Why does my key fob not work first thing in morning?

How is the CR2032 button battery in the fob?

What is the voltage of the 12 V battery in the right rear first thing in the morning?

 

When the original 12V battery in our Prius began to age, fob reception problems were one of the symptoms. The fact you are seeing trouble first thing in the morning somewhat fits a 12 V problem, as overnight the battery voltage will have dropped down a bit.

If that is really the case, probably things will get worse when it gets colder.

 

When you walk up to the car with the fob, does the red light on the fob come on? This indicates that the fob detects the car and is active so if the red light does come on, it's a problem with the car system, not the fob.
How old is the 12 volt battery? I've heard that many (most?) strange electrical problems start with the 12-volt battery going out.

 

This part should have nothing to do with the battery, if you are testing while the car is READY and running. When READY, the 12V system is powered and regulated by an inverter from the traction battery. This is the Prius version of the alternator & voltage regulator of a traditional car.

There is a scroll wheel brightness adjustment to control if, and how much, the dashboard lights dim at night when the headlights are turned on. If you don't want them to dim, e.g. when using the headlights in daytime, roll that adjustment wheel all the way up, then 'clicks'.

 

Measure the 12 volt battery with engine off first thing in the am before starting the car.

Measure it at the front jump under the hood with car off. It is useless to measure it directly.

If your not close friends with the 12 it probably has a corroded ground connection too. This can be detected by measuring at the front jump point. Then measure directly at the battery. If more than exactly .5 volts less at front JP there is a connection/corrosion issue in the hatch.

12 volt battery is probably sitting in water too. Very leaky back there.

 

post #3 nailed it

 

what was the 12v reading?

how old is it?

 

was that the problem?

what load tester? did it show voltages?

 

does it?