# Catalytic Converter and push starts **URL:** \ **Category:** Factual Questions\ **Created:** [March 22, 2004, 1:08pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715 "2004-03-22T13:08:32Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 22, 2004, 1:08pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/1 "2004-03-22T13:08:32Z")
I was reading the manual of my car and noticed, in the section concerning the catalytic converter, that you should “never start the engine by pushing or pulling the vehicle” Why the heck not? ---
**Author:** ![Starguard](https://avatars.discourse-cdn.com/v4/letter/s/8dc957/32.png) [@Starguard](https://boards.straightdope.com/u/Starguard)\ **Post date:** [March 22, 2004, 1:13pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/2 "2004-03-22T13:13:36Z")
What type of car do you have? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 22, 2004, 1:39pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/3 "2004-03-22T13:39:08Z")
I doubt it’s relevant, but it’s a Subaru Outback. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [March 22, 2004, 1:41pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/4 "2004-03-22T13:41:12Z")
Presumably they don’t want you to flood your catalyst with a large amonut of unburned gas/air mixture which gets sucked into the exhaust any time you do this. Also, upon starting the car sometimes there is a backfire or minor explosion in the exhaust, which could, in theory, damage the catalyst. However, in just the case of unburned fuel entering it your catalyst will recover from it after it heats up enough and has time to clear itself. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [March 22, 2004, 1:42pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/5 "2004-03-22T13:42:42Z")
Sorry about the typo. My hands don’t work completely well until another hour or so these days. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [March 22, 2004, 2:01pm UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/6 "2004-03-22T14:01:13Z")
> [@Una Persson](#): > > Presumably they don’t want you to flood your catalyst with a large amonut of unburned gas/air mixture which gets sucked into the exhaust any time you do this. Also, upon starting the car sometimes there is a backfire or minor explosion in the exhaust, which could, in theory, damage the catalyst. However, in just the case of unburned fuel entering it your catalyst will recover from it after it heats up enough and has time to clear itself. Exactly what Una Persson said. My old Seat Toledo ('92 model) manual says that is is okay to push-start the car, but not for too long (ie. there’s some serious engine problem and the car won’t start). I have noticed that newer car manuals do not have this warning. I believe that catalytic converters have changed dramaticaly since then. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 23, 2004, 3:59am UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/7 "2004-03-23T03:59:50Z")
I have a bad feeling I’m completely missing something here, but why would this flood the CC with unburnt fuel air mix? When you start the car normally you turn it over till it catches using the starter motor. When you push start the car you roll it till it’s moving then drop the clutch and the driveshaft turns the engine over and it catches. From the perspective of producing unburnt fuel/air mix, what’s the diff? Or is it just that when push starting you usually have a flat battery so it takes longer to start, so more fuel air mix gets through? ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [March 23, 2004, 5:18am UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/8 "2004-03-23T05:18:51Z")
> [@Princhester](#): > > When you start the car normally you turn it over till it catches using the starter motor. When you push start the car you roll it till it’s moving then drop the clutch and the driveshaft turns the engine over and it catches. From the perspective of producing unburnt fuel/air mix, what’s the diff? > > Or is it just that when push starting you usually have a flat battery so it takes longer to start, so more fuel air mix gets through? I’m not sure it’s as real a concern as they make it out to be, especially with fuel-injected cars and carburetted cars with a positive fuel shutoff system. But the thought is that there is a larger quantity of unburnt fuel when you’re having to push or pull start (as opposed to when you are hitting the starter, and the cylinders are firing, albeit not quite efficiently) which could enter the catalyst. This can temporarily “overload” the catalyst and make it not function correctly for a while. Or possibly cause a small backfire. I think the “danger” is over-rated and the note is more of a CYA. However, in a carburetted car, I’ve been able to purposefully make some horrendous exhaust explosions by shutting the engine off on the highway while in gear, then turning the ignition on again. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 23, 2004, 6:04am UTC](https://boards.straightdope.com/t/catalytic-converter-and-push-starts/235715/9 "2004-03-23T06:04:49Z")
Thanks Una. > [@Una Persson](#): > > I think the “danger” is over-rated and the note is more of a CYA. Yeah, in the same section they tell you that under no circumstances should you \*drive over grass[i/], as the CC is hot and may start a fire. This is the manual for a _semi off road vehicle_, mind you.🙂