P1118
VWManifold Absolute Temperature Circuit Low Input indicates that the Engine Control Module (ECM) has detected a voltage signal from the Manifold Absolute Temperature (MAT) sensor circuit that is consistently lower than the expected range, suggesting an open or short circuit condition.
Medium SeverityThis code means your engine's computer isn't getting a proper temperature reading from the Manifold Absolute Temperature (MAT) sensor, which measures the air temperature going into the engine. This incorrect information can mess with your engine's fuel mixture and overall performance.
⚠️ Before You Begin (Safety First)
Always make sure the engine is cool before working on any components, especially those related to the intake system. Disconnect the battery's negative terminal to prevent accidental shorts or electrical shocks during inspection or replacement. Use appropriate eye protection and gloves.
Common Symptoms
- Check Engine Light illuminates on the dashboard
- Rough idle or poor engine performance
- Decreased fuel economy
- Engine may run rich or lean
- Hesitation or stumble during acceleration
- Difficulty starting the engine, especially in cold weather
Common Causes
- Faulty Manifold Absolute Temperature (MAT) sensor itself, providing an inaccurate or no signal. This is the most common cause because sensors can simply wear out or fail internally over time.
- Damaged wiring or connectors in the MAT sensor circuit. A low input usually points to a short to ground or an open circuit in the signal wire. This could be due to chafing, corrosion, or impact damage.
- Corroded electrical terminals at the MAT sensor connector or within the wiring harness, leading to poor electrical contact.
- An internal issue with the Engine Control Module (ECM), though this is much less common than sensor or wiring problems.
Frequently reported on: Volkswagen Jetta (2000-2005), Volkswagen Golf (1999-2006), Volkswagen Passat (1998-2005), Audi A4 (2000-2006), Audi TT (2000-2006)
Diagnostic Steps
- Start by locating your Manifold Absolute Temperature (MAT) sensor. On many VW models, it's often integrated with the Manifold Absolute Pressure (MAP) sensor or located in the intake manifold or air intake tube after the air filter box.
- Once you've found it, carefully inspect the electrical connector and wiring harness leading to the MAT sensor. Look for any signs of damage, such as frayed wires, exposed copper, or corrosion on the pins within the connector. The goal here is to identify obvious physical damage.
- Disconnect the electrical connector from the MAT sensor. With the key off, use a multimeter to check for proper voltage supply and ground at the connector terminals, following your vehicle's specific wiring diagram. A typical sensor might receive a 5-volt reference signal. A low input code often means the sensor isn't pulling the signal line up properly or the signal wire is shorted to ground.
- Next, you'll want to test the resistance of the MAT sensor itself if you can access its terminals. You're looking for an open circuit (infinite resistance) or a short circuit (very low resistance) that doesn't change with temperature, which would indicate a bad sensor. Refer to your service manual for the correct resistance values at various temperatures.
- If the sensor tests bad or the wiring appears compromised, the next step is to replace the MAT sensor or repair the damaged wiring/connector. Make sure to use an OEM equivalent part to ensure proper function.
- After replacement or repair, securely reconnect all electrical connectors. Reconnect the battery. Then, use an OBD-II scan tool to clear the P1118 code from the ECM's memory.
- Finally, perform a test drive to ensure the code does not return and that engine performance has improved. This helps confirm the repair was successful and that the ECM is now receiving accurate temperature data.
👨🔧 When to Call a Professional
If after following these steps the code persists, or if you're uncomfortable checking electrical circuits, it's best to consult a qualified Volkswagen mechanic. They have specialized diagnostic tools and knowledge to pinpoint more complex electrical issues within the ECM or wiring harness that might be beyond a simple sensor replacement.
Commonly Asked Questions
What exactly does the MAT sensor do?
The Manifold Absolute Temperature (MAT) sensor measures the temperature of the air entering your engine. This information is crucial for the Engine Control Module (ECM) to calculate the correct amount of fuel to inject, ensuring optimal combustion and emissions. Think of it like telling the computer how 'dense' the air is.
Can I drive with a P1118 code?
While you might be able to drive your vehicle with this code, it's not recommended for extended periods. The incorrect air temperature reading can lead to an improper fuel mixture, causing issues like reduced fuel economy, increased emissions, rough idling, and potentially long-term engine damage if the engine runs too rich or too lean for too long.
Is the MAT sensor the same as the IAT sensor?
Good question! Yes, for many vehicles, the Manifold Absolute Temperature (MAT) sensor and the Intake Air Temperature (IAT) sensor perform the same function: measuring incoming air temperature. The naming often depends on its location or integration. For this code, P1118, we're talking about the sensor that reports air temperature after the air has entered the manifold.
How do I test the MAT sensor with a multimeter?
To test the MAT sensor, you'd typically disconnect it and measure its resistance across its two terminals while it's at ambient temperature. You can then gently heat it (e.g., with a hairdryer, carefully!) and observe if the resistance changes smoothly. A higher temperature should result in lower resistance for most NTC (Negative Temperature Coefficient) thermistors used in these sensors. Always compare your readings to the manufacturer's specifications found in a service manual.
Why is the P1118 code 'low input'?
A 'low input' code usually means the voltage signal the ECM is receiving from the sensor circuit is lower than expected. In many cases, this happens when the sensor or its wiring is shorted to ground, or when the sensor itself has failed in a way that causes it to read extreme cold (which would result in a higher resistance and potentially a lower voltage signal back to the ECM on a pull-up circuit, depending on design). It indicates the circuit isn't behaving as the ECM anticipates for a healthy sensor at normal operating conditions.