murphysf

Hello

On my 2000 RX300 AWD the check engine light is on. Scanned and got the following codes.

P0125, P1150, P1155

I understand that the P0125 is for the coolant temp beign too low for close loop and P1150 and P1155 are for the Air Fuel Sensors. I did read that the P0125 could be caused due to the bad Air Fuel sensors.

Should I just replace the two Denso 234-9009 pre cat air fuel sensors? And hope that all three codes go away?

Thanks

d72polara

I'm only a "shadetree mechanic" but I'm pretty sure you shouldn't do what you're suggesting. As I recall, you are supposed to start with the lowest numbered code as they can set off the higher numbered codes. Lets start with our coolant temperature...is your engine coolant temperature actually low? Do you recall where your temperature gage normally reads on the dash? Does it seem like it's now reading cooler than normal? If that seems normal I'd check the operation of the coolant temperature sensor. Normally this is done by a voltage drop test. If you're not familiar with it let me know and I'll see what I can find online to help walk you through it.

hypervish

Welcome to Club Lexus!

First thing I would do is reset the CEL, and wait for it to come back on. If/once it does see which code pops up first again.

How many miles on the RX?

murphysf

Thanks, will reset the codes later today. The RX has 230,000 miles.

murphysf

thanks for the tip about the lowest numbered code, will clear them and see if and how they reappear.

I will also watch the temp gauge to see if warms up, I guess for the running temp the needle should be mid scale. I will also feel the radiator hoses to see how the engine warms up to check for a stuck open thermostat. thanks

murphysf

More info

When I got home from work I unplugged the ECU-B 7.5 amp fuse. Left it unplugged for about 30 minutes. Plugged the fuse back in and started the car and the check engine light was still on. I scanned the codes and they were still there. I then used the scan tool to erase the codes and the check engine light went out.

I drove for about 2-3 minutes and the CEL came on with a P1150 Fault and P155 Pending. I drove about 30 minutes more and checked it again and now I had two faults and three pending P1150 fault, P1155 Fault, P0171 Pending, P1150 Pending, and P1155 Pending.

Note P0125 did not resurface , maybe because the car never got cold, will rescan after driving after a cold start in the morning. Also note that P0171 appeared, which it was never there before.

I erased all the faults and will rescan after driving it to work tomorrow morning.

As for the running water temperature, the needle stayed at the second mark about the Cold Mark, note the dash water temp gauge has a Cold Mark and a Hot Mark and there are 4 marks in between the H and C marks, the temp stays steady at the second mark above the C mark this was after 30 minutes of freeway and city driving. Is this the normal running temperature. I ask this as I originally posted a P0125 (coolant temp too low for closed loop)

murphysf

More Info Part II

Erased all Fault and Pending codes last night.

Started car cold (55F outside temp) and drove to work, around 4-5 minutes after starting the CEL came on.

I drove 20 miles to work.

When I got to work I read the codes, 1 fault and 3 Pending.

P0125 Fault
P0171 Pending
P1150 Pending
P1155 Pending

so as I suspected last night the car needed a cold start to trigger the P0125. Perhaps since this occured first that this is causing the other codes. Now I need to investigate if it is my thermostat or my coolant temp sesor.

carguy07

I would replace the front A/F sensor first (it looks like that is the one that is showing up first). Don't chase the P0125, it will go away after the A/F sensor is replaced. One bad A/F sensor can trip codes for both, so do the one that shows up first). Denso only too.

murphysf

carguy07 thanks for the reply

the one that shows uo first is P0125 (coolant not up to temp for closed loop), and it happens around 4-5 minutes after a cold start which is when the ecu must timeout if temp is not reached.

So shouldn't I change the coolant temp sensor?

carguy07

My experience has been that the P0125 is a false code that pops up along with a dead A/F sensor. Replace the A/F sensor and the P0125 disappears. Anything is possible, but history seems to repeat itself on these things.

murphysf

carguy07, thanks again

So I drove home from work tonight and the check engine light did NOT come on, however I did scan it when I got home and had the following pending P0171, P1150 and P1155.

BearsLexus

Thanks Carguy... I just had these same codes pop - at least the 0125 and the 1150. I'll replace the A/F sensor first and see where we go from there.

Bear

djeddieo

anyone still subscribed to this old thread? my daughter just got the same 4 codes in her 99 RX300 (she's about 2000 miles away at college). she reports that the car is not running rough so is the A/F sensor all that critical to replace?

artbuc

Nonsense. First code up will be determined by the cause of the problem not the number of the code.