I have recently noticed a distinct increase in mpg while 2 codes are on (P1600 / P1433). Obviously my battery is on the decline and I plan to do a grid charge this next weekend (I have schematics and will have pictures of the charger/discharger if someone can suggest a good forum thread to post them on). But back to the main topic of this thread, lately my 2005 HCH1 w/ 215K has been averaging roughly 38-39 mpg with no CEL/IMA lights (used to be easy to maintain 40-41 mpg). Then about 2 weeks ago I didn't drive it for a little over a week and then the CEL/IMA lights came on. At first I figured it was because it hadn't been driven and tried clearing the codes, then disconnecting the 12V battery. The codes would come back within 10 miles of driving. Being my only vehicle I didn't have a choice so I continued driving. During the last few days I stopped trying to clear the codes and noticed that my mpg had improved quite significantly without any driving behavior change (around town/rural around 41-42 range). Yesterday the IMA light turned off on its own while on a 50 mile highway drive (CEL still on). As an experiment I had my wife clear the codes while I was driving (have a wifi OBDII always plugged in so no crazy driving involved), and instantly my mpg dropped from the 44-45 range (hwy) to the 39-40 range. Flat highway driving at about 65 mph and no change in driving style. For the rest of the drive it was a struggle to maintain 40 mpg. Near the end of the drive both codes tripped again and my mpg improved again. My question is what would cause the improvement in mpg while the codes exist? My first thought was that maybe the codes were causing the computer to run in more of an open loop and maybe one/both of my O2 sensor(s) were failing (not sure how old they are, but I have put roughly 50K on them). Maybe once a year I get a P2000 code, so maybe that is an additional indicator. Only other thought was maybe the battery was needing some additional charging that wasn't indicated and put extra strain on the engine during normal operation but not while the codes were tripped (feels like a long shot).

 

we're in exactly the same boat. I let mine sit for a week and same thing except my ima and cel stay on unless I clear them.
I'm regard to your post, I think maybe the dash gauge just lying to you. Did you fill up and do math? Mine is usually 2 or 3 mpg higher on Gauge than reality... will report ima light on mpg in a couple weeks as I only drive 30miles a day and just reset it for 3rd time. (Interestingly, when I parked car SOC was full, after reset it's maybe 4 from top, wonder if it's more accurately guessed post reset)

​​​​​certain times I swear I get more power with the ima lite on but more often it's uses less assist or acts close to normal.

 

I have been too lazy lately to compare the dashboard mpg indicator with fill/drive mpg. All of my numbers are based on the dashboard indicator. Even if the dashboard is slightly off from actual its ok as long as its consistent. I acknowledge that there could be something fishy affecting the reported value, but somehow that seems unlikely. I have a pretty good feel for the mpg in certain situations, locations, speeds (driving alone or with others, outside temperature, winter/summer ethanol mix, etc).

For sure my battery is struggling. Before the week off, the battery was working pretty good, good assist and charging, stable SoC. It would really only force the charge when SoC was about 2 bars below half. Now it has completely changed, full one minute, then nearly empty the next, forced charging when 3 bars from top. I have noticed significantly less assist and charging, as expected in degraded mode, which has made acceleration noticeably weaker.

 

I'm telling you, the dash gauge is off.
I HAVE done the math at the filling station.
All 4 tanks its been 2-3 MPG under what the gauge said it should be. It lies to make you feel better.
Most upsetting is that its barely better than my 98 civic with over 200k miles on it.

I wonder what resetting the computers CEL does... perhaps reset something in the MPG calc too? maybe you were trending down but the reset cleared the prior higher values.

 

You are describing recalibrations. They're frequency increases as the battery deteriorates. They can destroy mileage by 20-30%.

 

That has been my experience too.

Regarding the apparent increase in MPG - it could be a false correlation. The MPG is definitely correlated with the ambient temperature and is best when it is fairly warm out, but not blazing hot. I don't have exact numbers, but somewhere in the 75-85F range seems to be optimal. If the temperature in the OP's location went from say 50F to 75F at the same time as the warning light changes it might "look like" the MPG change resulted from the lights, when in fact the improvement followed from the change in the weather.

Also in some instances relatively small changes in driving habits can cause big changes. For instance, if one normally drives 6 miles to work, and then 6 miles back the car may have been running optimally for about 4 miles and suboptimally for the first 2 miles each way. If the routine changes to a 10 mile commute it becomes 8 optimal, and since 8/10 > 4/6 the MPG will go up. Similarly if the commut changes to just 2 miles each way it will go way down, since the car will be running open loop most of the time it is in motion.