Huge MPG Loss

Discussion in 'Prius c Technical Discussion' started by CMJAnew, Jan 2, 2026 at 9:45 AM.

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  1. CMJAnew

    CMJAnew Member

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    I haven’t eliminated the EV system….however, having driven C’s for better than 600,000 miles combined (three cars) I’ve not experienced an MPG loss like this - WELL OVER 10mpg loss happened pretty much overnight - but not last night - LOL
    I have eliminated brake drag and wheel bearing issues. The car has new spark plugs and air filter.
    There is a noice that sounds similar to ‘traction control/stability control’ activation coming from the passenger front only when the ice engine is activated. At least that is where I believe it’s coming from.
    Also, when watching the onboard “home” screen it seems to get extremely poor mileage - like 35ish mpg at 65mph (history says it should be getting closer to 50 than 40 at those speeds).

    ThanksMuch, and happy new year
     
    #1 CMJAnew, Jan 2, 2026 at 9:45 AM
  2. bisco

    bisco cookie crumbler

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    so you're not tracking it at the pump?
     
    #2 bisco, Jan 2, 2026 at 2:16 PM
  3. Mendel Leisk

    Mendel Leisk MMX GEN III

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    The dash display, albeit skewed, is at least consistently skewed.

    Any new tires in the mix?

    Gas up somewhere other than the usual?
     
    #3 Mendel Leisk, Jan 2, 2026 at 2:34 PM
  4. bisco

    bisco cookie crumbler

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    12 volt or hybrid battery weak?
     
    #4 bisco, Jan 2, 2026 at 4:13 PM
  5. BiomedO1

    BiomedO1 Senior Member

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    If your only hearing a noise when the engine is running; there's a high likelihood that's an engine noise your hearing. Traction/stability control, wheel spin issue, suspension issue; you should also hear the noise when the engine is OFF too, since the car is also moving.
    You'll need to put the car into maintenance mode to track down that noise. Could be something as simple as, something rubbing up against the harmonic balancer.

    Good Luck..
     
    #5 BiomedO1, Jan 2, 2026 at 7:07 PM
  6. CMJAnew

    CMJAnew Member

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    I’ve confirmed the MPG with calculator too. I ALWAYS reset trip A at fuel up.
     
    #6 CMJAnew, Jan 3, 2026 at 3:24 PM
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  7. CMJAnew

    CMJAnew Member

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    Four New Tires AFTER the mileage drop. I gas up all over the place - no specific brand.
     
    #7 CMJAnew, Jan 3, 2026 at 3:25 PM
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  8. CMJAnew

    CMJAnew Member

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    This may sound stupid, but what is “maintenance mode”?
     
    #8 CMJAnew, Jan 3, 2026 at 3:26 PM
  9. rjparker

    rjparker Tu Humilde Sirviente

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    Maintenance Mode
    Continuously Idle Engine

    Press Start button 2 times without Brake
    (ignition On)

    Press Gas Pedal 2 times

    Press Brake and shift to Neutral

    Press Gas Pedal 2 times

    Press Park

    Press Gas Pedal 2 times

    (hybrid warning light will start flashing)

    Press Brake and Start button

    (starts engine, shutdown returns to normal)


    Cliff Notes:
    Ignition Mode, 2Gas, Neutral, 2Gas, Park, 2Gas, Start Engine
     
    #9 rjparker, Jan 3, 2026 at 3:30 PM
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  10. CMJAnew

    CMJAnew Member

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    Here’s an addendum or two to my original query;

    1. the issue started when a driver side wheel bearing was replaced.

    2. So today I put my cheap diagnostic tool on the car. I monitored the live wheel speed - the driver front is reading some 5mph off when compared to the other three. Like the fool I am, I didn’t check the speedo — nor do I recall if it’s reading faster or slower (I’ll try again with a passenger this evening).

    thanks for trying to help….i know I’ll not be fixing the issue myself, but I like to have some idea when ‘going in’
     
    #10 CMJAnew, Jan 3, 2026 at 3:33 PM
  11. rjparker

    rjparker Tu Humilde Sirviente

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    Continuous idle maintenance mode was suggested to allow you to find the noise; not locate the mpg issue.

    If you had a 5 mph difference from that wheel's speed sensor you would get a specific code for that sensor.

    Most Toyotas of that vintage display high mph to the driver and accurate mph to a scanner. At 65 it displays around 3 mph high.

    Typically drastic mpg drops are usually caused by a weak hybrid battery.

    I would also clean the throttle body and perhaps suspect a clogged catalytic converter.
     
    #11 rjparker, Jan 3, 2026 at 3:57 PM
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  12. Brian1954

    Brian1954 Senior Member

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    Use a scanner and look at the short-term and long-term fuel trims. If they look good, my guess is the hybrid battery is getting weak (losing capacity).
     
    #12 Brian1954, Jan 3, 2026 at 4:33 PM
    Last edited: Jan 3, 2026 at 4:43 PM
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  13. CMJAnew

    CMJAnew Member

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    No Code Specific to the driver front wheel sensor. With a passenger reading the scan tool, we determined the wheel is reading about 5mph slower than the other three AND the speedo.
     
    #13 CMJAnew, Jan 4, 2026 at 9:42 AM
  14. CMJAnew

    CMJAnew Member

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    Just to reiterate, the mileage drop was immediate….not a gradual change. And it happened the day of the bearing replacement. On another thread I mentioned unintended wheel spin due to the bottle jacks on the lift not being fully retracted (free spin in reverse). That is also the exact time the noise started. I’ll do the maintenance mode check today. Probably won’t learn anything …. due to my lack of mechanical aptitude.
     
    #14 CMJAnew, Jan 4, 2026 at 9:48 AM
  15. CMJAnew

    CMJAnew Member

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    I don’t understand the “….fuel trims….”
     
    #15 CMJAnew, Jan 4, 2026 at 11:09 AM
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  16. CMJAnew

    CMJAnew Member

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    I did this procedure….the dash indicated “maintenance mode” + “trac off” in rotation. But, this did not lock out EV. It still cycled into EV mode while running.
    So, rather than waste any more of you guy’s time, I’m just gonna contemplate my next move.
    I will report back with any findings going forward.

    thanks again
     
    #16 CMJAnew, Jan 4, 2026 at 11:14 AM
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  17. bisco

    bisco cookie crumbler

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    you had a bearing replaced and mpg immediately dropped? Occam's_razor
     
    #17 bisco, Jan 4, 2026 at 4:08 PM
  18. CMJAnew

    CMJAnew Member

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    Yes Sir. And that’s the same time the noise on the opposite side started.
     
    #18 CMJAnew, Jan 4, 2026 at 5:00 PM
  19. bisco

    bisco cookie crumbler

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    and that isn't worth taking a look at?
     
    #19 bisco, Jan 4, 2026 at 8:44 PM
  20. CMJAnew

    CMJAnew Member

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    Other than a manual turn of the wheel(s) while jacked up and in neutral?
     
    #20 CMJAnew, Jan 5, 2026 at 4:19 AM
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