A question about modern electronics

[Bren]

When our vectra goes into limp mode why does turning it off and on again get rid of limp mode for a period of time?

Surely if something is kaput it would be in limp mode constantly?

Any knowledge appreciated?

[cobblers]

Things can be various "levels" of kaput, and sometimes the ECU needs a while to decide that it's sure a sensor value is definitely wrong before dropping into limp.

It could also be an intermittent fault, so again the ECU will take some time to be sure that there's an issue before dropping into degrade mode.

 

For example, on an ECU test jig at work I can run a truck ECU with 25PSI of boost at zero throttle and on idle. It takes about 10 seconds before it decides that the boost pressure sensor is faulty and falls into degrade.

I can run it at 3PSI of boost for 5 minutes before it realises something is amiss - and it decides that the fault is probably the turbo actuator rather than the pressure sensor.

[Guest Hooli]

You have to have the right conditions for the fault to appear too.

 

Random example, duff temp sensor that always reads cold. The ECU won't decide it is faulty until it's been running long enough to know it should be at temperature,

[SiC]

Some manufacturers keep a code active and stay in limp until the codes are cleared. However others will store the code as historic and not active after a ignition cycle. Mostly because sometimes if it's an intermittent fault like a damaged cabling, dodgy sensor, etc then there is no need to force limp mode permanently for a brief blip. Also as others have said, it can be difficult for an ECU to figure that a value it's receiving is indeed not valid.

 

The idea of limp mode is that the ECU isn't getting enough sensor data to allow it to run properly - be that efficiently or cleanly. But instead of dumping you dead by the side of the road, it'll let itself run by making safe assumptions on what the failed sensor could be, so you can get home. That safe assumption generally means a lot less power - especially in the case of turbo'd cars where limp mode generally means little or no boost allowed.