I had an issue when sometimes my 2000 1.8L Corolla stalls if I roll in reverse. A few weeks later, I got a check engine code pointing to Bank 1 Sensor 1 as an issue. I replaced it, cleared the codes and thought it was cured.

A few weeks later, the light came back with a new set of codes pointing to the MAF sensor. Aside from a barely perceptible vibration at idle, it drove and continues to drive fine. I cleaned the MAF sensor and the check engine light disappeared. Again, I assumed the problem was cured.

Today, the check engine light comes back on with the same code. I am wondering whether there's a leak in the system. Perhaps, the 02 sensor isn't snug enough on the exhaust? I've included the freeze frame for the code, I also included MAF volume vs the 02 sensor response (long term) graph when tapping the gas while in park. The last images are two pages of sensor data while holding the RPM.

Freeze frame data

MAF vs Bank 1 02 sensor (long term fuel trim

all sensors pg 1 all sensors pg 2

  • Welcome to Motor Vehicle Maintenance & Repair! Just curious why you were running the engine at redline? 7431rpm is quite high for most autos ... really high in fact. Oct 18, 2017 at 22:31
  • We need more data, Is this the 1.8l engine? What year is your Corolla? What's happening in the two graphs? What were the driving conditions? What happens to fuel trim when you snap the throttle to WOT? Does it reflect the graph posted? Does LTFT go high or does LTFT zero out? What kind of voltage are you seeing at the front AFR or O2S? What kind of load are you seeing at WOT? Can you perform a volumetric efficiency test?
    – Ben
    Oct 18, 2017 at 23:05
  • @Pᴀᴜʟsᴛᴇʀ2 According to the freeze frame data, I was only going around 32 mph. Perhaps, the app I am using is buggy.
    – ATL_DEV
    Oct 18, 2017 at 23:30
  • @SteveRacer I cleaned the sensor and installed it. The code went away and the readings were more down to earth. The code came back. The freeze frame data triggered at 30mph. A high reading like that would indicate a big vacuum leak correct? If it is a big one, I don't see it. If it is a small one, it should not be heard.
    – ATL_DEV
    Oct 21, 2017 at 19:33
  • Which codes are coming back? I'm not sure what you mean, but I don't think the MAF problem and the PO171 are exectly the same issue - unless it's the intake hose leak as I described in my answer. The PO017x codes are not set instantly, and require a "trend" before becoming a permanent code. But I would beleive the high LTFT and the O2 code is related. At those LTFT+STFT trims, you are really getting to the point the engine management simply cannot compensate enough.
    – SteveRacer
    Oct 21, 2017 at 19:42

1 Answer 1


LTFT at 29.7% seems mighty high. I'd start by looking for unmetered air leaks aka "vacuum" leaks.

Unmetered air might be fooling the primary O2 sensor - so called "false lean". Easy to check and a standard part of my KISS rule/Occam's Razor mantra.

Interesting about the "reverse" stalling comment. My spidey sense says to check any rubber or plastic intake hoses after the air metering for cracks or splits, especially any flexible "accordion" sections. What I have seen many times is that such a split is mostly closed, but still causes an unmetered air leak, which eventually leads to high LTFT and poor running and fuel efficiency.

However, reversing causes the engine to torque over in a different way, and potentially opens up that crack very wide ... creating huge bucking or stalling. This is from the airbox being mounted to the car, and yet the intake hose mounted to the intake - possibly compounded by weak engine/transmission mounts.

  • Thanks for your assistance. I checked all the hoses and tested the harness connector. Everything appeared fine short of a smoke test. I did a bit of research and discovered to my amazement that the ECU lies about the MAF sensor. It is possible for the sensor to fail, yet have the live data appear "normal." My repair manual suggests back testing the MAF sensor under operation. On some engines you can check it by disconnecting the harness after starting the motor. If the engine doesn't stall, it means it is no longer part of the closed loop and the ECU is compensating for it.
    – ATL_DEV
    Oct 22, 2017 at 10:31
  • I hope you found it, but I can't beleive the ECU is in a "limp" mode without throwing a permanent code. I think that would violate OBD rules.
    – SteveRacer
    Oct 22, 2017 at 23:59
  • @ATL_DEV what you've experienced is the ECU setting a simulated signal.
    – Zaid
    Feb 19, 2018 at 6:03

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