What causes fault code P056D?

• Most likely: Internal electrical failure within the NOx sensor control module (Bank 1) • Likely: Wiring issues (voltage drop or high resistance) in the sensor supply circuit • Possible: Contaminated or damaged NOx sensor probe causing logic errors in the module • Rare: Corrosion in the NOx sensor connector pins

Powertrain (Engine & Gearbox)Standard EOBD

P056D - NOx Sensor Controls Aftertreatment Device Hardware Performance (Bank 1)

Q: What are the symptoms of fault code P056D?

• Check engine light (MIL) illumination • 'Service DEF system' or 'Remaining starts' countdown message • Reduced engine power (Limp mode) • Vehicle will fail emissions inspection

Q: How do you fix fault code P056D?

STEP 1: Inspect the NOx sensor connector for moisture or corrosion. Clean as necessary. STEP 2: Measure voltage supply (typically 12V or 24V) and ground at the sensor connector. Voltage drop should not exceed 0.5V. STEP 3: Verify CAN-High and CAN-Low signals using an oscilloscope to ensure stable communication between sensor and ECM. STEP 4: If wiring is intact and power is stable but the code persists after clearing, the NOx sensor unit (including the module) must be replaced. STEP 5: Perform a NOx sensor adaptation reset using a diagnostic tool (e.g., OEM software).

Last updated:1 February 2026

Related:NOx Sensor (Bank 1), SCR Control Module, Engine Control Unit (ECU), CAN-bus system

System

Powertrain (Engine & Gearbox)

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Technical Description

P056D is an SAE-standard DTC indicating an internal hardware or performance fault within the NOx sensor control module for Bank 1. This control logic is typically integrated into the sensor's own electronic heat sink/processor box. When the Engine Control Module (ECM) detects that the sensor's internal self-test or power monitoring does not meet calibrated thresholds via CAN bus communication, P056D is stored. This directly impacts the SCR system's ability to accurately meter Reductant (DEF/AdBlue) as the closed-loop feedback for NOx levels is compromised.

Possible Causes

Internal electrical failure within the NOx sensor control module (Bank 1)

Wiring issues (voltage drop or high resistance) in the sensor supply circuit

Contaminated or damaged NOx sensor probe causing logic errors in the module

Corrosion in the NOx sensor connector pins

Symptoms

Check engine light (MIL) illumination

'Service DEF system' or 'Remaining starts' countdown message

Reduced engine power (Limp mode)

Vehicle will fail emissions inspection

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P056D - NOx Sensor Controls Aftertreatment Device Hardware Performance (Bank 1)

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P056D - NOx Sensor Controls Aftertreatment Device Hardware Performance (Bank 1)

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