CLAAS 777Ah – ENG A15: Input Voltage of the Fresh Air Intake Temperature Low

The Engine Control Unit (ECU) has detected an abnormally low voltage signal from the fresh air intake temperature sensor, which it interprets as an extremely low air temperature. In response, the system may default to a pre-set temperature value, such as 50°C, and deactivate certain engine protection functions related to exhaust gas recirculation (EGR) and intake air temperature. An incorrect signal from this sensor can lead to an improper air-fuel mixture, potentially causing reduced engine performance, increased fuel consumption, and rough idling.

1. 1

   Visually inspect the wiring harness and connector associated with the fresh air intake temperature sensor for any signs of damage, chafing, corrosion, or loose connections.

2. 2

   Disconnect the sensor and inspect the connector terminals for contamination or physical damage.

3. 3

   Check for a short to ground in the wiring: with the sensor disconnected and ignition on, measure voltage between the signal wire and chassis ground (a reading near 0V indicates a short). Alternatively, check for continuity to ground on the signal wire with the ECU disconnected.

4. 4

   Test the sensor's resistance: gently warm the sensor and observe if the resistance changes smoothly. An open circuit (infinite resistance) or a short circuit (zero resistance) indicates a faulty sensor.

1. 1

   Isolate the fault

   Perform the diagnostic checks on the wiring and sensor to determine if the root cause is a wiring short or a failed sensor.

2. 2

   Repair wiring

   If a short or break is found in the wiring harness, repair the affected section. Ensure the repair is properly insulated and protected from future damage.

3. 3

   Replace sensor

   If the wiring is confirmed to be intact but the sensor fails the resistance test, replace the fresh air intake temperature sensor.

4. 4

   Clear fault codes

   After completing the repair, use a diagnostic tool to clear the fault code from the engine module's memory.

5. 5

   Verify repair

   Start the engine and let it run. Check the live data from the fresh air intake temperature sensor to confirm it is providing a plausible reading and ensure the fault code does not reappear.