What is Hyundai HX220 NL EPFC Fault Code 691 (1172-3)?

Fault Code 691 (1172-3) on the Hyundai HX220 NL EPFC indicates a malfunction in the engine coolant temperature sensor circuit, specifically reporting an abnormal voltage condition (Circuit High). This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects a voltage signal from the coolant temperature sensor that exceeds the manufacturer's specified upper threshold, typically above 4.5 volts.

The coolant temperature sensor plays a critical role in the HX220's engine management system by monitoring operating temperatures and allowing the ECM to adjust fuel injection timing, idle speed, and emission controls. When this sensor circuit fails, the excavator's performance, fuel efficiency, and emission compliance are directly compromised, potentially leading to engine protection modes that limit machine productivity.

Common Symptoms

• Engine warning light or check engine light illuminated on the instrument panel
• Hard starting conditions, especially during cold starts when the ECM cannot accurately determine engine temperature
• Rough idling or irregular engine speed fluctuations due to improper fuel mapping
• Reduced engine power or activation of derate mode as a protective measure
• Excessive black smoke from the exhaust caused by incorrect fuel-to-air ratios

Potential Causes

The most common technical causes for Code 691 (1172-3) on used HX220 excavators include:

• Open circuit in the coolant temperature sensor wiring harness, often caused by wire chafing near the engine block or valve cover where heat and vibration are concentrated
• Failed coolant temperature sensor with internal resistance breakdown, common in machines with over 5,000 operating hours
• Damaged or corroded connector pins at the sensor plug, particularly on excavators operated in wet or corrosive environments
• Shorted sensor ground wire creating artificially high voltage readings
• ECM internal fault (rare but possible in older machines with electrical system history)

How to Troubleshoot and Fix Code 691 (1172-3)

Step 1: Visual Inspection Locate the coolant temperature sensor on the engine block near the thermostat housing. Inspect the wiring harness for visible damage, particularly checking for wire insulation wear at contact points with the engine. On used excavators, pay special attention to connector corrosion and pin integrity—green or white oxidation indicates moisture intrusion.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms. Compare readings against Hyundai's specifications. Next, check circuit voltage: with ignition on and sensor disconnected, the ECM supply voltage should read approximately 5 volts at the harness connector.

Step 3: Circuit Continuity Check Test for open circuits or high resistance in the wiring between the sensor and ECM. Check ground circuit integrity—poor grounds are extremely common in used machines. Inspect harness routing for friction points where wires may have worn through insulation against metal surfaces.

Step 4: Component Replacement If sensor resistance is out of specification or wiring shows damage, replace the faulty component. Always use OEM or quality aftermarket sensors designed for Hyundai specifications. After replacement, clear codes using diagnostic software and perform a cold-to-warm temperature cycle to verify proper operation.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Hyundai service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.