P1118
HondaManifold Absolute Temperature Circuit Low Input
Medium SeverityThis code tells us that the engine's computer, the ECM, is seeing a voltage signal from the Manifold Absolute Temperature (MAT) sensor that's too low. This low signal indicates a problem with the sensor or its circuit, which means the ECM isn't getting accurate information about the air temperature entering the engine, potentially affecting fuel delivery and engine performance.
⚠️ Before You Begin (Safety First)
Always remember to disconnect the negative battery terminal before working on electrical components to prevent accidental shorts or shocks. When handling sensors, avoid touching the sensing element as oils or contaminants can affect their accuracy. Always allow the engine to cool down completely before working in the engine bay to avoid burns.
Common Symptoms
- Check Engine Light illumination: This is the most direct symptom, as the ECM detected an out-of-range signal.
- Poor fuel economy: The ECM might incorrectly enrich or lean the fuel mixture if it doesn't know the actual air temperature, leading to inefficient burning.
- Rough idle or hesitant acceleration: Incorrect air temperature readings can disrupt the air/fuel ratio, causing the engine to run poorly.
- Increased emissions: An improperly mixed fuel charge will result in higher levels of pollutants being released.
Common Causes
- Faulty Manifold Absolute Temperature (MAT) sensor: The sensor itself could be failing internally, sending an incorrect low voltage signal to the ECM.
- Damaged wiring or connectors: Corrosion, fraying, or a break in the wiring between the MAT sensor and the ECM can cause an open circuit or a short to ground, resulting in a low input.
- Poor electrical connection: A loose or corroded connector at the MAT sensor or ECM can impede the signal flow, leading to a low voltage reading.
- Short to ground in the sensor circuit: This is when the signal wire inadvertently connects to a ground point, pulling the voltage down too low.
- Faulty Engine Control Module (ECM): While less common, an internal issue within the ECM's input circuit for the MAT sensor could cause it to incorrectly interpret the signal as low.
Frequently reported on: 2006-2011 Honda Civic, 2003-2007 Honda Accord, 2002-2006 Honda CR-V, 2005-2010 Honda Odyssey
Diagnostic Steps
- Locate the Manifold Absolute Temperature (MAT) sensor: On most Hondas, this sensor is often integrated into the intake manifold or the intake air tube.
- Inspect the sensor and its wiring: Look for any obvious signs of damage, such as frayed wires, corrosion on the connector, or a physically broken sensor.
- Disconnect the sensor's electrical connector: Press the tab and carefully pull it off.
- Test the MAT sensor resistance: Using a multimeter set to ohms, measure the resistance across the two terminals of the sensor. Compare this reading to your Honda service manual's specifications for various temperatures. A very low or open circuit reading indicates a bad sensor.
- Check the sensor's voltage supply and ground: With the ignition on, measure the voltage between the appropriate power and ground pins at the disconnected connector. You should typically see a reference voltage (e.g., 5V) and a good ground.
- Test for continuity in the signal wire: Ensure there's no open circuit or short to ground in the wire running from the sensor connector back to the ECM.
- If the sensor or wiring tests indicate a fault, replace the MAT sensor: Install the new sensor, ensuring it's properly seated and the electrical connector is firmly attached.
- Clear the P1118 code: Use an OBD-II scanner to clear the diagnostic trouble code from the ECM's memory.
- Perform a test drive: Monitor if the Check Engine Light returns and if the engine's performance has improved.
👨🔧 When to Call a Professional
If after testing the sensor and its wiring you're still getting the P1118 code, or if you're not comfortable using a multimeter for diagnostics, it's best to consult a professional technician. They have specialized diagnostic tools to pinpoint more complex electrical issues within the ECM or harness that might be beyond a simple sensor replacement.
Commonly Asked Questions
What exactly does the MAT sensor do and why is it important for my engine?
The Manifold Absolute Temperature (MAT) sensor measures the temperature of the air entering your engine's intake manifold. This information is crucial for the Engine Control Module (ECM) to calculate the correct amount of fuel to inject. Think of it like a weather report for your engine's air intake; without an accurate report, the ECM can't prepare the right fuel mixture, impacting performance and efficiency.
Can I drive my Honda with a P1118 code?
While you generally can drive your Honda with a P1118 code, it's not advisable for long periods. The engine might run rich or lean due to incorrect air temperature readings, which can lead to reduced fuel economy, poorer performance, and potentially increase wear on components like the catalytic converter over time. It's best to address it promptly to avoid further issues.
Is the MAT sensor the same as the IAT sensor?
That's a great question for an apprentice! The Manifold Absolute Temperature (MAT) sensor and the Intake Air Temperature (IAT) sensor essentially perform the same function: measuring incoming air temperature. However, the MAT sensor is typically located in the intake manifold, measuring air temperature after it's been compressed (if turbocharged/supercharged) or closer to the combustion chambers. The IAT sensor is usually located earlier in the intake tract, like in the air filter box or intake tube. Some vehicles use one or the other, or integrate both functions into a single sensor.
How do I properly test the MAT sensor with a multimeter?
To test the MAT sensor, you'll first disconnect its electrical connector. Then, set your multimeter to the ohms (resistance) scale. Place the meter leads onto the two terminals of the sensor itself. You'll observe a resistance reading that should change with temperature. For instance, as the sensor gets warmer, its resistance should decrease. Compare your readings with the specific resistance-temperature chart found in your Honda's service manual to determine if the sensor is functioning within its expected range. A very high (open circuit) or very low (shorted) reading often indicates a faulty sensor.