Understanding the P0668 PCM Internal Temperature Sensor Circuit Low

The P0668 diagnostic trouble code indicates a problem with pcm internal temperature sensor circuit low. This code is part of the P06XX family of internal control module and circuit-related codes that affect your vehicle's engine management system.

When your vehicle's Powertrain Control Module (PCM) detects a fault in this system, it stores the P0668 code and illuminates the check engine light. This particular code requires prompt attention as it affects critical engine management functions that can impact vehicle operation, fuel economy, and emissions.

What Does the P0668 Code Mean?

The PCM internal temperature sensor is reading below the expected voltage. This indicates the module may be running too cool internally.

This code specifically relates to the pcm internal temperature low system in your 2023 Ram 1500. The PCM constantly monitors all inputs from sensors and actuators in this circuit to ensure proper engine operation. When the signal falls outside the expected range or the circuit fails its self-test, this code is triggered.

The frequency of this code appearing varies by vehicle application and driving conditions. Some vehicles may experience this code after a battery disconnection or jump start, while others may develop this issue due to gradual degradation of components over time.

Common Symptoms of P0668

When your 2023 Ram 1500 stores the P0668 code, you may notice one or more of the following symptoms:

Check Engine Light Illumination: The most obvious symptom is the check engine light appearing on your dashboard. In some cases, the light may be steady, while in others it may flash intermittently.

Engine Performance Issues: Depending on the specific circuit affected, you may experience reduced engine power, hesitation during acceleration, rough idle, or stalling. The PCM may enter a reduced-power mode to protect the engine from potential damage.

Starting Problems: If the circuit affects the starting system or certain sensors required for engine operation, you may experience difficulty starting the vehicle, especially in cold weather.

Poor Fuel Economy: Many P0668-related issues cause the engine to run richer or leaner than optimal, resulting in decreased fuel efficiency.

Transmission Shifting Issues: Since the PCM controls transmission operation, some P0668 codes can affect shift quality and timing.

Brand-Specific Causes for Ram Vehicles

Ram trucks commonly experience PCM power relay issues, especially in the 6.7L Cummins diesels. The glow plug control module failures are frequent causes of P0670-P0680 codes.

Use wiTECH for Ram diagnostics. For Cummins engines, check the OEM glow plug relay and resistor grid assembly.

For 2023 Ram 1500 specifically, this code is often related to:

1. Electrical Connections: Corroded, loose, or damaged electrical connectors in the affected circuit can trigger this code. Inspect all related connectors for signs of water intrusion, bent pins, or corrosion.

2. Wiring Harness Issues: The wire harness in your vehicle experiences constant thermal cycling and vibration. Over time, this can cause wires to chafe, break, or develop high resistance connections.

3. Ground Connections: Poor ground connections can cause voltage drops that affect sensor readings and circuit operation. Check all ground points associated with this system.

4. Component Failure: Like all mechanical and electrical components, the affected part has a finite service life and will eventually require replacement.

Step-by-Step Diagnostic Procedure

Step 1: Verify the Code Use a quality OBD-II scanner to retrieve the P0668 code and any accompanying codes. Freeze frame data can provide valuable information about the conditions present when the code was set. Note any other codes present, as they may be related to the root cause.

Step 2: Visual Inspection Perform a thorough visual inspection of all wiring and connectors in the affected circuit. Look for:

• Damaged, chafed, or exposed wiring
• Corrosion on electrical terminals
• Loose connections
• Signs of water intrusion
• Burnt or melted connectors

Step 3: Check Technical Service Bulletins (TSBs) Before beginning extensive diagnosis, check for any TSBs related to the P0668 code for your specific vehicle. Manufacturers often release service updates that address common issues.

Step 4: Circuit Testing Using a digital multimeter, test the affected circuit according to manufacturer specifications. This typically includes:

• Checking for power and ground at the component
• Testing circuit continuity
• Checking for shorts to ground or power
• Verifying signal voltage at the PCM

Step 5: Module Reprogramming In some cases, especially with codes like P0600-P0607, the PCM may need to be reprogrammed or updated with the latest software from the manufacturer. This can resolve issues caused by software bugs or calibration errors.

BCM vs. ECM Replacement Scenarios

Understanding when to replace the Body Control Module (BCM) versus the Engine Control Module (ECM/PCM) is critical for proper repair:

ECM/PCM Replacement Indicators:

• Codes P0600-P0607 (internal memory/processor faults)
• Multiple sensor-related codes appearing simultaneously
• Complete loss of communication with the PCM
• The vehicle will not start due to PCM not controlling fuel injection

BCM Replacement Indicators:

• Codes affecting exterior lighting, horn, wipers
• Interior electrical systems affected while engine runs normally
• Security system malfunctions
• Communication codes between BCM and other modules

Key Consideration: Never automatically replace either module without thoroughly checking all wiring, grounds, and related circuits first. Many module failures are actually caused by external circuit issues.

Repair and Replacement Options

Once the root cause has been identified, repair options may include:

Circuit Repair: If wiring or connectors are damaged, repair or replace the affected components. Use proper wire connectors and sealing methods to prevent future issues.

Component Replacement: Replace the failed sensor, relay, solenoid, or actuator as needed. Always use quality OEM or equivalent replacement parts.

Module Replacement: If the ECM/PCM or BCM is confirmed faulty, replacement with a new or remanufactured unit is required. Note that replacement modules typically require programming to your specific vehicle.

Module Reprogramming: In some cases, updating the module's software can resolve the issue without physical replacement. Check with your dealer for the latest software updates.

Prevention and Maintenance Tips

To help prevent P0668 codes from occurring in your 2023 Ram 1500:

1. Maintain Electrical Connections: Periodically inspect and clean electrical connections, especially in the engine bay where exposure to heat and moisture is highest.

2. Address Battery Issues Promptly: Weak or failing batteries can cause voltage fluctuations that damage sensitive electronic circuits. Replace batteries before they fail completely.

3. Use Quality Parts: When replacing sensors or actuators, use quality parts from reputable manufacturers. Cheap replacement parts often fail prematurely.

4. Follow Maintenance Schedules: Regular maintenance helps identify potential issues before they cause code setting failures.

5. Protect Against Water Intrusion: Ensure all engine compartment seals are intact and functioning to prevent water damage to sensitive electronics.

Cost Considerations

The cost to diagnose and repair the P0668 code varies significantly depending on the root cause:

• Diagnostic Inspection: $75-$150 at a qualified repair shop
• Wiring Repair: $100-$300 depending on accessibility and extent of damage
• Sensor/Actuator Replacement: $150-$500 depending on the specific component
• BCM Replacement: $400-$800 plus programming
• ECM/PCM Replacement: $500-$2000 plus programming and keys

Always obtain a complete diagnosis before approving any repairs, as symptoms that appear to be one issue may actually be caused by something else entirely.