Mystery performance gain after maint (ATF change, Rear O2 extend/clear 0420 code)

Friend's car : 01 208k miles. Did ATF change (5 quarts Type IV, pickup tube o-ring) and added an extender to rear O2 sensor + clear the 0420 code (been coming on for awhile)

Friend swears the car has more power on hiway and is happy with the performance gain.

My understanding is the rear O2 ONLY checks for cat efficiency and doesn't change ECU otherwise as front O2 handles air/fuel ratio (AFR). Perhaps Prius's ECU decides to do something different if it thinks there is a failing cat? I've heard of extenders on front O2 improving performance by fooling ECU into leaner AFR but not the rear O2 which just measure cat efficiency.

Drained ATF looked pretty good (still some redness) and the typical film of mud on bottom of tranny pan.

We are happy with the gains but why?

 

haha yeah, I initially thought I put the extender on the front O2 after the report but it was indeed on the rear O2. Owner seems pretty convinced of the noticeably better performance.

 

Did a bit more research. This Prius have 2 Catalytic Converters. Here is the layout

Upstream O2 -> First CAT -> downstream O2 -> HCAC Assembly (VSV -> CAT or HC (hydrocarbon) absorber)

Here is an article that explains the HCAC.

Hydrocarbon Adsorber Catalyst System - Toyota Sequoia 2001 Repair

Here is a picture and diagram




Cylinder before HCAC's actuator is the first CAT, HCAC's CAT and HC absorber is behind the actuator.

When the HCAC's CAT is cold, VSV opens and sends HC to the absorber. After the CAT is warm, VSV closes and send the exhaust to the CAT.

So the question is... how does the ECU know if HCAC's CAT is warm? Only way is guess by engine temperature or does the downsteam O2's voltage readings somehow indicate temperature?

My friend also said he used to smell fuel on cold startup but not anymore after the downstream O2 extender installation . Does it make sense the the downsteam O2 extender somehow made the ECU think HCAC's CAT is warm and bypass the HC absorber?

Another thought is if VSV somehow remains open after car is warmed up. Don't have a very good diagram but the HC absorber channel looks narrower than the CAT. If so, maybe its creating back pressure at highway operation. My friend said he use to have to depress the accelerator nearly to the bottom and car would just stay at highway speeds (note this is all hearsay)

Or I suppose a possible explanation is the persistent 0420 code cause the ECU to detuned the engine in some way. ECUs and TCMs often go into limp home mode with bad codes to protect the engine. Although 0420 code is strickly emission and should not affect performance.

Car gets 45mpg so seems like pretty normal fuel burn (My Gen 2 gets 45 and Gen 3 gets 50)

Anyway, I think I'll check the VSV's operation and see if I can get some readings on the rear O2.

UPDATE 12/11/15

VSV works perfectly. Can see it move on engine start. Techstream diag tool can manually move it back and forth.