Komatsu PC1250-11R Fault Code CB4952: Complete Diagnostic Guide

What is Komatsu PC1250-11R Fault Code CB4952?

Fault Code CB4952 indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormal voltage signal or open circuit condition in the ECM (Engine Control Module) monitoring system. This code triggers when the ECM receives voltage readings outside the normal operating range (typically 0.5-4.5V) from the coolant temperature sensor, which is critical for proper fuel injection timing, engine protection, and hydraulic system performance.

For the PC1250-11R's SAA6D170E-5 engine, accurate coolant temperature monitoring is essential. The ECM uses this data to adjust fuel delivery, control fan speed, and prevent overheating damage to this high-output powerplant. Ignoring this code can lead to engine derate, reduced hydraulic performance, and potential catastrophic engine failure.

Common Symptoms

• Engine warning lamp illuminated on the monitor panel with reduced engine power output
• Automatic engine derate to protect components, limiting maximum RPM to approximately 1,500-1,800 RPM
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Difficulty starting in cold conditions or rough idle during warm-up cycles
• Inaccurate temperature gauge readings showing extreme high or low temperatures that don't match actual conditions

Potential Causes

The most common causes for CB4952 on used PC1250-11R excavators include:

• Coolant temperature sensor failure due to age-related degradation or thermal cycling damage (sensor located on engine block near thermostat housing)
• Wiring harness damage at known rub points near the engine mount brackets and along the right-side frame rail
• Corroded or loose connector pins at the sensor plug (3-pin Deutsch connector), especially on machines operating in coastal or high-humidity environments
• ECM internal circuit failure affecting the 5V reference voltage supply to the sensor
• Damaged sensor ground circuit causing erratic voltage readings
• Previous improper repairs using non-OEM sensors with incorrect resistance curves

How to Troubleshoot and Fix Code CB4952

Step 1: Visual Inspection and Data Verification Using Komatsu KOMTRAX or a compatible diagnostic tool, check freeze frame data to determine if the fault is current or intermittent. Inspect the coolant temperature sensor and its wiring harness for obvious damage, corrosion, or coolant contamination around the connector. On used excavators, pay special attention to harness routing near heat sources and moving components.

Step 2: Electrical Testing Disconnect the sensor connector and measure resistance across the sensor terminals using a digital multimeter. At 68°F (20°C), resistance should read approximately 2,400-2,700 ohms. Test the wiring harness by measuring continuity from the ECM connector to the sensor plug, and check for shorts to ground. Verify the ECM supplies proper 5V reference voltage at the disconnected harness connector.

Step 3: Component Replacement and Verification If sensor resistance is out of specification or wiring shows damage, replace the faulty component with genuine Komatsu parts (sensor P/N: 6261-81-6510). After replacement, clear codes using diagnostic software, run the engine through a complete heat cycle, and verify the code doesn't return. For used machines, apply dielectric grease to connectors and secure harnesses with new zip ties away from wear points.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC1250-11R service manual and consider professional assistance for complex electrical diagnostics. Improper repairs may void warranties or cause additional damage.