what does fuel trim mean? vauxhall zafira 1.8, 2002

kieran, May 2011

Fred & Raz thank you so much for your help, it is the lambda sensor. know my question is can i replace it myself or am i better taking it back to garage?

thanks again

kieran, May 2011

I assume you have a P0170 fault code. In practical terms this usually means there is unmetered air getting in to the system. Check the plastic trunking between the air filter and the throttle body. It can also be caused by an exhaust leak (the leak does not just let exhaust gases out, it also allows extra air to be sucked in which sends the lambda sensor out of range).
Can you access live data and check if the fuel trim is showing a positive or negative deviation?

fred, May 2011

Hi Kieran your car runs a system of checks called open or closed loop. if you start your engine and the engine management system finds a fault the system stays in open loop and then puts a fault symbol on the dash if everything is ok the loop is closed its a circle of checks if the circle is broke by a faulty component the the dash light comes on and stores a fault in the vehicle computer.This is normally a code eg po1234 you need this code to diagnose the problem.The code given is only what the car thinks is wrong and not always the fault try typing in fuel trim fault on google for your car you may get the answer.Hope this helps.

Raz, May 2011

thanks Raz, but this is just foreign to me. i have no idea about cars, car went in to garage yeat and thats what they came up with and they didn't know what it meant either,
will it be easy to fix and will it be expensive?

thanks again

kieran, May 2011

It is a term used for the measurement of fuel/air ratio via the lambda sensor in the exhaust. Short term fuel trim is the measurement in real time you can see this switching from rich to lean if you have a scanner. Long tern fuel trim is the average measurement over a period of time.Hope this helps.

Raz, May 2011