Hey so I have a 2003 Lexus is300. I absolutely love this car and I’ve been spending quite a lot of money in getting it fixed but it just keeps getting back to me in the same condition. So far I’ve replaced spark plugs, ignition coils/wires, fuel injectors and I’m still getting misfires in cyl 4-5-6. I also have a P0031 code for the O2 sensor that most likely needs replacement. I’ve spent almost $2,000 In trying to repair it. I was wondering what’s the best thing I should do and avoid spending unnecessary expenses. So far I’ve fixed an oil leak from the VVTi cam, and I’ve replaced the VVTi solenoid + filter. The car is STOCK EXCEPT for the K&N Air Intake. If someone could help me find the solution that’d be great!

 

I see you're new to this forum. This place is not a really good resource for IS300 info, especially if you're in a hurry.
That is my.IS and the majority of IS300 owners are all in there.
There are a few of us still active on here but that's because we also have other Lexus cars.

P0031 is oxygen sensor heater control circuit low (Bank 1 Sensor 1 - cylinders 1-3) This means there is low or no voltage in the heating element of this O2 sensor. Current of 0.2A or less.

This means there is a short or a cut wire in the wiring, or the heating element in the sensor is burned out. If it is neither of those things, it could be the EFI relay or even the ECU (but with no other codes this is unlikely)
The wire colors for the O2 sensor heating circuit of B1S1 are light green and brown on the harness. The light green wire goes directly to the ecu and the brown wire merges with all the other O2 sensor heating element wires and grounds to the intake manifold