What is Caterpillar Fault Code 1590-5?

Caterpillar Fault Code 1590-5 indicates a "Current Below Normal" condition on the Engine Speed/Timing Sensor #2 circuit. This means the ECM (Engine Control Module) has detected that the electrical current flowing through the secondary engine speed sensor circuit is lower than the expected threshold defined by Cat's factory specifications.

This sensor plays a critical role in monitoring crankshaft or camshaft position, allowing the ECM to precisely control fuel injection timing. On used Caterpillar equipment, this fault can directly impact engine performance, fuel efficiency, and in severe cases, trigger a derate mode that limits machine productivity on the jobsite.

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Common Symptoms

When Fault Code 1590-5 is active, operators and mechanics typically observe the following:

• Warning lamp illuminated on the operator display panel
• Engine derate or reduced power output during operation
• Rough idling, misfiring, or unstable engine RPM
• Difficulty starting or extended cranking before the engine fires
• Cat ET diagnostic software logging an active or logged fault against the speed/timing circuit

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Potential Causes

On used and older Caterpillar machinery, this code commonly results from:

• Damaged or corroded wiring harness connections at the sensor or ECM connector
• Faulty Engine Speed/Timing Sensor #2 due to wear or contamination
• Open circuit in the signal or ground wire caused by harness chafing against the frame
• Moisture intrusion into the sensor connector causing resistance spikes
• ECM connector pin damage or pushed-back terminals from previous repairs

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How to Troubleshoot and Fix Code 1590-5

Step 1 – Inspect the Harness and Connectors Begin with a thorough visual inspection of the wiring harness running to Engine Speed/Timing Sensor #2. On used excavators, look specifically for chafe points where the harness contacts the engine block, frame rails, or hose bundles. Check connectors for corrosion, bent pins, or moisture before any component replacement.

Step 2 – Perform Resistance and Voltage Checks Using a quality multimeter, measure the resistance across the sensor terminals. Compare readings against Cat's specification (typically 150–1,000 ohms depending on model). Next, verify supply voltage at the connector with the key-on — you should see approximately 5V or 12V depending on circuit design. A reading significantly below spec confirms a wiring or power supply issue.

Step 3 – Use Cat ET Diagnostic Software Connect Cat Electronic Technician (Cat ET) software to the machine's service port. Monitor live sensor data to confirm whether the signal is absent or erratic under cranking conditions. This eliminates guesswork and pinpoints whether the fault is in the sensor, harness, or ECM.

Step 4 – Replace the Sensor if Necessary If wiring checks out clean, replace Engine Speed/Timing Sensor #2 with a genuine or OEM-equivalent Cat part. Clear the fault code and perform a functional test.

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Disclaimer: This guide is intended for informational purposes only. Always consult a certified Caterpillar dealer or licensed heavy equipment mechanic before performing repairs, especially on machines under warranty or in safety-critical applications.