2011 D4 'Gearbox Fault' warning
The ZF procedure for the 6HP26 says the level is correct when a small stream comes out at 40degC.
You start checking between 30-35degC, with the engine idling cycle between D and R (pause briefly at each) then back to P, then remove filler plug and wait for the temperature to hit 40degC where only a small stream should come out.
If you check above 50degC then the resulting level will be too low.
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2011 D4 'Gearbox Fault' warning
It is the ZF 8HP that has the different procedure and you need to get above 75degC first to open a thermostat valve, but then wait for it to cool down below 30degC to start the level checking - which is then done between 30-50degC but has more steps involved where you rev the engine at 2000rpm for 30 seconds once above 30degC and also hold it in D, D1, D2, and R for 10 seconds each before then checking the final level.
Whilst 40degC would be the mid point for the correct level the ZF 8HP procedure does not set an exact point like the ZF 6HP as long as you are above 30degC when you start, and still below 50degC by the time you are finished.
With the ZF 6HP26, after an oil change and level check as per the ZF procedure, I still then take it for a drive to get fully hot, then redo the level check procedure starting from cold the next day and rechecking at 40degC. It then may need another top up suggesting the ZF6 procedure probably should add the same “get it hot” step as per the ZF8.