Emergency Mode, Back-Up Mode "Limp-Home"

Emergency mode, back-up mode "Limp-home"
When a malfunction has occurred in the system that is confirmed (permanent malfunction) and is registered by the control module, modes are activated for certain systems and functions to handle the malfunction.
The purpose of these actions is to "protect" the control system and at the same time retain as much functionality or driving function as possible.
The minor malfunctions do not activate any back-up modes, there are different programs depending on type of malfunction and which control system to which it applies. All control systems do not have emergency modes or back-up modes.
Sometimes emergency modes must be activated immediately when a malfunction is detected, even if there has been no time to confirm and store any diagnostic trouble code. This is to maintain certain function, e.g., in case of malfunction of the mass air floe sensor for the engine management system, when the emergency program tries to prevent the engine from stopping.

Hint: Limp-home, emergency mode or back-up mode that may appear in case of malfunction is described in diagnostic trouble code information under replacement values.

For, e.g., automatic transmission TF-80SC AWD (XC90, B8444S) there are two back-up modes:
- Failsafe action (temporary action)
- Emergency/limp-home mode
Failsafe action is activated at the first detection of the malfunction, if the malfunction disappears the system returns to normal function.
Emergency mode is activated in case of less serious malfunctions Limp-home mode is activated for the most serious malfunctions.
The warning light in the driver information module lights up, and a text message is shown in the text window in the driver information module that emergency/limp-home mode has been activated. When the ignition is turned off and on again, no text is shown until the malfunction is detected again.

Note! If the malfunction disappears (intermittent malfunction) the control module returns to normal function first when the ignition is turned on the next time.

For automatic transmission's emergency mode, the following may take place, e.g.:
- Adaption function is blocked.
- Lock-up function is blocked.
- Function slipping lock-up is blocked.
- Function neutral check is blocked.
- The transmission only shifts to 3rd gear and reverse gear. All other shifting is blocked.
The above actions will be noticed by the driver since only one gear can be sued. Some may also notice that lock-up is not engaged. Then the vehicle comes to the workshop with a symptom for some form of lost driving function, probably also with the warning light on.
This symptom shall not be confused with the malfunction itself, instead it is a symptom of the back-up mode. The symptom that the malfunction itself causes (it it causes any) may be noticed briefly or not at all.

Note! If the malfunction is confirmed (permanent) it may be that the control module resumes normal function at ignition off and on again (symptom does not exist any longer). First when the malfunction is detected again (diagnostic trouble code test started and conditions fulfilled), the back-up mode is activated. The malfunction may then first be interpreted as if it was intermittent (non-existent or not active).