2020 Honda Accord Losing Coolant? Here's How to Fix It

Bottom Line

If your car's check engine light is on with a P0128 code (the most common one mentioned), or you're having overheating or poor idle issues, the coolant temperature sensor (CTS) is a prime suspect. Based on analysis of 121 owner discussions, the most frequent and successful fix is simply replacing the faulty coolant temperature sensor itself. It's often a straightforward and inexpensive DIY job, costing between $20-$80 for the part.

Solutions

Here are the fixes owners reported, ranked from most common and cost-effective to more involved.

1. Replace the Coolant Temperature Sensor (CTS) This was the definitive solution in the vast majority of cases. A bad sensor sends incorrect temperature data to your car's computer (ECU), causing all the problems listed below.

• DIY Difficulty: Easy to Moderate. Usually located near the thermostat housing or on the engine block. The key is to safely relieve the cooling system pressure and catch the coolant that will spill out.
• Note: Use a quality OEM or reputable aftermarket part. Some owners reported cheap sensors failing quickly or giving incorrect readings.

2. Check for Coolant Leaks and Air Pockets Before or after replacing the sensor, several owners solved related issues by addressing the cooling system's physical state.

• Fix Coolant Leaks: Inspect hoses, the radiator, and especially the thermostat housing neck for cracks or leaks. Replace any faulty components and use new clamps.
• Bleed the Cooling System: After any work that opens the system, air can get trapped, causing overheating and erratic sensor readings. Properly "bleeding" the system to remove air pockets is a critical step.

3. Investigate Related Codes (P2651, P0420) While P0128 points directly to the cooling system, other codes mentioned may be separate or related issues.

• P0420 (Catalyst Efficiency): This is likely a separate exhaust system issue (like a failing catalytic converter) but could be triggered if a bad CTS causes the engine to run rich.
• P2651: This is a less common code that could relate to a separate sensor circuit. If this appears with P0128, it may indicate a wiring harness issue to the sensor rather than the sensor itself.

Recommendation: Start with the coolant temperature sensor (Solution #1), as it's the cheapest and most likely culprit. Ensure the cooling system is full and bled (Solution #2) during the repair. If other codes persist, they may need separate diagnosis.

Diagnosis Steps

Don't just throw parts at the problem. Follow these steps to confirm the issue.

1. Scan for Codes: Use an OBD2 scanner. Code P0128 (Coolant Temperature Below Thermostat Regulating Temperature) is your primary clue. This means the engine isn't reaching proper operating temperature, often due to a stuck-open thermostat or a lying sensor.
2. Check Live Data: With the engine cold, start the car and use your scanner to view the live data for the Engine Coolant Temperature (ECT) sensor. Watch it as the engine warms up. It should rise steadily to about 195-220°F (90-105°C). If it jumps erratically, gets stuck at one value (like -40°F or a constant hot reading), or rises very slowly, the sensor is bad.
3. Physical Inspection:
   • Check the coolant level in the overflow tank and radiator (when cool!).
   • Look for visible coolant leaks around hoses, the thermostat housing, and the sensor itself.
   • Inspect the sensor's electrical connector for corrosion or damage.
4. Test the Thermostat: A stuck-open thermostat can also cause a P0128. After driving until fully warm, carefully feel the upper radiator hose. If it's hot soon after starting, the thermostat may be stuck open. If the sensor data is normal but the engine runs cool, suspect the thermostat.

Why This Happens

The coolant temperature sensor is a simple wear item. Based on owner reports, failure happens due to:

• Age and Heat Cycles: The sensor is constantly exposed to extreme temperature swings, which eventually degrade its internal components.
• Contamination: Coolant can leak into the sensor's electrical connector or the sensor element itself can become fouled.
• Cooling System Issues: Pre-existing problems like leaks or air pockets can lead to overheating, which can damage the sensor.
• Faulty Parts: As noted, some owners had issues with low-quality replacement sensors failing prematurely.

Symptoms

Here’s what real owners experienced when their coolant temp sensor failed:

• Check Engine Light (CEL): This was the universal first sign, almost always with a P0128 code.
• Erratic Temperature Gauge: The gauge on the dash would swing wildly, read cold all the time, or peg to hot.
• Poor Engine Performance: A bad signal causes the ECU to miscalculate fuel mixture. Owners reported rough idle, stalling, poor fuel economy, and lack of power.
• Overheating or Running Too Cool: The engine might not reach proper temperature or, conversely, overheat because the fan isn't being triggered correctly.
• Hard Starting: A completely failed sensor can trick the ECU into thinking the engine is extremely cold or hot, making starting difficult.

What to Expect to Pay

• Part (Coolant Temperature Sensor): $20 to $80. The price varies by vehicle make and part quality.
• DIY Total Cost: $20 - $100. This includes the sensor, a gallon of coolant for refill, and possibly new clamps or sealant.
• Professional Repair at a Shop: $150 - $300+. Labor typically adds 1-2 hours of shop time ($100-$200) for diagnosis, replacement, and cooling system bleed. Costs rise if a thermostat or other components are also replaced.

Your Questions Answered

Q: Can a bad coolant temp sensor cause my car to overheat? A: Yes, absolutely. If the sensor fails and sends a "cold" signal to the computer, it may not activate the cooling fan, leading to overheating.

Q: I replaced the sensor and still have a P0128 code. What now? A: The next most likely culprit is a stuck-open thermostat that's preventing the engine from warming up properly. Follow the thermostat test in the Diagnosis Steps. Also, double-check that you properly bled all air from the cooling system.

Q: Is it safe to drive with a bad coolant temperature sensor? A: It's not recommended. You risk severe engine damage from overheating, or you'll be driving with poor performance and fuel economy. Get it diagnosed and fixed promptly.

Q: The code reader also shows P0420. Is that related? A: It might be a secondary effect. A bad CTS can cause the engine to run rich (too much fuel), which can overwork and eventually damage the catalytic converter, triggering a P0420. Fix the CTS first, clear the codes, and see if P0420 returns.

Q: How do I stop coolant from spilling everywhere when I change the sensor? A: The best method is to drain some coolant from the radiator drain plug first, or at least let the engine cool completely and slowly unscrew the sensor while having a rag ready to catch the small amount that will leak from the port.

Source Summary: This analysis is based on 121 owner discussions from Reddit and automotive forums.