Long Term Fuel Trim Question
Thread Starter
Super Member
Joined: Jul 2017
Posts: 525
Likes: 59
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Long Term Fuel Trim Question
I have an M278 and had the CEL come on last fall. I have an iCarSoft scanner and it appears the B1 LTFT was high, anywhere from 12.5-19. A local mechanic diagnosed it as a faulty O2 sensor. I had both B1 O2 sensors replaced, and while the CEL went off it did not solve the high numbers on the B1 LTFT. I took it to another mechanic, but he didn't think it was an issue worth looking at. I have since noticed while the B1 LTFT is high at idle, it returns to normal while driving. Could this be a faulty injector(s) maybe sending to much fuel to a cylinder?
Super Member
Joined: Aug 2004
Posts: 970
Likes: 250
E550 Coupe 2wd (2016)
Mine, and others, typically read 5 to 10. I also noticed mine reads even higher a idle, but only under certain conditions and typically it's the STFT that does it. Short and Long work together so you really need to watch both, live, to see what they're up to.
For example, it could be your LT is 15 but your ST is -10, so really you're at 5.
When you say B1 O2 it doesn't tell me which Bank 1 sensor. You have to change the wideband sensors, which is Bank 1 Sensor 1. If you change that you should also change the other (Bank 2 Sensor 1) because they have similar life spans.
The #2 sensors, Bank 1 Sensor 2 & Bank 2 Sensor 2 don't have anything to do with trims. If your check eng light said to replace one of those, and he did, that would explain why there was no change.
There could be other issues, like a vacuum leak, dirty injectors or who knows what, but I'd first focus on ensuring there are two new wideband sensors up front (Sensor 1). If I recall correctly, your car has some miles on it, so it's possible they're original and dying.
For example, it could be your LT is 15 but your ST is -10, so really you're at 5.
When you say B1 O2 it doesn't tell me which Bank 1 sensor. You have to change the wideband sensors, which is Bank 1 Sensor 1. If you change that you should also change the other (Bank 2 Sensor 1) because they have similar life spans.
The #2 sensors, Bank 1 Sensor 2 & Bank 2 Sensor 2 don't have anything to do with trims. If your check eng light said to replace one of those, and he did, that would explain why there was no change.
There could be other issues, like a vacuum leak, dirty injectors or who knows what, but I'd first focus on ensuring there are two new wideband sensors up front (Sensor 1). If I recall correctly, your car has some miles on it, so it's possible they're original and dying.
Thread Starter
Super Member
Joined: Jul 2017
Posts: 525
Likes: 59
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Mine, and others, typically read 5 to 10. I also noticed mine reads even higher a idle, but only under certain conditions and typically it's the STFT that does it. Short and Long work together so you really need to watch both, live, to see what they're up to.
For example, it could be your LT is 15 but your ST is -10, so really you're at 5.
When you say B1 O2 it doesn't tell me which Bank 1 sensor. You have to change the wideband sensors, which is Bank 1 Sensor 1. If you change that you should also change the other (Bank 2 Sensor 1) because they have similar life spans.
The #2 sensors, Bank 1 Sensor 2 & Bank 2 Sensor 2 don't have anything to do with trims. If your check eng light said to replace one of those, and he did, that would explain why there was no change.
There could be other issues, like a vacuum leak, dirty injectors or who knows what, but I'd first focus on ensuring there are two new wideband sensors up front (Sensor 1). If I recall correctly, your car has some miles on it, so it's possible they're original and dying.
For example, it could be your LT is 15 but your ST is -10, so really you're at 5.
When you say B1 O2 it doesn't tell me which Bank 1 sensor. You have to change the wideband sensors, which is Bank 1 Sensor 1. If you change that you should also change the other (Bank 2 Sensor 1) because they have similar life spans.
The #2 sensors, Bank 1 Sensor 2 & Bank 2 Sensor 2 don't have anything to do with trims. If your check eng light said to replace one of those, and he did, that would explain why there was no change.
There could be other issues, like a vacuum leak, dirty injectors or who knows what, but I'd first focus on ensuring there are two new wideband sensors up front (Sensor 1). If I recall correctly, your car has some miles on it, so it's possible they're original and dying.
t I think I am going to change the vacuum pump valve this weekend. I currently have no codes showing.
Last edited by Davery; Yesterday at 09:05 PM.
Super Member
Joined: Aug 2004
Posts: 970
Likes: 250
E550 Coupe 2wd (2016)
I would still be wary of that old sensor potentially reading incorrectly and upsetting things, but of course it could be one or more injectors on Bank 1 are not supplying enough fuel. If it were me I'd swap out that O2 and dump in a bunch of injector cleaner, and cross my fingers.
