P1118
ToyotaManifold Absolute Temperature Circuit Low Input
Medium SeverityThis diagnostic trouble code indicates that the Engine Control Module (ECM) is detecting a voltage signal from the Intake Air Temperature (IAT) sensor circuit that is below its specified operational threshold. This low voltage input can lead to inaccurate air density calculations, directly impacting fuel delivery and ignition timing strategies.
⚠️ Before You Begin (Safety First)
Always ensure the ignition is off and the battery is disconnected before performing any electrical diagnostics or component replacement to prevent accidental shorts or injury. Allow the engine to cool completely before working near hot components. Wear appropriate personal protective equipment, including gloves and eye protection.
Common Symptoms
- Malfunction Indicator Lamp (MIL) illumination
- Elevated engine idle speed
- Reduced fuel economy due to incorrect fuel trim adaptation
- Delayed throttle response
- Potential for increased exhaust emissions
Common Causes
- Faulty Intake Air Temperature (IAT) sensor, exhibiting an internal short or incorrect resistance characteristics
- Short circuit to ground within the IAT sensor signal wire or reference circuit
- Open circuit condition in the IAT sensor reference voltage or ground circuit
- Corroded or loose electrical connections at the IAT sensor or ECM harness
- Rarely, an internal ECM fault preventing proper voltage signal interpretation
Frequently reported on: 2005-2010 Toyota Corolla, 2002-2006 Toyota Camry, 2005-2015 Toyota Tacoma, 2003-2009 Toyota 4Runner
Diagnostic Steps
- Using a diagnostic scan tool, review the Intake Air Temperature (IAT) sensor PID data to observe the reported temperature value. A very high or constant value, regardless of actual engine temperature, suggests a fault.
- Locate the IAT sensor, typically situated in the air intake duct or manifold. Visually inspect the sensor and its electrical connector for any visible damage, corrosion, or loose terminals.
- With the ignition off and the sensor disconnected, use a multimeter to measure the resistance across the IAT sensor's terminals. Compare this reading to specified resistance values for your vehicle's ambient temperature.
- With the ignition on (engine off) and the sensor disconnected, measure the voltage between the IAT sensor's signal wire and ground at the harness connector. Expect a reference voltage (typically 5V). Also, check for continuity to ground on the sensor's ground circuit.
- If the sensor resistance is out of specification or voltage/continuity tests indicate circuit issues, replace the Intake Air Temperature sensor. Ensure the new sensor is an OEM equivalent or direct replacement part.
- Clear the P1118 diagnostic trouble code using a scan tool and perform a drive cycle to verify the repair.
👨🔧 When to Call a Professional
If the initial sensor replacement does not resolve the P1118 code, or if the circuit integrity tests show no anomalies, then further investigation into the ECM's input monitoring circuit or potential intermittent wiring faults is warranted. Complex electrical diagnostics or persistent fault conditions should be addressed by a certified professional.
Commonly Asked Questions
What is the typical voltage range for the IAT sensor signal when functioning correctly?
The IAT sensor utilizes a 5V reference circuit. Under normal operating conditions, the signal voltage will vary inversely with temperature. For instance, at 20°C (68°F), the signal might be around 2.5-3.0V, while at 80°C (176°F), it could drop to 1.0-1.5V, and a 'low input' means the voltage is significantly below the expected operational minimum.
How does a low IAT sensor input affect engine performance logic?
A 'low input' voltage is typically interpreted by the ECM as an extremely high intake air temperature. The ECM will then reduce fuel injection duration and retard ignition timing to compensate for the perceived hot, less dense air, potentially leading to lean conditions, reduced power, and increased NOX emissions.
Can a P1118 code be caused by an issue with the Manifold Absolute Pressure (MAP) sensor?
While both sensors provide critical air input data, P1118 specifically targets the Intake Air Temperature circuit. Although a faulty MAP sensor can cause similar drivability issues and related codes, it is distinct from the IAT circuit fault indicated by P1118. However, ensure no vacuum leaks or other issues are affecting both sensors if their wiring runs in close proximity.
Is it possible for a dirty air filter to trigger a P1118 code?
A dirty air filter primarily restricts airflow and doesn't directly cause an electrical 'low input' condition for the IAT sensor. While it can indirectly affect engine performance, it will not directly trigger a P1118. The P1118 code points to an electrical fault within the IAT sensor circuit itself, not an airflow restriction.
What is the primary diagnostic PID to monitor for P1118?
The most relevant Diagnostic Trouble Code (PID) to monitor is 'Intake Air Temperature' (IAT). A functioning sensor should show ambient temperature when the engine is cold and rise as the engine heats up and draws in warmer air. A 'low input' will likely register an abnormally high or fixed temperature value.