P1118

What Does Code P1118 Mean?

The OBD-II diagnostic trouble code P1118 indicates a condition where the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an abnormally low voltage input from the Manifold Absolute Temperature (MAT) sensor circuit. This “low input” signal is outside the sensor’s expected operational range, and it typically translates to an extremely high reported air temperature reading. The MAT sensor, often a Negative Temperature Coefficient (NTC) thermistor, measures the temperature of the air within the intake manifold, which is crucial for the ECM to accurately calculate air density. This air density information is then used to precisely adjust fuel injection pulse width, ignition timing, and idle air control strategies to optimize combustion efficiency, control emissions, and ensure proper engine performance. When the ECM detects a continuous low voltage signal from the MAT sensor circuit that deviates significantly from plausible values, it registers the P1118 code and may resort to a predetermined substitute value (failsafe mode) for manifold air temperature, which can lead to various drivability issues due to inaccurate air density calculations.

Common Symptoms

• Check Engine Light Illumination: The primary and most direct indicator of the fault.
• Reduced Fuel Economy: The ECM may default to a richer fuel mixture in failsafe mode or miscalculate air density, leading to excessive fuel consumption.
• Rough Idling or Stalling: Inaccurate air temperature data can cause the ECM to mismanage the fuel-air mixture, leading to unstable idle speed or engine stalling.
• Hesitation or Poor Acceleration: Sub-optimal fuel delivery and ignition timing, resulting from incorrect temperature input, can lead to a noticeable lack of power during acceleration.
• Increased Exhaust Emissions: An imbalanced fuel-air mixture can result in incomplete combustion, leading to higher levels of unburned hydrocarbons and other pollutants.
• Difficulty Starting: Particularly noticeable during cold starts, as the ECM uses MAT data for cold-start fuel enrichment strategies.
• Black Smoke from Exhaust: A symptom of an overly rich fuel mixture, which can occur if the ECM interprets the low voltage as an extremely hot manifold temperature and reduces fuel delivery excessively (or if it defaults to a rich mixture in failsafe). Note: While “low input” in a voltage sense often corresponds to high temperature on an NTC thermistor, the fault itself, such as a short to ground, can cause erroneous readings that might lead to unexpected engine behavior beyond just high-temp interpretation.

What Causes the Code P1118?

• Faulty Manifold Absolute Temperature (MAT) Sensor: The most common cause, including an internal short circuit within the thermistor or the sensor housing, or a complete failure of the sensing element.
• Wiring Harness Issues:
  • Short to Ground: The signal wire from the MAT sensor is inadvertently shorted to the vehicle’s chassis ground or a sensor ground, causing the voltage to drop below the ECM’s operational threshold.
  • Open Circuit/High Resistance: While P1118 specifically indicates “low input,” an open circuit in the sensor’s signal return path or an issue with its 5V reference voltage supply can also present as an abnormally low signal voltage to the ECM.
  • Damaged or Corroded Wires: Frayed, pinched, cut, or corroded wiring along the MAT sensor circuit can lead to signal degradation or loss.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the MAT sensor connector or the ECM/PCM connector due to corrosion, bent pins, or insufficient tension.
• ECM/PCM Failure: Though rare, an internal fault within the ECM/PCM affecting its input monitoring circuit for the MAT sensor can lead to an erroneous P1118 code.

How to Diagnose and Troubleshoot

A systematic diagnostic approach using a professional OBD-II scanner and a Digital Multimeter (DMM) is essential:

