Isuzu 4JJ1 Engine – DTC P0112: Intake Air Temperature Circuit Low Input

System Overview

The Intake Air Temperature (IAT) Sensor is located between the air filter and the turbocharger. It functions as a variable resistor, measuring the temperature of incoming air. This sensor has two circuits: a signal circuit and a ground (GND) circuit, both connected to the ECM.

• Low intake air temperature: Higher sensor resistance → Higher signal voltage.
• High intake air temperature: Lower sensor resistance → Lower signal voltage.

When the ECM detects a signal voltage below a specified threshold, DTC P0112 is triggered.

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DTC P0112 Setting Conditions

Prerequisites

• Ignition switch is ON.
• DTCs P060B, P0651, and P160B are not active.

Trigger Conditions

• The ECM detects the IAT sensor signal voltage is below 0.1 V for at least 5 seconds.

System Response to DTC P0112

1. Error Display: The machine monitor or diagnostic lamp illuminates. (May vary depending on the manufacturer.)
2. Default Value: ECM uses a substitute value for intake air temperature.
3. Injection Limitation: ECM reduces fuel injection amounts to protect the engine.

Diagnosis Steps

Step 1: Priority DTC Check

1. Connect a diagnostic scan tool.
2. Turn the ignition switch OFF for at least 30 seconds, then turn it ON.
3. Check for DTCs using the scan tool.

• If DTC P0651 is set:
  → Diagnose DTC P0651 first.
• If DTC P0651 is not set:
  → Proceed to IAT Sensor Signal Voltage Check.

Step 2: Intake Air Temperature Sensor Signal Voltage Check

1. Use the scan tool to read the IAT sensor value.
2. Compare the reading with the specified threshold:

• If the reading is ≤ 0.1 V:
  → Proceed to IAT Sensor Signal Output Voltage Check.
• If the reading is > 0.1 V:
  → Diagnose Intermittent Engine Condition.

Step 3: Intake Air Temperature Sensor Signal Output Voltage Check

1. Turn the ignition OFF.
2. Disconnect the IAT sensor harness connector.
3. Turn the ignition switch ON.
4. Observe the IAT sensor value on the scan tool.

• If the value is ≥ 4.5 V:
  → Replace the IAT sensor and proceed to Repair Verification.
• If the value is < 4.5 V:
  → Check the IAT sensor signal circuit for a short to ground.

Step 4: Signal Circuit Ground Fault Check

1. Inspect the signal circuit between the ECM and the IAT sensor for:
   • Short to ground.
   • Short to the GND circuit.

• If the circuit is normal:
  → Proceed to inspect the ECM harness connector.
• If a fault is found:
  → Repair the circuit and proceed to Repair Verification.

Step 5: ECM Harness Connector Inspection

1. Check the ECM harness connector for loose or damaged connections.

• If the connector is normal:
  → Replace the ECM and perform programming.
• If the connection is faulty:
  → Repair as necessary and proceed to Repair Verification.

Repair Verification

1. Clear DTCs using a diagnostic scan tool.
2. Turn the ignition switch OFF for at least 30 seconds.
3. Start the engine and perform a test drive under conditions that could trigger the DTC.
4. Confirm no DTC is set.