Manifold Absolute Temperature Circuit Low Input
Your engine's manifold air temperature sensor is sending a voltage signal that's too low, like a thermometer stuck at the cold end of the scale. The ECU can't properly adjust fuel and ignition timing because it thinks the intake air is colder than it actually is.
The Manifold Absolute Temperature (MAT) sensor provides a voltage reading that decreases as intake air temperature increases. The ECU monitors this analog voltage signal and expects it to fall within a specific range based on operating conditions. A low input voltage indicates either a sensor failure, wiring issue, or genuine extremely cold air conditions.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| MAT Sensor Voltage | 0.5V - 4.5V (varies with temperature) | Below 0.1V or stuck near 0V |
| Air Temperature Reading | -40°C to 120°C (-40°F to 250°F) | Sensor reads abnormally cold or no signal |
Code P1123 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1123 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.