Hello, 99 ES300, 215K miles. P0125 code thrown, and a P1135 code buried in the Lexus diagnostic. Interwebs reading prompts me that the underlying 1135 code causes the 0125. 1135 is the A/F sensor code, is this the same as the O2 sensor, just called a different name? Don’t think the “02” sensors have ever been replaced.

As to the 0125, there is a possible defective ECT sensor somewhere? Don’t know where that is located; the FSM wasn’t very explicit there. I have recently replaced the thermostat, hose and o-ring, due to a leaking radiator, also replaced. I guess it is possible the thermostat is stuck open, but the gauge works fine. I will say I have recently been driving the car only 1 day a week, and a short 4 mile round trip. Haven’t had these codes before. I have reset the codes, but have yet to drive it.


Any BTDTs or other guidance? Thanks in advance!

Just a follow up, reset the codes, and after about 10 ignition cycles, 2 codes popped up, though 1 is different this time.
P0125 insufficient coolant temperature for closed loop fuel control, and P1133, air fuel sensor circuit response malfunction. Interesting 1133 this time and not 1135.

Any BTDTs? Guessing an O2 sensor? Thx

​​​​Both 1133/1135 are Heater and Circuit response malfunction on B1S1 AF sensor.
Not knowing what your AF readings at idle or freeze frame data and assuming your wiring are good,
Its more than likely a bad AF sensor Bank 1.
As for the Po128, replace the t-stat usually resolves it.

Thank you both for the responses. Just received both Denso AF sensors; interesting the Denso site has different numbers for the AF sensors and the O2 sensors. Since this is a California car, I ordered the AF sensors.

I will try and install both over the weekend and go from there. I’m hoping the heater in the sensors will take care of the P0125
code. New Tstat, O ring, radiator, hoses and coolant in the past year, so thinking it’s not the Tstat. Temp gauge responds better after the cooling work.

Kind of dreading the firewall sensor, I will report back...

I went to tackle the front “easy” O2 sensor, behind the radiator. I had previously hit it a couple of times with PB Blaster, to ease the removal.

Bought a O2 sensor removal tool, and was able to just barely budge the sensor. Tried an 18 inch breaker bar, no help. I have a 22mm 12 point box end wrench, which fit the sensor perfectly. I was barely able to move it another 1/8 of an inch. I tried another couple blasts of PB Blaster, but got no further. I know these are the original sensors.
And this is the easier access O2 sensor!.

Not sure how to progress any further, may have to bite the bullet and find a place that will do the labor on these.... ugh! I have been vanquished!