Caterpillar Fault Code 5700: Complete Diagnostic Guide

What is Caterpillar Fault Code 5700?

Caterpillar Fault Code 5700 indicates an Intake Air Heater Relay Circuit malfunction, specifically detecting an abnormal voltage condition in the relay control circuit managed by the Electronic Control Module (ECM). This code triggers when the ECM detects that the intake air heater relay is not responding correctly to control commands or shows voltage readings outside the acceptable range.

The intake air heater (also called the grid heater) is critical for cold-start performance in diesel engines, particularly in Caterpillar excavators operating in low-temperature environments. This component preheats incoming air to facilitate easier combustion during engine startup. When this system malfunctions, the ECM cannot properly regulate engine warm-up sequences, potentially leading to hard starts, increased white smoke, and extended cranking times in cold conditions.

Common Symptoms

When Code 5700 is active, operators typically experience:

• Difficult cold starts requiring prolonged cranking, especially in temperatures below 40°F (4°C)
• Check Engine Light or diagnostic warning lamp illuminated on the instrument panel
• Excessive white or gray smoke during initial startup that persists longer than normal
• Rough idle immediately after cold starts until engine reaches operating temperature
• No audible click from the intake air heater relay during key-on position (pre-glow cycle)

Potential Causes

The most common technical reasons for Code 5700 in used Caterpillar excavators include:

• Failed intake air heater relay due to internal contact degradation or coil failure
• Damaged or corroded wiring harness between the ECM and relay, particularly at frame mounting points where vibration causes wire chafing
• Poor ground connections at the relay mounting bracket or ECM ground points (common in machines with 3,000+ hours)
• Faulty intake air heater element creating a short circuit or open circuit condition
• ECM internal relay driver failure (less common but possible in high-hour machines)
• Corroded relay socket or connector pins due to moisture intrusion in the electrical cabinet

How to Troubleshoot and Fix Code 5700

Step 1: Visual Inspection Begin by locating the intake air heater relay (typically in the main fuse/relay panel near the battery compartment). Inspect all connectors for corrosion, bent pins, or moisture. On used excavators, check the wiring harness along the frame rails for signs of rubbing, cracking, or previous repairs with electrical tape—these are common failure points.

Step 2: Relay Testing Remove the relay and perform a bench test using a multimeter. Check for coil resistance (typically 70-90 ohms on Caterpillar relays) between the control terminals. Apply 12V directly to the coil terminals and verify you hear an audible click and measure continuity across the load terminals. Replace the relay if readings are out of specification.

Step 3: Circuit Voltage and Ground Testing With the relay removed and key in the ON position (engine not running), use a multimeter to check for battery voltage (11-14V) at the relay power supply terminal. Next, verify the ECM control signal wire shows approximately 12V during the pre-glow cycle (first 3-7 seconds after key-on). Check ground circuit resistance to chassis ground—should read less than 0.5 ohms.

Step 4: Intake Air Heater Element Testing If relay and circuits test good, disconnect the heater element and measure resistance. Typical values range from 0.1 to 0.5 ohms depending on element design. An open circuit (infinite resistance) or short to ground indicates element replacement is necessary.

Step 5: Advanced Diagnostics Connect Caterpillar Electronic Technician (CAT ET) or compatible diagnostic software. Monitor real-time data for intake air heater command status versus actual relay feedback. For used machines, check for intermittent faults by wiggling harness connections while monitoring live data—this often reveals failing connectors.

Critical Note for Used Equipment: Before replacing expensive components, thoroughly clean all electrical connections with contact cleaner and apply dielectric grease. Many Code 5700 issues on older excavators resolve simply by addressing corroded connections that developed over years of exposure to dirt, moisture, and vibration.

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Disclaimer: This guide provides general diagnostic information for Caterpillar fault code 5700. Always consult your machine's specific service manual and consider professional diagnostic services for complex electrical issues. Improper repairs can lead to ECM damage or safety hazards.