This is another post that will only be of use to G.M. technicians at the present time.

If you see a 2015 or 2016 CNG Bi-fuel Impala with a complaint of SES on and will not operate in CNG mode, check for P125D and P01A9 set current.

P125D is for the alternate fuel high pressure system small leak detected and P01A9 is for the alternate fuel rail pressure too high. If you see these codes, command the alternate fuel pressure regulator through the complete range of the control valves. If the up response is not immediate and the down response does not stabilize within 2 seconds, the alternate fuel rail pressure regulator is sticking.

The leak code is due to the high pressure bleeding through the regulator when the vehicle is off, causing a leak detection fault and the increase in rail pressure due to the regulator leak is causing high rail pressure on start up after sitting for a time. This sets the P01A9 code. G.M. is aware of this problem and a recall is forthcoming.

James from Florida