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I have a 2010 Forester with nearly 100k miles, and it just started giving the "emissions" check engine code (P0420, Catalytic Converter Efficiency Below Threshold).

When that code is tripped in the system, it responds by flashing 'cruise' and solid light indicating the traction control system is disabled. These systems are disabled - turning at speed is noticeably worse, and cruise certainly doesn't work.

Why is that? Is there a reason to disable the traction control system and cruise control when the catalytic converter efficiency is below expected levels? If it's just a matter of having to do something to show the driver what kind of problem there is, why disable traction control (which is a valuable safety mechanism)?

One Subaru dealer offers a potential answer, though it is sufficiently vague to be more like marketing speak:

To work properly, the cruise control system and vehicle dynamics control system require a powertrain operating at peak efficiency. When the check engine light senses a problem, your Subaru deactivates those other systems to avoid erratic performance.

If that's the case, I'd love to know why this is so important - in particular, why vehicle dyanmics control ("traction control" above) is affected here.

|improve this question
  • 2
    Great question. – Zaid Jan 27 '16 at 19:45
  • Are you sure the traction control is disable because of the CEL? A quick way to check this would be to clear the code and confirm that your traction control is back online. – Zaid Jan 27 '16 at 19:46
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    @Zaid Yes. We have had it cleared (once by accident once on purpose) and both times the traction control comes back. Subaru also states that this is the case - with a vague comment about engine efficiency that seems like marketing speak. – Joe Jan 27 '16 at 19:49
  • Maybe our resident Subie expert, @BobCross, has an answer for you. – Pᴀᴜʟsᴛᴇʀ2♦ Jan 27 '16 at 23:32
  • Am I correct then in assuming this is something specific to Subaru, and other cars don't have this happen when the CEL turns on? – Joe Jan 27 '16 at 23:34
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Because it's dangerous with erroneous data

This is actually very smart on Subaru's part. I have to give them credit.

If you have sensors that are faulting you could be giving an ECE false or eroneous information. The ECU would then take that false data and make adjustments to traction control and engine power. You could be on a wet or icy surface, step on the gas and spin out of control into a ditch with alligators, witches and zombies.

In order to prevent that, Subaru deactivates certain systems upon sensor fault to ensure your safety. This is also a great motivator for the operator of the vehicle to get the car serviced or somehow remediate the issue as quickly as possible.

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  • While erronoeus readings are dangerous, I don't think it answers the question. Why should the traction control system be tied to the operation of the catalytic converter? – Zaid Jan 28 '16 at 7:21
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    In embedded computer systems like the ECU resources are limited and sometimes shortcuts must be taken. In this case I'm guessing that the ECU doesn't analyse which code it is, only that some code has been thrown. Maybe it filters out a few of the simpler ones so traction control didn't get turned off because your fuel cap is loose lol. But I'm guessing this is not one of the filtered ones. – cdunn Jan 28 '16 at 13:37
  • cdunn is correct. All Subarus with an electric throttle do this. There is no filtering, any code disables cruise/traction. – Ben Jan 28 '16 at 22:27
  • Let me ask how much Subaru asks for a new catalytic converter for this car, because for the Toyota Prius they ask $1300 and is only found on dealers while most aftermarket will go for less than $300. – Gabriel Diego Mar 10 '16 at 18:12
  • @gabrieldiego you have posted a question as a comment. if you go here mechanics.stackexchange.com/questions/ask you can post your question. It would be out of scope as a price shopping question so you may want find a way to reword it. – DucatiKiller Mar 10 '16 at 18:17

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