Case IH P26 – Engine Management Unit (EMU) / Vehicle Electronic Control Unit (VECU): Boost Air Temperature Sensor (PHY), Grounded Or Current Above Normal Range

The controller detects a voltage or current signal on the Boost Air Temperature Sensor's circuit that exceeds the maximum expected threshold, which points to a direct short to ground in the wiring harness or an internal short inside the sensor itself.

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190

1. 1

   Visual Inspection: Inspect the wiring harness leading to the Boost Air Temperature Sensor for rubbed-through insulation, melted wires, or pinch points that could cause the signal wire to short to the engine block (ground).

2. 2

   Connector Check: Disconnect the sensor. Inspect the pins on both the sensor side and the harness side for corrosion, moisture, or bent pins that could be bridging the circuit.

3. 3

   Sensor Resistance Test: Using a multimeter, measure the resistance across the sensor's terminals. If the multimeter reads 0 ohms (full continuity) at ambient temperature, the sensor is internally shorted and must be replaced.

4. 4

   Harness Continuity & Ground Test: With the sensor and the ECU disconnected, test the signal wire for continuity to ground. If there is continuity to ground while unplugged, the wiring harness has a short and needs to be repaired.

1. 1

   Safety First

   Park the tractor on level ground, apply the parking brake, turn off the engine, and disconnect the negative battery terminal to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the Boost Air Temperature Sensor on the engine's intake manifold piping, downstream of the turbocharger/intercooler.

3. 3

   Disconnect

   Unplug the electrical connector from the sensor.

4. 4

   Wiring Repair (If Applicable)

   If your diagnostic checks revealed a short to ground in the harness, splice in a new section of wire or apply heat-shrink tubing to the exposed wire, ensuring it is routed away from hot exhaust components or sharp metal edges.

5. 5

   Sensor Replacement (If Applicable)

   If the sensor failed the resistance test, use a deep socket or wrench to unthread the faulty sensor from the intake manifold.

6. 6

   Install New Sensor

   Thread the new sensor into the manifold by hand to prevent cross-threading, then tighten to the OEM torque specification. Reconnect the electrical harness.

7. 7

   Recalibrate / Clear Codes

   Reconnect the battery. Turn the ignition key to the ON position. Enter the tractor's H-Menu (diagnostic mode) using the dash buttons to clear the stored T26 fault code.

8. 8

   Test

   Start the engine and verify that the code does not return and that the tractor operates without defaulting to a derated protection mode.