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I’ve been having this issue with my isf that’s been really bugging my brain. When I purchased the car it had a Cel for The Air/Fuel sensor. Long story short, I replaced the extension harness and air/fuel sensor leading to bank 1 which was throwing the code. Fast forward the car is still throwing P0031 for low heater circuit. New sensor and harness, fuse and relay are good. Has anyone dealt with this issue before? And if so what should I look to? I was hoping to get the wiring harness diagram leading to the ecu so I can assess the air fuel wiring…
Though you've replaced the air/fuel sensor and its harness, it's essential to look at the power and ground circuits for the heater element of the sensor itself. A poor ground connection or a break in the signal wire can result in low voltage readings and trigger the code. Additionally, an issue with the Engine Control Unit (ECU) could manifest as an incorrect signal to the sensor, so ensure that the ECU is functioning correctly. Another unlikely possibility is a problem with the fuel trim; if the PCM is not correctly adjusting based on sensor inputs, it might falsely report heater circuit issues.
Though you've replaced the air/fuel sensor and its harness, it's essential to look at the power and ground circuits for the heater element of the sensor itself. A poor ground connection or a break in the signal wire can result in low voltage readings and trigger the code. Additionally, an issue with the Engine Control Unit (ECU) could manifest as an incorrect signal to the sensor, so ensure that the ECU is functioning correctly. Another unlikely possibility is a problem with the fuel trim; if the PCM is not correctly adjusting based on sensor inputs, it might falsely report heater circuit issues.
how would I ensure the ecu is in working order? Also where can I find the ground and power connections for the sensor itself? Would they be in the location of the sensor?
There are two blue wires coming from the O2 heater circuit. If you turn the car on, but don't start the engine, you'll see 12v on those wires if your wiring is in good condition. I just recently worked on a GS F where the ECM failed to provide ground to this circuit when the engine was running. Something caused the semiconductor responsible for managing the heater circuit to fail, so a new ECM was the only fix, and that worked for this car. This is pretty obvious if you have Techstream or some other OBDII monitoring tool where you can see lambda from the wideband O2 sensors. When the heater circuit fails, the signal stops.
There are two blue wires coming from the O2 heater circuit. If you turn the car on, but don't start the engine, you'll see 12v on those wires if your wiring is in good condition. I just recently worked on a GS F where the ECM failed to provide ground to this circuit when the engine was running. Something caused the semiconductor responsible for managing the heater circuit to fail, so a new ECM was the only fix, and that worked for this car. This is pretty obvious if you have Techstream or some other OBDII monitoring tool where you can see lambda from the wideband O2 sensors. When the heater circuit fails, the signal stops.
would the blue wires be going into the ecu harness? Are the wires the same color as the other air fuel sensor? If so how will I know which are bad? I’m sorry I’m not to keen on wiring and such so this is a foreign issue for me.
Oops. Wire colors are different on the IS F. The wires are black with a red stripe on the IS F. They are in pins 5 and 6 of the third connector from the front of the car. If you are not comfortable with wiring, you really should engage with someone who is. It's not difficult to damage things electrically with even what seem like small mistakes.
Last edited by lobuxracer; Oct 23, 2024 at 11:50 PM.
Lexus Isf P0031 issue
