Hard to say. With your recent IMA light, that may mean you're experiencing IMA battery issues with recalibrations.

How does that number compare to what it was a month ago?

 

1 month ago, I saw a gas consumption of 30 mpg. Before that loss of power starting 1 month ago, gas consumption was normal (38 mpg). When I drove to Orlando on the highway 1 month ago, it was 37 mpg.
I still see the loss of power, but it's less bothering now.

 

The reduced MPG is likely due to loss of power. Are you seeing any recalibrations on the battery gauge?

negative recalibration: assist is disabled. SoC gauge "marches" down to TWO bars at 1 bar per second. Car forces charging even when you would expect it to assist. Power and economy are significantly reduced.
positive recalibration: usually follows negative recalibration. charging terminates and gauge "marches" to full at 1 bar per second. Normal operation resumes.

This video depicts a negative recalibration starting at about 2:30 with forced charging followed by a positive recalibration starting at about 7:30.


If you are seeing the above gauge behavior regularly (more than once a month or so), you likely have a failing IMA battery. If you are not seeing that behavior, it may be the cat.

Other concerns:
Why did you get an IMA light?
If your 12V hasn't been replaced within the last 2-3 years, is it due?
Have you ever replaced the (-) battery cable or the ground straps from the engine and CVT to the mounts?

 

I will try to see any recalibrations on the battery gauge, I'll let you know.
I got the IMA light after driving from Miami to Orlando on the highway. I didn't drive for so long since 6 years.
My 12V has been replaced 7 months ago.
About your last question, I never did it myself. I gave the car to the garage for many maintenance and repairs and I don't know if they did that.

 

I drove this afternoon, and unfortunately, I saw a recalibration on the battery gauge.
What will happen next?

 

Did you notice that after the battery finished recalibrating (showed 3-8 bars after forced charging stopped) that your power improved?

Based on the information you've provided, your IMA battery is deteriorating and will eventually fail. It is the likely cause for your loss of power. It's also the likely cause of the IMA light you received during your trip.

You have three options:

1. Do nothing as it deteriorates and eventually fails. With inoperative IMA battery, you will not pass emissions if required in your area.
2. Replace IMA battery - about $2K
3. Purchase and install grid charger/discharger - about $500 and recondition your battery - no guarantees this will fix your battery, but given the progressive failure you're experiencing, it will likely be successful.

 

After the battery finished recalibrating, the gauge was at full, and I could see the assist bars reappearing when accelerating, and the charge bars enabled when not accelerating.

I was planning to sell this car in September so I can buy a new one.

If I decide to Purchase and install grid charger/discharger, which one do you recommend?

 

This is the nice one with the automatic discharger that removes the need to mess with light bulbs:

https://hybridautomotive.com/product...g-package.html

Here's the "value" one that uses light bulbs to discharge:

https://hybridautomotive.com/product...g-package.html

Note that I am an authorized installer for these items. I receive no compensation for recommending them, and I receive no payments of any kind from Hybrid Automotive. As an authorized installer, when I purchase a Prolong system, I receive a small discount.

 

I looked at the instructions on the web site you gave me: https://hybridautomotive.com/pages/install-hc2-1pnp
The problem is that I live in a condo with my car in a parking lot outside, so if I install the car harness following the instructions above, I cannot plug the charger to the wall.
I think this is good when you park your car inside your house, with access to a wall plug.
Am I right to think that?

 

Correct. If you can't plug in overnight either in a your garage or driveway, it's difficult.