Sany SY6HK1 Fault Code P1655(59): Technical Guide

What is Sany SY6HK1 Fault Code P1655(59)?

Fault Code P1655(59) indicates a malfunction in the Engine Coolant Temperature (ECT) sensor circuit, specifically signaling an intermittent or out-of-range voltage condition detected by the Engine Control Module (ECM). This code is part of Sany's proprietary diagnostic system and typically points to electrical issues within the coolant temperature monitoring circuit rather than actual overheating.

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending voltage signals to the ECM. The ECM uses this data to adjust fuel injection timing, idle speed, and cooling fan operation. On the SY6HK1 excavator, which commonly uses Isuzu or Yanmar diesel engines, accurate temperature readings are essential for preventing engine damage and maintaining optimal hydraulic system performance. When this circuit fails, the ECM defaults to conservative operating parameters that can significantly reduce machine efficiency.

Common Symptoms

When P1655(59) is active on your Sany SY6HK1, operators typically experience:

• Check Engine Light (CEL) illuminated on the instrument panel, often accompanied by a warning buzzer
• Engine derate mode where the excavator operates at reduced power (typically 60-75% of normal output)
• Cooling fan running continuously at maximum speed, even during cold starts
• Rough idle or extended warm-up periods as the ECM compensates for missing temperature data
• Difficulty starting in cold weather due to improper fuel enrichment calculations

Potential Causes

The most common technical causes for P1655(59) on used SY6HK1 excavators include:

• Corroded or loose connector pins at the ECT sensor harness (common failure point on machines with 3,000+ operating hours)
• Damaged wiring harness near the engine block where vibration and heat cause insulation breakdown
• Failed ECT sensor due to internal resistance drift or element contamination from coolant additives
• ECM internal fault or corrupted calibration data (less common but possible on older units)
• Poor ground connection at the sensor mounting point or ECM ground strap
• Aftermarket coolant incompatible with sensor materials, causing accelerated sensor degradation

How to Troubleshoot and Fix Code P1655(59)

Step 1: Visual Inspection and Connector Check

Begin with a thorough visual inspection of the ECT sensor located on the engine's thermostat housing or cylinder head. On used excavators, pay special attention to connector corrosion and harness chafing where wires contact the engine block. Disconnect the sensor connector and inspect pins for green corrosion, bent terminals, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease before reconnection.

Step 2: Sensor Resistance Testing

Using a digital multimeter (DMM), measure the ECT sensor resistance with the sensor disconnected. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). If resistance is infinite (open circuit) or near zero (short circuit), replace the sensor. For used machines, also submerge the sensor in heated water and verify resistance decreases smoothly as temperature rises—erratic readings indicate internal sensor failure.

Step 3: Circuit Voltage and Wiring Verification

With the ignition on and sensor disconnected, measure reference voltage at the harness connector. You should see approximately 5 volts on the signal wire. If voltage is absent, trace the wiring harness back to the ECM, checking for breaks, shorts to ground, or damage at common rub points near engine mounts. On SY6HK1 models, the harness often rubs against the fuel filter housing bracket—a known wear point requiring inspection.

Step 4: Ground Circuit Testing

Verify the sensor ground circuit by measuring continuity between the ground pin at the sensor connector and a known good chassis ground. Resistance should be less than 1 ohm. High resistance indicates a poor ground connection, often caused by corrosion at the ECM mounting bolts or deteriorated ground straps common in used equipment operating in harsh environments.

Step 5: Component Replacement and Code Clearing

If testing confirms sensor or wiring failure, replace the defective component using OEM or equivalent quality parts. After repairs, clear the fault code using Sany diagnostic software or a compatible OBD-II scanner. Operate the machine through a complete heat cycle and verify the code does not return. For persistent codes after component replacement, ECM reprogramming or replacement may be necessary.

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Disclaimer: This guide provides general diagnostic information for Sany SY6HK1 excavators. Always consult the manufacturer's service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs can cause additional damage or create safety hazards.