This person had same issue with sensors and their posts to themselves suggest loose wire https://www.toyotanation.com/threads/o2-sensor-issue-code-p0125.322160/
this post was submitted on 18 Apr 2025
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My 99 Corolla ran fine until 100k when I noticed excessive oil consumption. Over a period of months it became slightly underpowered and then dramatically lost power at high speed when cylinder three was permanently misfiring. Huge cloud of smoke indicated failed piston rings which explained the oil consumption during previous months. Apparently a relatively common problem. I wonder if this would foul the O2 sensor and cause the MIL to trigger eventually?
It hasn’t been consuming oil or misfiring, so I don’t think it’s that, it is showing more signs of an air leak from what I can tell
I think you're correct, more likely to be a leaky joint or sensor out of calibration. The post was triggering because of my experience with a 99. I spent months tracking down a water leak into the passenger footwell and then the engine blew 😂
That sounds about right! And about average for every older vehicle I’ve had once they start causing these rabbit holes type issues
The skills of buying, maintaining and disposing of used vehicles are various and judging when to let go is critical. Sent my much cherished Volvo V40 to the crusher in running condition because it needed a cam chain and a catalyst that would have doubled the value of the vehicle 😅 Still unsure it was the correct call.
Oh I know! I about have everything diagnosed and despite the rabbit trails the fixes are all pretty cheap. Smoke was 2 separate issues: clogged pcv hose, and dying cdi boxes causing incomplete ignition. The check engine light is the upstream O2 sensor, so now I’m just waiting on parts.
Late to the party but if it is a vacuum leak you can use some kind of flammable aerosol to find it. Quick little spurts around the throttle body and vacuum lines with the motor running and you'll hear the idle increase if it gets sucked into the engine and you'll identify where the leak is.
It sounds like a sensor to me though. Don't know which, but if it runs okay without smoke for a while after clearing codes then starts smoking after a drive and throws the MIL it sounds like it's fine when the computer is in open loop using predefined fuel mixtures, but when it goes into closed loop it starts dumping more fuel into the engine than is needed. If that's the case and the computer is forcing a rich condition you'll wanna get it figured out before the catalytic converter plugs and needs to be replaced.
If your familiar with motorcycles are you familiar with reading spark plugs? Probably worth pulling a few to try and see if the engine is experiencing a rich or lean condition.
What did the code reader said beside the code? From my experience, bad o2 sensor doesn't always lead to performance issue, and usually it will just bad fuel consumption. Maybe get a used one and see if it fixed the issue
It's a very basic code reader and only had stats listed as "ready" or "not ready." the O2 sensor heater monitor (device says HO2S) says ready but the O2 sensor says "not ready." I'll see if the coolant seems to be heating up this am and try reseating the cable for the O2 sensor and if the radiator hoses heat up probably focus on the O2 sensor first if the light comes back on. Worst case I get an actually decent obd2 scanner!
Do you have and obd scanner? Maybe you could look at the o2 sensor live data to rule it out or not. The running rich seems in line with the open loop fuel management. Maybe it’s something electrical like thermostat or temp sensors. You can test the sensors with a multimeter I think. Here’s what somebody said about testing the thermostat:
Feel the upper radiator hose after the engine has warmed up and determine if it is very hot or not. If it feels relatively cool, the thermostat is most likely the cause.
You can also use the obd to see what temp the radiator fluid is getting to.
Good luck!
I would need a better scanner unfortunately, relooking at the manual mine will only say if a device is reporting. O2 is listing as "not ready" but there is nothing for coolant. I'll check the connection on the O2 sensor and clean the housing this am and drive for a bit and feel the radiator hose and see what I can find.
I have a pretty affordable Foxwell ($50) scanner that shows live data but there are a lot of other options. You could go to an auto store and ask to borrow theirs. You should see both O2 sensor jumping between 0.1 and 1. I’m not a pro though so should probably watch some videos or ask somebody else about how to text the O2.
Good luck!
I got one from Walmart for $50 that allowed for a phone app to record while I was driving home. Coolant hung from 174-184 (F), and it's the upstream O2 sensor that is showing issues. Specifically running lean, never getting over .065v and reading 0 several times at idle, which was also about 20 rpm low sitting at roughly 685 instead of 700.
So googling tells me it could be:
- Vacuum leak at the throttle body (the common reason)
- Dirty MAP sensor
- Old O2 sensor
- Exhaust leak
- Weak fuel pump
- Dirty fuel injection
- Restricted fuel line
Only the first 3 really seem likely so I'll probably start with those and reply to this thread generally once I have a final fix. Thankfully none seem all that major.
Nice job chasing it down. Hopefully it’s one of those, but either way a learning lesson and fun if you like working on cars
Definitely a learning lesson! I really don’t care for any vehicle work larger/more complicated than my old motorcycles but I also prefer to not pay a mechanic to fix this stuff as often as possible.
For sure. I imagine working on a bike might be easier in that there’s a lot of open space to get to what you need to?
Usually yes. The bike I work on the most the whole motor falls out the bottom if you remove five bolts so even large overhauls are manageable. Other routine maintenance things are also placed in convenient places.
The clutch is on the side of the motor (usually the bottom left) and come out with just that cover removed.
The suspension I replaced without any tools besides a socket wrench as I could put them on the wheel and just lift the wheel by hand to place it correctly onto the other side of the frame.
That sounds so nice haha. I’ve done some jobs on my cars where removing a single bolt took almost 30 min.
So there is an exception to my past statement. Yamahas. The last one I took the engine apart on every bolt was a different damn socket size!
Haha wtf? Any idea why?
No clue. Swore I’d never buy another Yamaha…but I failed and have another one. This one I’ve had to do nothing but normal maintenance on and it looks like the same thing. 2 sizes of Allen keys and a socket wrench to remove the plastic fairings just to get to the oil filter for an oil change.
I’ve just started to budget for a mechanic doing all the larger maintenance on it this time!
Haha Yamaha makes good bikes? Riding a motorcycle sounds fun but I’m afraid of hurting myself or getting hit by somebody
They do, though admittedly I’d steer anyone towards Honda over anyone else for price and longevity. We have a 1978 Honda in the garage that runs almost like new still!
Now I won’t say motorcycles are as safe as cars (statistically they’re not), but I will say it is less risky than many people think. The majority of fatal crashes involve no helmet and a very large portion involve alcohol unfortunately. The average commuter who gear up is fairly safe if they’re conscious of the weather and road. Plus they make vest worn airbags now that are showing incredibly promising things if you do know someone who rides. Something like a 89% reduction in spinal injuries. (YouTube link)
Yeah Honda’s are nice. It just sucks cuz even if you don’t get into a major accident, a small hit could fuck up your day vs a car you just carry on.