Diagnostic guide: P0112 - IAT Sensor Circuit Low Input Intake Air Temperature Sensor

Overview

• What the code means: P0112 indicates the Intake Air Temperature rather than necessarily indicating a vacuum or air flow issue by itself.
• Where it fits in OBD-II: P-codes are Powertrain Codes used by the engine management and emissions controls. OBD-II codes are standardized as part of modern vehicle diagnostics and Powertrain Codes cover engine management related faults (including sensor circuits). The IAT circuit fault is typically treated as a powertrain sensor circuit issue that the PCM uses to adjust fueling and timing.

Key reference

• The description is reflected in the open-source code definition: Circuito do sensor IAT 1 - Sinal muito baixo. This aligns with P0112 meaning in the standard code set.
• For general context on how DTCs work and how Powertrain codes are organized, see the OBD-II Diagnostic Trouble Codes and Powertrain Codes sections on Wikipedia.

Symptoms

Note: Symptoms can be caused by the core fault (IAT circuit low input) or by related wiring/PCM issues. Typical customer-reported symptoms that technicians often observe include:

• Check Engine/Service Engine Soon light illumination.
• Idle instability or rough idle.
• Hesitation or reduced acceleration response.
• Possible richness/leaness feel or fuel economy changes during warm-up.
• In some cases, no obvious driveability symptoms beyond a stored/pending code.

Important: Symptoms are influenced by ambient temperature, engine load, and other sensor inputs. The IAT is used by the PCM to adjust fueling and air-fuel mixture, so a fault here can alter mixture especially during cold starts or rapid temperature changes.

Probable Causes

• Faulty IAT sensor itself (open, short, internal failure): 40-50%
• IAT sensor wiring harness fault (damaged insulation, broken/Loose wire, connector corrosion, reverse polarity, short to ground): 25-30%
• Connector or harness issues at the IAT sensor (poor connection, corrosion, bent pins): 15-20%
• PCM/ECM input fault (rare but possible, internal fault or communication issue on the IAT input): 5-10%
• Contamination or oil/solvent intrusion on the sensor element (less common, but can cause readings to be erratic): 5% (part of the wiring/sensor fault family)

Note: If multiple faults are present (e.g., sensor plus wiring), combined symptoms may be more pronounced.

What to check (diagnostic plan)

1) Verify DTC and data context

• Confirm P0112 is present, and note whether it's current, pending, or history. The OBD-II framework defines DTC storage as part of the system status (Diagnostic Trouble Codes) and Powertrain Codes are the usual repository for this fault.
• Record freeze-frame data and any available live data related to IAT, intake air temperature, ambient air temperature, engine coolant temperature, and MAF or MAP readings to establish a context for the fault.

2) Visual inspection

• Inspect the IAT sensor and its wiring harness for obvious damage: cracked insulation, melted insulation from heat, oil contamination, or signs of moisture ingress.
• Check connector condition: bent pins, corrosion, loose locking tab, and ensure a clean, snug connection.
• Inspect for electrical issues that could cause a short to ground or an open circuit in the IAT circuit.

3) Electrical checks (with engine off and ignition off)

• Using a multimeter and the service manual wiring diagram for your vehicle, verify:
  • 5V reference circuit to the IAT sensor is present (if your vehicle uses a 5V reference for IAT). Check for a valid reference voltage from the PCM to the sensor.
  • Ground circuit continuity from the IAT sensor to PCM ground.
  • Signal wire continuity from the IAT sensor to the PCM; check for shorts to ground or open circuits.
• If possible, unplug the IAT sensor and measure resistance of the sensor element at room temperature (or according to the service manual's spec) to assess the thermistor behavior. Compare to the expected resistance versus ambient temperature curve in the service information.

