And here is a Non-AI response.
Remove the Lambda sensor and test it to see whether it is actually working.
Remove it and place it on a flameproof surface. Connect a multimeter (set to the smallest voltage scale it has) to the black and white wires.
Now heat the tip of the lambda sensor. It's not an issue to get it red hot. You should typically see the voltage climb to around 900mV when at its hottest.
This will also clean the sensor of any carbon deposit or fouling. If it gives you the correct voltage reading, put it back and test with the GS911 again.