My RX400h was at the dealer for a recall issue when they recommended a new 12V battery. I went ahead with replacing it. Immediately afterwards, my around-town mileage dropped from ~28 mpg to ~24 mpg. Since it was winter, I thought it might just be the cold. But the consistent lower mileage persisted even after the weather warmed up.
I discovered this thread about the ISC "learning procedure" and I did it today while the car was fully warm. It's a strange sequence of things to do, but I followed the instructions very carefully. Immediately afterwards, my wife drove us around about 30 miles of mixed driving and got over 30 mpg! I drove it, and still got over 28 mpg. The car seems back to normal. I know this car is more muscle than Prius, but I appreciate the little bit of extra efficiency.
If the dealer replaced the battery and did not run this procedure, I feel they should be faulted. On the other hand, they changed the battery during a bitterly cold winter, and it might have been difficult and inconvenient to get the engine fully hot. Incidentally, I also had to reset all my tire pressure limits myself, first deflating all my tires, setting the lower pressure limit, and then putting all the tire back to the pressure I like. I must say that replacing the battery is a pain on this car!
Thanks to all for this useful thread.
—Carl
