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Can faulty cat converter cause battery problems?

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My Prius Gen 2 Battery has not been charging properly recently, and the car is therefore running more on the engine than it should. Toyota did some diagnostics today and said it was the cat converter (£1500 to replace). I'm suspicious, though... how can a faulty cat converter cause Battery charging problems, with the knock-on effect of the motor not running properly so that the engine is in use more than normal?

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Sounds fishy to me. A cat could be damaged by too much Oil or unburnt fuel getting through to the exhaust, but there'd have to be something else significantly wrong with the engine or fuelling to cause either of those. The ICE just running more shouldn't do that, unless it's already burning a load of Oil (piston rings tend to go on these).

Has the 12V Battery been changed recently? If it's bad it can cause various spurious issues, and I've seen it lead to various mis-diagnoses in the past.

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Thanks Aaron. The 12V Battery was changed about a year ago, so probably not an issue. Interestingly, the engine light was coming on intermittently some time ago - Toyota immediately blamed the cat converter then too, but another (non-Toyota) mechanic said it was almost definitely a faulty sensor, and that the cat converter was fine. He proved the point by taking the car to a testing station and getting an emissions report that was normal. The car passed its MOT a month after that, again proving that emissions were in an acceptable range. Not that I'm saying this implies a perfect cat converter, but it does seem to me that Toyota want to blame the cat converter for everything!

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Am I right in saying (I seem to recall seeing it somewhere) that the Prius is exempt from emission tests in the MOT.

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9 hours ago, kithmo said:

Am I right in saying (I seem to recall seeing it somewhere) that the Prius is exempt from emission tests in the MOT.

My mate with a Gen3 Prius takes his car for MOT to the local test station where taxis go for MOT which is more rigorous then “normal” MoT and he tells me they don’t check emissions. 

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As per Catlover and Kithmo, at my MOT testing station, the helpful chappie told me that there is no emissions test on these.  I think that when the first Priuses came into the UK it was discovered that you couldn't easily and reliably force the engine to run for the emission test, so they exempted it .

But I can't see any reason why the MOT emissions equipment shouldn't be used on the car for your situation (i.e. cat. verification), you just need to turn the heater right up with the windows open, or put the engine into the 'diagnostic' mode to make it run.  I have used the diag. mode, but it was a couple of years ago, the button pushing sequence escapes my memory, but no tools were required to put the car into it.

The figures you obtain for the engine exhaust will be the same as any modern catalyst-equipped petrol car, as long as it's warmed the cat. up.

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Thanks for these replies relating to whether or not an emissions test can be done... very interesting, and something I hadn't actually thought of. Getting back to my original question, however, is it possible for a faulty cat converter (in the exhaust system of the engine) to cause the primary Battery (for the motor) to not charge properly, and therefore for the motor to not operate properly? Two weeks ago, my average MPG was around 55 - now it is around 47, which indicates what I am feeling when driving the car, that the engine is doing more work than normal because the Battery is not keeping its charge, so the motor can't operate optimally (and the engine has to do extra work in its attempts to charge the Battery when it gets down to the last bar!)

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May be worth paying Toyota the £40 for them to do the HV Battery health test, may thow up that one or more cells have expired. If that is the case Toyota can replace individual cells or there are now specialist doing the same, at a reasonable cost. In other words, if it is a HV Battery fault, it is not always necessary to replace the whole Battery

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Thanks Joe. Toyota were meant to do a FULL diagnostic when my wife took it in yesterday, so I would have presumed they would have done a HV Battery health test, especially since she told them that the Battery wasn't charging properly. Maybe I shouldn't presume such a thing!!! Up until about 2 years ago, I was always 100% satisfied with my local Toyota dealership/workshop... that has dropped significantly since then, so I really no longer trust them, hence my questions here! Maybe I should drive it to another town and get the whole diagnostic done again!

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I have no experience of invoking the 'Hybrid Health Check' battery warranty that should come as part of a normal Toyota dealer service, but this hybrid Battery guarantee scheme has been (newly) extended to run for cars up to 15 years.  If you had had a Toyota service recently enough to be in that scheme (unlikely from your recently described events?) then you might be covered for hybrid Battery failure.

