Can someone explain to me what is limp mode?

[RS6]

How do you know if its on and in use? is there a dash light?

just interested don't see anything in the handbook

[sambryant]

the turbo becomes restricted. you get only half boost, so the car feels like a 1.1 corsa after a while i think the eml comes on the dash, some times it takes a while, howere there will be a fault code on the ecu.

[MRA23]

turbo?

[darkhorse]

Limp mode initiates when the ecu detects a problem that can cause damage to the engine or components. The car engages limp mode and allows it to run in a simplified state to enable the driver to take it a short distance to safety or garage etc

[Muttley]

Limp mode is a description shortened from "limp-home mode", and is a catch-all term to describe the ECU's response to failures of the sensors or actuators. There are many limp modes, depending on what has failed.

For instance, if the accelerator pedal sensor fails, the ECU runs the engine at a high idle speed to allow you to reach a safe stopping point. Once you switch the engine off, it won't start again until the fault is fixed. This failure will light up the CEL on the dash.

If the MAF fails, then a fixed value for air mass is substituted, resulting in power reduction. This failure does not trigger any dash light.

[RS6]

so the indications of this would be a dash light & reduced power

can't see how a light would not show up on the dash registering a fault if you can only tell by plugging it in to vag com

you would never know sort of thing if you could only read the error code off the ecu

[Muttley]

It's been reported many times on here that MAF failure does not cause the engine light to come on, but does restrict performance i.e. drops the ECU into a limp mode. Perhaps this is because it is not a critical issue, the engine will not suffer and the emissions are not compromised by the reversion to a fixed value for air mass?

Intermittent faults are logged to the ECU but also don't always trigger the engine light.

All limp modes reduce performance, since the ECU is responding to a sensor failure which reduces its ability to control the engine precisely. Some failures will shut the engine off completely (modulating piston sender in a distributor-pump diesel, for instance).

[wayne lcr]

its like when you add to much beer and your lass wants sum nooky and you lad goes in limp get the jist only kidding its a protection for your car if theres a problem sum were it does this so you dont total your engine and gets you home untill you can do the logs and find the problem

[wayne lcr]

also you would no for defo cos it slow and there will be lights on the dash like a xmas tree

[ibizacupra]

MAF under reading is not limp mode.. nor drops it into limp mode..
Limp is actuator pressure only, when ecu decides its unhappy, and turns off the n75 valve.
ign off on often resets it, as can the ecu itself on continued driving for a while.

vag-com is essential to interrogate what it did'nt like. not all liimp modes will trigger a dash light

[djgurps]

I agree.. My LC often looses the turbo fully! No light on the dash, just switch off the engine and back on again its the boooost is back! Nothing also showing on VAG-COM! Weird?

[Muttley]

i've always read the phrase "substitute function" in the tech brochure as equivalent to limp mode, as it is a limitation of the engine power imposed by the ECU. On MAF failure the ECU substitutes a fixed value for air mass.

Are you talking about the MAF reading low, but still within expected limits? I guess this would not be seen as a failure. But I am guessing, and working from technical descriptions rather than real-life experience, which is the best guide.

[andycupra]

Originally Posted by Muttley

i've always read the phrase "substitute function" in the tech brochure as equivalent to limp mode, as it is a limitation of the engine power imposed by the ECU. On MAF failure the ECU substitutes a fixed value for air mass.

Are you talking about the MAF reading low, but still within expected limits? I guess this would not be seen as a failure. But I am guessing, and working from technical descriptions rather than real-life experience, which is the best guide.

To clarify regards MAF: (this sensor measures the quantity of air entering the intake)

If it 'fails' ie stops providing a reading or one thats detected as faulty as its out of limits, then you WILL get a light on the dash (ESP/traction control light).
The car will then run using alternative data but not in limp mode.. it will actually run near to normal power, some people wouldnt even notice. Its likely to run a little rich.

However the other problem people may report with the maf is reading low, in this case its providing a reading which is within limits so the ecu does not detect an error, but the figure provided is lower than it should be so the engine runs down on power.
As said above, this is not limp mode, but rather a sensor reading incorrectly (less air, less power) and as a result the engine produces less power than it should as it uses the incorrect data from the sensor.

Limp mode is where the ecu has detected an error and limits boost to prevent further damage. Limp mode depending on the fault may or may not reset after ignition off/on.

[Tomoose_86]

Originally Posted by andycupra

To clarify regards MAF: (this sensor measures the quantity of air entering the intake)

If it 'fails' ie stops providing a reading or one thats detected as faulty as its out of limits, then you WILL get a light on the dash (ESP/traction control light).
The car will then run using alternative data but not in limp mode.. it will actually run near to normal power, some people wouldnt even notice. Its likely to run a little rich.

However the other problem people may report with the maf is reading low, in this case its providing a reading which is within limits so the ecu does not detect an error, but the figure provided is lower than it should be so the engine runs down on power.
As said above, this is not limp mode, but rather a sensor reading incorrectly (less air, less power) and as a result the engine produces less power than it should as it uses the incorrect data from the sensor.

Limp mode is where the ecu has detected an error and limits boost to prevent further damage. Limp mode depending on the fault may or may not reset after ignition off/on.

I seam to have developed this problem after fitting a NEW MAF. I changed the MAF because I had uneven and sometimes lumpy idol and the engine management/excessive emissions light was coming on with error code P0171 - system too lean bank 1. This seams to have fixed this problem because the dash light hasn't come back on and runs better but still the occasional waver on the idol.
But now I have the new problem of random loss of power, usually when I put my foot down a bit and sometimes the ESP/traction control light comes on. I have tried disconnecting the battery to rest the ECU but that doesn't sort it so is my new sensor a doggy one that doesn't work properly or does something need resetting using vag com.

I'm going to try looking with my mates laptop end of this week to see if there is an error code that's being thrown up in the memory but not sure what to do next any ideas?