Lambda changing with electrical load

If i'm sitting in my car idle and a high electrical load is switched on the lambda changes, what could be causeing this. I noticed it when the fans switched on but it will also do it if I turn the headlights on

Injector battery compensation (injector characterisation doesn't correctly account for reduced voltage from the load). This change in "dead time" is particularly important at small injector pulses, like using large injectors at idle.

I'll try and get a data log showing the difference in lambda. The injectors are stock 540cc items and the dead times are the standard setting link put on the base map which I was told from them should be pretty close.

As well as Davids suggestion, another common cause of error is if using an analog wideband device that doesnt have a dedicated signal ground, then you will have a ground offset that will vary depending on how much electrical load is passing through the chassis.

There are a couple more possible problems if this is a speed density (MAP sensor) setup. If the electrical load changes the Manifold Pressure, the ECU might be referencing fuel map cells that need some adjustment. Also, if there is an Idle Air Control Valve adding airflow that isn't measured well by the MAP sensor, you might need to simply add a bit more fuel to the cells so it runs a bit rich without the electrical load. Some cars have Electrical Load sensors that are measured by the OEM computer, possibly to account for this sort of situation.

The car does have an analogue wideband fitted, however I've tried my best to mitigate any ground ofset. I spoke to link prior to wiring it in an was told not to wire it into the sensor ground incase it drew too much current so I wired in into the main ecu ground. The wideband gets it's 12v supply from the same position and it's the 12v Batt+ supply for the ecu. Presumably this will be easy enough to check though if i log the raw voltage of the Analog in as well as lambda I should seee if there's a problem? I have wired my grounds into a star in the engine bay with only one cable running back to the battery (battery in the boot).

It's probably going to take some time before I can do anymore logging though as as a componenet on my board decided to let go so it's having to be sent back to Link to be repaired, hoepfully under warranty as it's only 2 months old.

In addition to that mentioned above, the Injector battery voltage or dead time compensation calibration could be out causing an actual fuelling problem but It could also be worth checking the actual lambda value with another externally powered source as ground loops can occur and wideband controllers are particularly sensitive to GL voltages