine Light (MIL) Illumination: The most immediate and common symptom.
• Poor Engine Performance: Reduced horsepower and torque, particularly under acceleration.
• Rough Idling or Stalling: Due to incorrect fuel-air mixture.
• Increased Fuel Consumption: The ECM may mistakenly enrich the fuel mixture, believing the air is denser (colder) than it actually is, leading to a rich condition.
• Hesitation or Misfires: Especially during cold starts or acceleration, as the ignition timing may be retarded incorrectly.
• Hard Starting: Engine may struggle to start, particularly in colder weather, as the ECM cannot properly adjust fuel for actual intake air temperature.
• Black Smoke from Exhaust: In severe cases of overly rich fuel mixture.

What Causes the Code P0112?

• Faulty Intake Air Temperature (IAT) Sensor: The most common cause. The sensor itself may be internally shorted, causing its resistance to be abnormally low, or it may be stuck at a very low resistance value, mimicking an extremely high temperature.
• Wiring Harness Short to Ground: The signal wire from the IAT sensor to the ECM may be chafed or damaged, causing it to short directly to the vehicle’s chassis ground. This will result in a near-zero voltage signal being sent to the ECM.
• Corroded or Damaged Connector: Corrosion or damage within the IAT sensor’s electrical connector can create a low-resistance path or intermittent short, leading to the low input signal.
• ECM/PCM Malfunction: While rare, an internal fault within the ECM/PCM’s IAT monitoring circuit could cause it to incorrectly read the sensor’s voltage, even if the sensor and wiring are functional.

How to Diagnose and Troubleshoot

Diagnosing P0112 requires a systematic approach using an OBD-II scanner, a digital multimeter (DMM), and visual inspection.

1. Initial Scan and Freeze Frame Data Review: Connect an OBD-II scanner and verify P0112 is present. Check for any related codes. Review freeze frame data to understand engine conditions (RPM, engine load, coolant temp) at the moment the code was set. Pay close attention to the reported IAT value; it will likely be excessively high (e.g., 280°F or 138°C) or stuck at an extreme value.
2. Visual Inspection:
   • Locate the IAT sensor. It may be integrated into the Mass Air Flow (MAF) sensor, located in the air intake duct, or mounted directly in the intake manifold.
   • Inspect the IAT sensor and its electrical connector for visible signs of damage, corrosion, bent pins, or proper seating.
   • Carefully trace the wiring harness leading to the IAT sensor, checking for chafing, cuts, signs of rodent damage, or areas where it might be shorting against a metal component or engine block.
3. Live Data Monitoring (KOEO – Key On, Engine Off):
   • With the engine cold and ambient air temperature stable, use the OBD-II scanner to observe the IAT sensor reading in live data.
   • Compare the IAT reading to the Engine Coolant Temperature (ECT) reading and ambient air temperature. If the IAT is significantly higher (e.g., 250°F / 120°C+) than the other temperature sensors or ambient temperature, it confirms the sensor or circuit is reporting an extremely high temperature. The corresponding voltage will be very low (near 0V).
4. Digital Multimeter (DMM) Testing (KOEO):
   • IAT Sensor Resistance Test: Disconnect the IAT sensor’s electrical connector. Set your DMM to measure ohms. Measure the resistance across the two terminals of the IAT sensor itself. Compare this reading to manufacturer specifications or a known good sensor’s resistance-temperature chart. A significantly lower-than-expected resistance for the current ambient temperature would indicate an internal short or faulty thermistor. As an example, at room temperature (e.g., 70°F / 21°C), a typical IAT sensor might read 2000-3000 ohms. A reading of 100-500 ohms would indicate a likely faulty sensor causing P0112.
   • Harness Voltage and Ground Test: With the IAT sensor disconnected and the ignition key in the “ON” position (KOEO), set your DMM to measure DC volts.
     • Carefully probe the 5-volt reference wire at the harness connector (refer to a wiring diagram for pin identification) and chassis ground. You should read approximately 5 volts. If not, investigate the 5V reference circuit from the ECM.
     • Next, probe the ground wire at the harness connector and chassis ground. You should read approximately 0 volts. Then, set your DMM to measure resistance (ohms) and check for continuity between the ground wire terminal at the connector and a known good chassis ground point. Resistance should be less than 5 ohms. If resistance is high or open, repair the ground circuit.
     • Finally, with the DMM still in DC volts, probe the signal wire at the harness connector (the wire that goes directly to the ECM) and chassis ground. This is critical for P0112. If the signal wire is shorted to ground, you will read near 0 volts here.
   • Signal Wire Short to Ground Test: Disconnect both the IAT sensor and the ECM/PCM connectors (if accessible and necessary per diagnostic procedure). Set your DMM to measure ohms. Place one lead on the IAT signal wire terminal at the harness connector and the other lead on a known good chassis ground. If you read very low resistance (near 0 ohms), it confirms a short to ground in the signal wire between the IAT connector and the ECM. If the resistance is infinite (open circuit), the wiring is likely good.
5. Wiggle Test: While monitoring live data with the scanner (IAT voltage/temp) or performing resistance checks with the DMM, gently wiggle the wiring harness and connectors associated with the IAT sensor. Observe if there are any sudden changes in readings, which would indicate an intermittent connection or short.

Recommended Repairs and Solutions

Based on the diagnostic findings, the appropriate repair typically involves:

• Replace the Intake Air Temperature (IAT) Sensor: If DMM tests indicate the sensor itself has abnormally low resistance or is internally shorted, replacing the IAT sensor is the most common and effective solution. Ensure the replacement sensor is an OEM equivalent or high-quality aftermarket part specific to the vehicle’s make and model.
• Repair or Replace Wiring Harness: If diagnostic tests confirm a short to ground in the IAT sensor’s signal wire, the damaged section of the wiring harness must be repaired. This often involves carefully cutting out the compromised section and soldering in a new piece of wire, protected by heat-shrink tubing. Alternatively, replace the entire segment of the harness if damage is extensive or multiple wires are affected.
• Clean or Repair Connector: If the IAT sensor connector is corroded, attempt to clean the terminals using electrical contact cleaner and a small brush. If pins are bent or the connector housing is cracked, replace the connector.
• ECM/PCM Replacement: Only consider ECM/PCM replacement as a last resort, and only after absolutely ruling out the IAT sensor, its wiring, and connector as the source of the problem. This is a costly repair and should be performed by a qualified technician, often requiring reprogramming.

Mechanic’s Tips:

• Always consult the vehicle-specific wiring diagrams and service information for accurate pinouts, voltage specifications, and sensor resistance values.
• When replacing an IAT sensor integrated into a MAF sensor, ensure the entire MAF sensor assembly is replaced with a genuine or high-quality equivalent, as the IAT component is often not separately serviceable.
• After completing repairs, clear the DTCs from the ECM/PCM memory. Perform a test drive, observing live data for the IAT sensor to confirm it is now reporting accurate temperatures and that the P0112 code does not return.
• Be cautious when probing electrical connectors to avoid spreading terminals, which can lead to future intermittent connection issues. Use specialized thin-tip probes if available.