I can't see a link between the catalyst and Battery problem, only in the *extremely* unlikely situation that the car turns off Battery charging whilst it attempts to further heat up, what it judge to be, an 'underperforming' catalyst, but I'm not aware of any situation where the car works in that way.  It would be interesting to know the error code behind the Engine Management Light that you had.

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...You probably know that for £15 you can buy a 'dongle' to plug into your car to read/erase the error codes (of any modern car) via your smartphone?

These are amazingly useful!

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I didn't, actually, Gerg! I'm one of those who uses a car to get me from A to B, and not much else (much to my mechanically-minded father's eternal disgust!), so have never thought of looking into this. Where does one obtain one of these magic £15 dongles?! (I suppose I could just search online!!! 😜)

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The key search words are:-  OBD2 ELM327 bluetooth dongle

Basically, this is:-  On Board Diagnostic Version 2, and, ELM327 is the protocol it works under, I think.

If you have an Apple phone then the compatability can get slightly more complicated, definitely check for this in the description of the item.

If you search this forum, or post the question 'What OBD2 dongles are recommended?', you should get a lot of replies.

Mine is a few years old now, else I'd suggest that (Amazon marketplace bought) one, mine was £10.  Sizewise, they are generally a bit larger than a box of matches.

You are looking to connect to the car's OBD port that is a mandatory fitment on all European cars since the early 2000s, it is usually somewhere near the base of the steering column, and looks like an old SCART/Peritel video socket.  This connection is the one the dealer always uses.

As an app to run on your phone, you might try 'Torque Lite', which is free.  But there are others.

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IF the cat is the problem, you could leave your Prius parked in one of those areas where the tealeafs jack the car up and cut out the cat. (they dont know its faulty) and you could claim for a new one on insurance. Just expressing my thoughts in type. 🤔

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Cats don't usually go "faulty" as such they either work or they get blocked and you would know about it if it was blocked. The bit that fails on the cat is the oxygen sensor that screws into it and can be replaced on its own. I think if that was duff then you would probably get the engine check light coming on and the ICE would run rough. Rough running can affect economy. The only link I can see between the cat and the car trying to run the engine more is in effect it may be trying to run a warm up cycle to get the cat up to temperature when the engine is actually already warm. Is there any soot in the exhaust ?

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Further to Kithmo's post above, just so you know, the engine check light comes on when there is an error but, will turn itself off after a preset amount of time and engine on/off cycles if the error has not re-occurred (or if the error is cleared by a technician etc.).  So if the error event is recurring often enough, or continuous, it will stay lit.  The time at which the light switches off (if it does) is not so critical - the fault has not repeated itself within the preset time 'window', but the error should be still logged in the ECU.  (Viewable with your OBD2 reader!)

Just to confuse matters, there is a pre-cat oxygen sensor (lambda sensor, lambda sond - all the same thing), and a post-cat oxygen sensor, this is used by the ECU to establish that the cat, is working correctly.  But for the sensors and the cat. to work they all have to be hot, the oxygen sensors often have a heater built-in and the cat. needs to be at around 400 centigrade at its (ceramic) core.  The pre-cat sensor is also used to adjust the fuel injection to achieve the correct mixture in normal use.

All this catalyst technology became a mandatory fitment to EU cars in 1991, which is the event that encouraged (forced, really) manufacturers to ditch the good, ol' carburettor (notable exception here for Austin Rover and their electronic carburettor and 2-way catalytic converter - nice try guys!).

I recommend the above 3 paragraphs are best read over and over before bedtime if you suffer from insomnia.

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 As regards the original post, I don't understand how a faulty part of the exhaust system would cause  trouble with the HV Battery unless the heat from the catalyst is affecting the Battery compartment. In instances like these, I find that it can be a useful ploy to act a bit "honest John" and just ask the diagnosing  technician to explain how a faulty catalytic converter will affect the engines ability to charge the Battery since you honestly find it difficult to comprehend. Its like saying that there's cornflakes for breakfast because your aunties name is Ethel!

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