New Holland ENG 3550 – Engine Control Unit (ECU): Humidity Sensor Signal Ratio failure - Signal Ratio below Limit

The signal voltage or frequency being returned by the intake air humidity sensor has fallen below the minimum acceptable threshold programmed into the engine control module. This condition is typically caused by a failed sensor, a wiring harness issue (such as a short to ground or high resistance), or the sensor element becoming heavily saturated with water droplets/condensation.

• T7 Series
• T7.185
• T7.220
• T7.225
• T7.235
• T7.250
• T7.260
• T7.270
• T8 Series
• CR Series
• CR6090
• CR9090
• CX Series
• CX8080
• CX8090

1. 1

   Visually inspect the air intake piping downstream of the air filter for excessive condensation, water pooling, or snow ingestion saturating the sensor element

2. 2

   Inspect the electrical connector at the humidity sensor for pushed-out pins, corrosion, or water intrusion

3. 3

   Trace the wiring harness back toward the ECU to check for chafing, breaks, shorts to ground, or high resistance

4. 4

   Connect a diagnostic tool to monitor live data for the Intake Air Humidity Sensor; if the reading is static at 0% or erratic, the sensor is likely dead

5. 5

   Check for related codes (3557 or 3558) which indicate the sensor is saturated with water droplets; drying the intake tract may resolve the issue without parts replacement

1. 1

   Preparation

   Turn off the engine, remove the key from the ignition, and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the intake air humidity sensor. It is mounted on the engine air suction pipe, directly downstream of the main engine air filter housing.

3. 3

   Removal

   Depress the locking tab on the electrical harness and disconnect it from the sensor. Unthread or unbolt the faulty humidity sensor from the intake pipe.

4. 4

   Clean the Intake

   Wipe the inside of the intake pipe port with a clean, lint-free rag to ensure no residual moisture or debris is present.

5. 5

   Installation

   Install the new humidity sensor into the intake pipe. Do not overtighten, as the plastic housing can crack.

6. 6

   Reconnection

   Plug the electrical harness back into the sensor until the locking tab clicks. Reconnect the negative battery cable.

7. 7

   System Reset

   Connect your diagnostic tool. Clear the ENG 3550 fault code. Note: If the tractor was placed into an engine derate/power limitation due to this emissions-related fault, you must use the diagnostic tool to perform a 'start-up counter reset' to restore full operational power.