1. Verify the Code and Freeze Frame Data: Connect an OBD-II scanner to the vehicle’s DLC. Confirm P1118 is present and review any accompanying freeze frame data. This data provides crucial information about engine operating conditions (engine RPM, load, coolant temperature, etc.) at the precise moment the fault was set. Clear the code and attempt to replicate the fault by driving the vehicle under similar conditions.
2. Visual Inspection:
   • Locate the MAT sensor, which may be integrated into the MAP sensor or IAT sensor depending on the vehicle’s design and year. Inspect the sensor itself for any physical damage, cracks, or signs of impact.
   • Carefully examine the MAT sensor electrical connector and the corresponding wiring harness. Look for signs of chafing, pinching, melting, corrosion on the terminals, or loose connections. Pay close attention to areas where the harness passes near sharp edges or high heat sources.
3. MAT Sensor Circuit Voltage Check (Key On, Engine Off – KOEO):
   • Disconnect the MAT sensor electrical connector.
   • Using a DMM, backprobe or connect the leads to the terminal pins within the wiring harness connector (not the sensor itself).
   • Identify the 5-volt reference (VREF) wire and verify approximately 5VDC.
   • Identify the sensor ground wire and verify a good ground connection (typically less than 0.1VDC to chassis ground).
   • If VREF or ground is absent or incorrect, diagnose the open or short circuit back to the ECM/PCM.
4. MAT Sensor Resistance Test (Sensor Bench Test):
   • Disconnect the MAT sensor and remove it from the intake manifold if possible.
   • Using a DMM set to ohms, measure the resistance across the sensor’s two terminals.
   • Compare this reading to the manufacturer’s specifications for resistance at a known ambient temperature (e.g., 20°C/68°F). A shorted sensor will show very low resistance (approaching 0 ohms), while an open sensor will show infinite resistance.
   • Gently warm the sensor with a heat gun (carefully, do not overheat) and observe if the resistance changes smoothly. For an NTC thermistor, resistance should decrease as temperature increases. Erratic or non-responsive readings indicate an internal sensor fault.
5. Wiring Harness Continuity and Short Test (ECM/PCM Disconnected):
   • With the MAT sensor disconnected and the ECM/PCM main connector disconnected (follow manufacturer procedures for ECM disconnection), perform continuity checks.
   • Use the DMM to check for continuity (low resistance, typically < 0.5 ohms) between the MAT sensor harness connector pins and their corresponding pins at the ECM/PCM connector for both the signal wire, VREF, and ground.
   • Check for shorts to ground: Place one DMM lead on the MAT signal wire pin at the harness connector and the other lead on a known good chassis ground. There should be infinite resistance (open circuit). Repeat for the VREF wire. A reading near 0 ohms indicates a short.
   • Check for shorts to power: With KOEO, backprobe the signal wire at the MAT sensor harness connector. Look for any stray voltage that shouldn’t be there (should be 0V if no sensor is connected).
6. Live Data Monitoring (OBD-II Scanner):
   • With the MAT sensor connected, access the “Manifold Absolute Temperature” or “Intake Air Temperature” (if combined) parameter in live data on the OBD-II scanner.
   • When the engine is cold and off, compare the reported MAT/IAT to the ambient air temperature. They should be relatively close.
   • Start the engine and observe the temperature readings. A P1118 code often corresponds to an extremely high temperature reading on the scanner (e.g., +140°C/+284°F) because low voltage in an NTC thermistor circuit means high resistance, which the ECM interprets as high temperature. If the reading is stuck at an implausibly high value or extremely low (-40°C/-40°F, which typically indicates an open circuit, but can present as low voltage depending on circuit design), or behaves erratically, it points to a faulty sensor or an intermittent wiring issue.

Recommended Repairs and Solutions

Based on the thorough diagnostic procedure, the following repairs are commonly indicated:

• Replace the Manifold Absolute Temperature (MAT) Sensor: If diagnostic tests confirm the MAT sensor itself is faulty (fails resistance tests, provides implausible live data despite good wiring), replacement is the most straightforward solution. Always opt for an OEM-quality sensor to ensure compatibility and long-term reliability.
• Repair or Replace Damaged Wiring Harness: If visual inspection or DMM tests reveal a short to ground, open circuit, high resistance, or damaged insulation in the MAT sensor’s wiring, the affected segment of the harness must be professionally repaired. This involves using appropriate gauge wire, soldering connections, and sealing them with marine-grade heat shrink tubing. For extensive damage, harness replacement may be necessary.
• Clean or Repair Corroded Electrical Connectors: If corrosion or poor terminal tension is identified at either the MAT sensor connector or the ECM/PCM connector, use a specialized electrical contact cleaner and a small wire brush to remove deposits. If pins are severely corroded, bent, or spread, the connector housing or individual pins should be replaced. Apply a thin layer of dielectric grease to the terminals upon reassembly to prevent future corrosion.
• ECM/PCM Replacement: This should be considered a last resort, only after all other potential causes (sensor, wiring, connectors) have been meticulously tested and confirmed to be fully functional. ECM/PCM replacement is a complex and expensive repair, typically requiring specialized programming or flashing to match the vehicle’s specific configuration.

After completing any repair, clear the diagnostic trouble codes using an OBD-II scanner. Perform a comprehensive road test under various driving conditions to ensure the P1118 code does not return and that engine performance (idle quality, acceleration, fuel economy) has returned to normal. Continuously monitor MAT/IAT live data during the test drive to confirm stable and accurate temperature readings.