P1118
GMManifold Absolute Temperature Circuit Low Input. This means the Engine Control Module (ECM) is detecting a voltage signal from the Manifold Absolute Temperature (MAT) sensor that is lower than what is expected for normal operating conditions. Think of it like a thermometer sending a reading that's unexpectedly cold.
Medium SeverityWhen the ECM sees a P1118 code, it indicates that the Manifold Absolute Temperature sensor circuit is reporting a voltage that's too low, suggesting either the sensor itself, its wiring, or its connection has an issue. This faulty reading can mislead the ECM about engine air density, affecting fuel delivery and potentially causing poor engine performance.
⚠️ Before You Begin (Safety First)
Always remember to disconnect the battery's negative terminal before working on any electrical components to prevent accidental short circuits or electrical shocks. Never work on a hot engine! Allow it to cool down completely before touching components in the engine bay. Wear safety glasses to protect your eyes.
Common Symptoms
- Check Engine Light illuminated on the dashboard. This is the ECM's way of telling you, 'Hey, something's not right here!'
- Rough idle or hesitation during acceleration. If the ECM thinks the air is colder than it actually is, it might add too much fuel, causing a rich condition.
- Decreased fuel economy. Consistently rich fuel mixture burns more fuel than necessary.
- Difficulty starting, especially in cold weather. Incorrect temperature input can throw off cold-start fuel enrichment.
- Engine may run rich, leading to a strong fuel smell. This happens when the ECM is adding more fuel than needed, based on the faulty temperature reading.
Common Causes
- A faulty Manifold Absolute Temperature (MAT) sensor. This is often the prime suspect when a circuit low input is detected because the sensor itself could be failing internally.
- Damaged or corroded wiring in the MAT sensor circuit. Wiring can get chafed, pinched, or corroded over time, leading to a high resistance or a short to ground, which the ECM interprets as a low voltage.
- Poor electrical connection at the MAT sensor or ECM. A loose or corroded terminal can prevent a proper signal from reaching the ECM.
- Short circuit to ground within the MAT sensor wiring. If the wire carrying the sensor signal touches a ground source, it will pull the voltage down, causing the 'low input' reading.
- Faulty Engine Control Module (ECM) (less common). While rare, the ECM itself could have an internal issue processing the sensor signal.
Frequently reported on: 2005 Chevrolet Silverado 1500, 2008 GMC Sierra 2500HD, 2010 Cadillac CTS, 2006 Pontiac Grand Prix, 2007 Saturn Aura
Diagnostic Steps
- Locate the Manifold Absolute Temperature (MAT) sensor. This sensor is typically found in the intake manifold or the air intake tube after the air filter, but before the throttle body. Consult your vehicle's service manual if you're unsure.
- Inspect the MAT sensor's electrical connector. Look for any signs of corrosion, bent pins, or loose connections. Unplug it and re-seat it firmly, ensuring a good connection.
- Examine the wiring harness leading to the MAT sensor for any visible damage. Check for frayed wires, cuts, or areas where the insulation might be worn through, which could indicate a short to ground.
- Test the MAT sensor's resistance using a multimeter. You'll typically find resistance specifications in your vehicle's service manual. A 'low input' often means the sensor's resistance is out of spec or it's shorted internally.
- If the sensor tests bad or you find significant wiring damage, replace the faulty component. Make sure you use the correct replacement part for your specific GM vehicle.
- Clear the P1118 trouble code using an OBD-II scanner after repairs are complete. Then, perform a test drive to confirm the repair and ensure the code does not return.
👨🔧 When to Call a Professional
If after checking the sensor and its connectors, you're still seeing the P1118 code, or if you suspect deeper wiring harness damage or an ECM issue, it's time to consult with a qualified technician. They have the advanced diagnostic tools to trace more complex electrical problems and test the ECM itself.
Commonly Asked Questions
What does 'low input' actually mean for a sensor circuit?
When the ECM says 'low input,' it means it's receiving a voltage signal from the sensor that is below the expected operational range. For a temperature sensor, a low voltage usually correlates to a higher temperature reading, but the 'low input' refers to the electrical signal itself being too weak or absent.
Can I drive with a P1118 code?
You can probably drive with it, but it's not advisable for long. The engine's fuel mixture will likely be off, leading to poor fuel economy, potential emissions test failures, and over time, could damage components like the catalytic converter. It's best to address it promptly.
Is the Manifold Absolute Temperature (MAT) sensor the same as the Intake Air Temperature (IAT) sensor?
Great question! They essentially do the same job of measuring incoming air temperature. In some vehicles, especially older ones or specific GM applications, it might be called a MAT sensor and is integrated into the manifold. In others, it's called an IAT sensor and is often in the air intake tube. Functionally, they're very similar thermistors.
How do I test the MAT sensor with a multimeter?
You'll want to disconnect the sensor and measure its resistance across the two terminals. The resistance value should change significantly with temperature. As the sensor gets warmer (e.g., from your hand or a heat gun), the resistance should decrease. Consult your specific vehicle's repair manual for the exact resistance-to-temperature chart. If it shows infinite resistance or zero resistance, it's likely faulty.
What if replacing the sensor doesn't fix P1118?
If a new sensor doesn't resolve the P1118, then your next step is to meticulously check the wiring. Look for continuity from the sensor connector back to the ECM connector, and check for any shorts to ground or power. This is where a wiring diagram for your specific vehicle becomes incredibly helpful for understanding the circuit.