What is Caterpillar Fault Code SPN1990?

SPN 1990 indicates a malfunction with the Diesel Particulate Filter (DPF) Differential Pressure Sensor circuit. This fault code signals that the Electronic Control Module (ECM) has detected an abnormal voltage reading, open circuit, or short circuit in the wiring or sensor that monitors pressure differences across the DPF.

The DPF differential pressure sensor measures the restriction level of exhaust flow through the diesel particulate filter by comparing inlet and outlet pressures. This data is critical for the ECM to determine when regeneration cycles are needed and to protect the engine from excessive backpressure. On Caterpillar excavators, especially Tier 4 Final emission-compliant models, this system is essential for maintaining proper exhaust aftertreatment function and preventing costly engine damage or severe performance derate.

Common Symptoms

When SPN 1990 is active, operators typically experience:

• Check Engine light or Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Engine derate or reduced power output, particularly during high-load operations
• Incomplete or failed regeneration cycles, leading to excessive soot accumulation
• Warning messages on the display panel indicating DPF system faults
• Possible rough idling or increased exhaust smoke if backpressure becomes excessive

Potential Causes

The most common technical reasons for SPN 1990 on used Caterpillar excavators include:

• Damaged or corroded wiring harness connections at the DPF pressure sensor, especially near exhaust components exposed to high heat
• Failed differential pressure sensor due to age, carbon buildup, or internal component failure
• Blocked or kinked pressure hoses connecting the sensor to DPF inlet/outlet measurement points
• Corroded or loose connector pins at the sensor or ECM side, common in machines with high operating hours
• ECM software issues or calibration errors requiring updates
• Physical damage to sensor mounting from vibration or improper maintenance on used equipment

How to Troubleshoot and Fix Code SPN 1990

Step 1: Visual Inspection Begin by inspecting the DPF differential pressure sensor location (typically mounted near the DPF canister). Check all wiring harnesses for signs of abrasion, melting, or rubbing against exhaust components. Examine connector pins for corrosion, bent terminals, or moisture intrusion—common issues on used excavators.

Step 2: Test Pressure Hoses and Connections Disconnect the pressure sensing hoses from the sensor and inspect for blockages, carbon deposits, or cracks. Blow compressed air through the hoses to verify they're clear. Check that hose connections to the DPF are secure and not leaking.

Step 3: Electrical Testing Using a digital multimeter, check sensor supply voltage at the connector (typically 5V reference from ECM). Test sensor signal output voltage while creating slight pressure differential. Compare readings against Caterpillar specifications in the service manual. Check for continuity in all wiring between sensor and ECM connector.

Step 4: Sensor Replacement and Calibration If sensor tests fail specifications or hoses are clear, replace the differential pressure sensor with a genuine Caterpillar or OEM-equivalent part. After replacement, use Caterpillar Electronic Technician (Cat ET) diagnostic software to clear codes and perform system calibration. For used excavators, always apply dielectric grease to connectors to prevent future corrosion.

Disclaimer: This guide provides general troubleshooting information. Always consult your Caterpillar service manual and consider professional diagnostic support for complex electrical issues or if you're unfamiliar with high-voltage systems.