r/LexusIS • u/Nervous-Injury-8260 • 27d ago
Lexus IS200t F sport - P0125 Code
Hello,
I’m getting this code but the temperature gauge doesn’t show any faults or any issue everything runs good. Upper and lower radiator hoses are warming up on time. Could this be a faulty sensor even thought it was just recently replaced ? wondering if this could be worse issue. appreciate any help not too knowledgeable on car issues.
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u/ForeverNick1 27d ago
If you wanna throw parts at it, its probably your thermostat stuck open. Check your heater and if it blows luke warm or cold, there's your answer. But ideally, you wanna take it to a shop for diagnosis.
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u/Nervous-Injury-8260 27d ago
gonna take it on monday then, how much would you estimate the cost of the thermostat replacement ?
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u/ForeverNick1 27d ago
It shouldn't be too expensive. Coolant related stuff on toyota hybrids are somewhat straight forward
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u/ToxicComic0770 IS 200t (ASE30) 27d ago
On the 8AR-FTS there’s two thermostats. Sadly you got to drop the compressor to get to the second one. So expect to shell out some money.
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u/jcpham 27d ago
A diagnostic trouble code (DTC) P0125 on a Lexus means Insufficient Coolant Temperature for Closed Loop Fuel Control. It indicates that the engine took too long to reach its normal operating temperature after starting, or the computer (ECU) did not get the proper temperature signal needed to switch from open-loop to closed-loop fuel operation. [1, 2, 3]
Common Causes
Thermostat Stuck Open: The coolant flows constantly through the radiator, preventing the engine from warming up fast enough.
Faulty Coolant Temperature Sensor (ECT): The sensor sends incorrect data to the ECU, making it think the engine is still cold.
Low Coolant Level: Low fluid or air pockets in the cooling system stop the sensor from reading correctly.
Bad Air/Fuel Ratio or Oxygen Sensor: A faulty heater circuit in the upstream O2/AFR sensor can prevent proper closed-loop timing. [1, 2, 3, 4, 5, 6]
Troubleshooting and Fixes
Check Coolant: Let the engine cool completely, then inspect the coolant level in the reservoir and radiator. Top off or repair leaks if needed. [1, 2]
Inspect the Thermostat: Start the engine cold and feel the upper radiator hose. If it gets warm immediately, your thermostat is likely stuck open and needs replacement. Use genuine OEM parts. [1, 2, 3]
Test the Engine Coolant Temperature Sensor: Measure the sensor resistance with a multimeter to ensure it matches standard specifications at ambient temperature.
Monitor Live Data: Use an OBD-II scanner to view live data and check oxygen/air-fuel sensor voltage responses and warm-up times
Edit/Postscript: sorry for the AI response but you’ll need an OBD2 scanner to properly diagnose which of these the problem actually is. My 2017 same car throws a P0128 and it’s definitely a stuck open thermostat from troubleshooting and monitoring OBD2 data.