Diagnostic Guide: OBD-II P1109 - Intermittent Intake Air Temperature IAT Sensor Fault

What This Code Means

• What P1109 generally represents: An intermittent fault related to the intake air temperature sensor input to the PCM. The open-source code definition in Portuguese lists (Intake Air Temperature - intermittent fault), which aligns with the idea of an IAT input issue that can be intermittent. Source indicating this mapping: Open Source definition for P1109.
• Context in OBD-II: P-codes are Powertrain/engine codes monitored by the PCM; OBD-II systems continuously monitor sensors including the IAT to help control air-fuel mixture and engine operation. Source context: Wikipedia - OBD-II, Diagnostic Trouble Codes; Powertrain Codes (general description of how codes are used and monitored).

Symptoms

• Check Engine/Service Engine Soon light is ON or UNSET, with P1109 stored or pending.
• Rough idle or fluctuating idle speed, particularly as engine warms or cools.
• Hesitation, reduced driveability, or poor acceleration at various RPMs.
• Unusual fuel economy changes (lean or rich conditions related to incorrect IAT input).
• Engine may run roughly or stumble during cold-start or warm-up as the PCM misreads intake air temperature.
• Occasional hard starts or extended cranking when ambient temperature rapidly changes.
  Note: These symptoms are commonly associated with IAT input issues or related PCM fuel-trim adjustments and are informed by typical engine-management behavior. Specific symptom intensity is vehicle-dependent.

Safety and basic precautions

• Park on a flat, cool surface; apply parking brake. Let the engine cool if you'll work around hot components.
• When inspecting wiring and connectors, avoid contact with hot parts and avoid shorting the electrical system.
• Follow standard safety practices for electrical testing: disconnect battery only as needed, use insulated tools, and avoid grounding yourself on hot engine surfaces.

Technical Context

• The IAT sensor provides ambient air temperature data to the PCM. The PCM uses this data to calculate air density and help determine fuel delivery, idle control, and sometimes compensation during cold starts.
• Some vehicles integrate the IAT function into the MAF sensor; others have a separate IAT sensor. Either way, the PCM relies on a valid IAT signal for accurate air-fuel calculations.
• A true intermittent fault means the IAT signal may read accurately at times and drift or jump at others, causing corrective fuel trims to swing and potentially illuminate the MIL.

Probable Causes

Note: Specific vehicle make/model can shift these probabilities. The ranges below reflect general field experience for intermittent IAT faults and are not derived from a single data source . If you have access to NHTSA complaints or manufacturer data, you can adjust these accordingly.

• Faulty IAT sensor itself or its wiring/connector (intermittent signal, short/open, corrosion): 45-60%
• Faulty or loose sensor electrical connection (connector, pin corrosion, damaged wiring harness near engine, grounding issues): 15-25%
• Electrical supply/ground issues to the PCM or IAT circuit (faulty 5V reference, poor ground): 5-15%
• PCM/ECU fault or software/calibration-related issue affecting sensor input interpretation: 5-10%
• Air intake system issues that indirectly affect readings (vacuum leaks, unmetered air entering the intake, MAF sensor misreadings in some setups): 5-10%
• Other sensor interaction effects (e.g., MAF or ECT fuel-trim interactions causing abnormal IAT interpretation): 0-5%

Diagnostic Approach

1) Verify the DTC and data

• Confirm P1109 is currently stored or pending with a scan tool.
• Check freeze frame data for the engine air temperature reading, ambient temperature, engine coolant temperature, engine load, and RPM at the moment the code was set. Look for IAT readings that are wildly different from ambient temperature or are unstable.
• If your tool provides live data, compare IAT to ambient air temperature under various conditions (cold start, warm engine, after a drive). A reliable IAT should roughly track ambient temperature with some offset under certain conditions.

2) Inspect the IAT circuit and connections

• Visually inspect the IAT sensor, its connector, and the wiring harness for signs of damage, chafing, heat exposure, oil/debris intrusion, or corrosion.
• Check for a loose or bent connector pins; unplug/plug the connector and reseat firmly.
• If there are multiple temperature sensors in the intake tract or an integrated MAF/IAT arrangement, verify the correct sensor is being read and that there are no cross-wiring issues.

3) Electrical tests (wiring and sensor)

• With the ignition ON (engine OFF), check for the presence of the sensor's reference voltage (often around 5V) on the signal wire and a good ground. Compare to service information for your exact vehicle.
• Inspect for shorts to ground or to 5V on the IAT signal wire.
• If you can access the IAT sensor circuit with a multimeter or oscope, test continuity and resistance per the vehicle's service data; note that resistance vs. temperature values should align with the sensor's spec. Any out-of-range readings or instability suggest a faulty sensor or wiring issue.
• Inspect for intermittent contact; wiggle the wiring near the connector and along the harness while monitoring the IAT signal in the data stream to see if the reading changes.

4) Sensor test and validation

• If the IAT sensor is accessible and tested within spec for resistance/voltage, but the PCM still sees inconsistent readings, consider replacing the sensor.
• If sensor data is out of range but the sensor circuit tests pass, consider PCM input conditioning or software interpretation issues (less common, but possible in older or updated calibrations).

5) Cross-check with related systems

• If the IAT signal is erratic, confirm that the MAF and/or ECT readings are reasonable. Inconsistent readings from multiple sensors can complicate fuel trims.
• Look for other DTCs that might indicate air-leak problems, MAF sensor issues, or ECU faults that can co-exist with P1109.

6) Perform a test drive and data recheck

• After any repair or connection fix, clear the DTCs and perform a cautious road test. Observe the IAT readings in real-world driving across ambient temperature changes and engine loads.
• Ensure the IAT value tracks ambient temperature and stabilizes, and verify that P1109 does not reoccur.

7) Repairs based on findings

• If the IAT sensor is faulty or its connector/wiring shows damage: replace the IAT sensor and repair or replace wiring/connectors as needed. Reconnect, reseal as required, and re-test.
• If wiring/connectors show corrosion or poor contact: repair or replace wiring harness sections, clean or replace connectors, and ensure proper locking and sealing.
• If the PCM or software is suspected: software update or reprogramming may be required per manufacturer service information.
• If there is an external air-leak contributing to inconsistent readings: repair intake leaks, torn hoses, or unmetered air paths.

8) Post-repair verification

• Clear DTCs, perform a complete drive cycle under various temperatures, and verify that P1109 does not reappear.
• Confirm IAT readings are stable and align with ambient temperature; monitor for normal fuel trims and engine performance.

Items to have on hand (tools and references)

• Diagnostic scan tool with live data capability (to view IAT, MAF, ECT, and fuel trim values)
• Multimeter or oscilloscope for inspecting sensor reference voltage, ground, and signal wire continuity
• Vehicle service information for exact IAT sensor type, resistance vs. temperature specs, 5V reference behavior, and wiring diagrams
• Basic hand tools for sensor removal, connector inspection, and wiring repair
• Replacement IAT sensor (if needed)

Documentation

• Exact DTC code(s) and any freeze-frame data
• Observed symptoms and driving conditions when the fault occurred
• What was found in the wiring/sensor inspection and the repair performed
• Parts replaced (sensor, connectors, harness sections) and any software/ECU updates
• Follow-up test results and whether the fault reoccurred during drive cycles

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.

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