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In the manual for my 2003 Opel Agila it says:

Do not start by pushing or towing, Your vehicle is fitted with a catalytic converter, it must NOT be started by pushing or towing.

Why & how would starting my car in this way damage the catalytic converter?

Is there a serious risk of damage?

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    I'm curious about this as well ... nothing in my pea brain which is telling me the why this happens. I can understand how a massive dump of fuel could cause an issue with the cat, but I'm not seeing as how a massive dump of fuel would happen. Nothing I've read really tells the reason why, only that it happens ... at this point I'm not believing it does happen. Maybe I can do further research into this, but maybe someone will actually have the answer and beat me to it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 8, 2015 at 0:41
  • Maybe it's just a disclaimer, so if you do damage it by push starting, they can say "we told you not to do this"
    – George
    Sep 8, 2015 at 6:30
  • Owners manuals seem to be written by lawyers that demand that some clause that is relevant to a 50 year old car be included. In the days of carburetors it could flood the exhaust with fuel and then back fire and damage the exhaust, and only then if the ignition was not turned on. Vehicle owner's manuals can be very dumbed down. For instance, you must never turn the ignition off while driving as stated in the manual. But if your throttle sticks, you may have to turn it off while driving. In fact, I think turning it off while driving should be in driver's ed to train for an engine stall.
    – Alex Cannon
    Oct 7, 2021 at 2:15

2 Answers 2

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unburnt fuel can damage the catalytic converter. By tow starting the car you could cause large amounts of unburnt fuel to be pushed through the engine and into the catalytic converter.

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    Could you perhaps add info on HOW the excess fuel gets there? Just something about "because there's no spark to ignite the mixture" and your answer would be great.
    – Captain Kenpachi
    Sep 8, 2015 at 8:53
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    That is right, If the engine is not starting due to no ignition (maybe flooded or faulty ignition), but the injectors or carb is fueling, then the fuel will be getting blown out of the exhaust ports. I guess there will be times that the engine will start quickly from tow starting and there will be no problems caused, but it is easier for the manufacturer to say not to tow start.
    – HandyHowie
    Sep 8, 2015 at 9:16
  • How does fuel get into the cylinders without the fuel pump running and the ECU opening the injectors? How do you get injectors opening and no spark to burn the fuel?
    – Mark
    Jul 6, 2017 at 10:00
  • @mark Who said the fuel pump is not running and the injectors are not injecting? The battery can be low enough that the starter motor will not work, but the fuel pump and ecu will still operate. There may well not be the voltage available from the battery to generate the high voltages required for the ignition system. These conditions will cause fuel to be pushed into the catalytic converter when tow started.
    – HandyHowie
    Jul 6, 2017 at 10:43
  • I'm assuming the injectors aren't running because the engine hasn't started. My thinking is that if the car is being push started the system voltage is higher by virtue of the starter motor not adding additional load. So I'm trying to understand under what circumstances you would get injectors adding fuel but no spark in the context of a push start but not crank start. I can see how this would be an issue for a carburetor engine, but even then I'm not clear on how this would cause the catalytic converter damage.
    – Mark
    Jul 13, 2017 at 8:47
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This is counterintuitive: Lean mixtures at high exhaust flow rates can damage a catalyst. The most common condition for this is the engine running out of fuel. But push starting could put the catalyst in the same condition. How this works it that the lean mixture does not burn in the cylinder but does in the catalyst if it is hot enough. It is the same process as misfire. The catalyst sees much more unburned fuel than it can handle.

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  • How does a lean mixture not completely burn in the cylinder? If the timing is wrong you might get a misfire but that is not dependent on lean mixture?
    – Mark
    Jul 6, 2017 at 10:03
  • The cylinder and the catalyst are two different combustion environments. The cylinder is under high pressure with a single ignition point. The catalyst is a low pressure environment with near infinite ignition points. By design the catalyst will burn fuels that could not ignite in the cylinder.
    – Fred Wilson
    Jul 7, 2017 at 5:46
  • It seems to me that in the context of a push start the catalytic converter would be cold, and have zero ignition points. Am I missing something here? Or is this warning assuming a push start with a hot catalytic converter? I also understand it is designed to react with byproducts of the combustion process, more than fuel.
    – Mark
    Jul 13, 2017 at 8:40

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