4) Functional tests with engine running (live data)

• With a scan tool, monitor IAT sensor readings while:
  • Engine at ambient temperature and not yet started to see the sensor readout when known ambient temp is appropriate.
  • After starting and warming up, observe how the IAT value changes as the intake air warms.
  • Compare IAT reading to ambient temperature and to other engine load data (e.g., MAF, MAP) and to coolant temperature.
• If the IAT reading stays abnormally low (relative to ambient or expected values) or does not respond to deliberate changes in air temperature, the sensor or its wiring is suspect.

5) Sensor replacement decision point

• If the sensor shows an anomalous reading (stuck low, no response to temperature change, or a resistance value outside spec when compared to ambient temperature), replace the IAT sensor and recheck.
• If wiring checks show a short/open or poor continuity in the harness or connectors, repair/replace the harness or connector as needed and re-test.
• If the sensor and harness test OK, but DTC still sets after clearing and testing, consider PCM input integrity or the need for a professional diagnostic with more advanced tooling.

6) Post-repair verification

• Clear DTCs and perform a road test or drive cycle.
• Confirm the IAT reading now tracks ambient temperature and that the PCM no longer logs P0112.
• Verify that driveability symptoms disappear or improve and that there are no additional misfire, hesitation, or fuel trim issues.

How to test and diagnose (step-by-step procedure)

1) Confirm DTC and collect data

• Use the scan tool to confirm P0112 and review freeze-frame data.
• Note ambient air temperature and IAT readings when the engine is off and at various operating conditions.

2) Inspect hardware

• Visually inspect IAT sensor and harness; look for damage, contamination, corrosion, or oil leaks.

3) Power and ground checks

• With ignition on (engine off), verify presence of Reference voltage at the IAT sensor input (per service data) and confirm a solid ground.
• Check for continuity on the signal wire from sensor to PCM; verify there's no short to ground or to voltage.

4) Sensor resistance check (per service data)

• Disconnect sensor and measure the sensor element resistance at known temperatures (as specified by the vehicle's service information). Compare readings with the manufacturer's spec curve or table.
• If readings are out of range or sensor shows no change with temperature, replace the sensor.

5) Reinstall and re-test

• After any repair or replacement, reinstall the harness and sensor, clear codes, and perform a drive cycle.
• Recheck live data to ensure IAT readings track ambient temperature and no P0112 returns.

Safety Considerations

• Always follow vehicle-specific service procedures; disconnect power and allow systems to de-energize before inspecting or handling electrical connectors.
• When probing circuits, use proper back-probe tools and avoid shorting adjacent circuits.
• If the battery is disconnected, ensure electronics are reinitialized as required by the vehicle maker.

Notes and clarifications

• The IAT sensor is a temperature-dependent input used by the PCM to adjust fueling. A low-input signal is interpreted as colder-than-expected air; the PCM uses this to adjust fueling. The exact electrical behavior (voltage vs. resistance) depends on the vehicle's IAT design, but the fault class is an IAT circuit low input, as described in the open-source IAT definition. (OSS code definition)
• OBD-II and Powertrain codes provide the structural framework for diagnosing issues like P0112, while specific sensor behavior and values are vehicle-specific and should be referenced against the service manual for the particular make/model.

Cross-referenced sources

• OBD-II general diagnostics and code structure: Diagnostic Trouble Codes; Powertrain Codes. These sources describe the standardized approach to engine/EMISSION codes and the grouping of powertrain diagnostic codes.
• Specific code description for P0112: IAT sensor circuit low input . This aligns with the standard interpretation that P0112 is an IAT circuit fault.
• Context on how DTCs function within OBD-II and the role of emissions testing in diagnostics: OBD-II sections on Emissions Testing.

Concluding guidance

• Start with the IAT sensor and its wiring harness. Given the relatively high likelihood of sensor or wiring faults in P0112 cases, a systematic electrical check of the sensor, harness, and connector is recommended before replacing PCM-related components.
• If the sensor and wiring test OK, monitor live data to ensure the IAT reading tracks ambient temperature and responds to engine temperature changes.
• Always clear codes after repair and validate with a drive cycle to confirm the fault is resolved and no new issues arise.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

---