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2009 Forester X Premium
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54 Posts
Discussion Starter #1
I have heard people talk about Limp Home or Failsafe Mode. I actually rang and asked Subaru what is was and I was told that if certain codes are thrown the ECU does in fact go into what they call failsafe mode. They said that what happens is that the ECU stops using its long term fuel correction tables and resorts to its defaults allowing only short term corrections. Reading here I have picked up that the flashing engine light means that the failsafe mode has been activated while if the light is static it has not and some less critical error has occurred. My car is throwing the P0420 code and going into failsafe mode. My question is exactly what happens in failsafe mode?

1. Does the ECU clear its long term tables (equivalent of a reset)
2. Does it limit the short term corrections that can be applied?
3. How exactly does it behave, as in what is the process?

The reason for the question is I have a tech looking at the car who says he has never heard of a failsafe mode and I am not sure he believes I have it right... come to that I am not sure I have it right... took Subaru a day to get back to me about it! Anyway when he plugs his scantool into the car it reports that all the fuel trims are in the +-2% range which is great for a car running on LPG. I'm a little suspicious that once the failsafe mode is activated what he is seeing is the ECU running with some very tight limitations and the scantool is not actually giving him the full picture. This of course makes it very hard for them to diagnose any LPG related issues.

This is the current sequence of events. First up the car drives very well but then is slowly starts to develop a dead throttle response, this gets worse until its quite noticeable. It appears that the fuel consumption is going up across this time and I am guessing that the ECU is taking the LPG system rich causing the dead throttle response. Then the CEL light comes on and the car starts to drive normally again, again I assume that is because the ECU has gone back to default settings and failsafe mode is not allowing it to wander to far. The car drives quite well in this state! Then we get back to the techs, they plugin the scanner and say that the fuel trims look fine, clear the code and away I go again.

I think I have had multiple issues with this car that has made the LPG install struggle from the start. I have replaced the A/F sensor and the O2 sensor so far hoping that the P0420 code would go, no cigar! However so far the new sensors seem to have dramatically reduced fuel consumption on LPG and marginally reduced it on ULP. They where not faulting but I think they where suspect, at least the AF one. Now I am working on getting rid of the P0420 code, hopefully without replacing the catalytic converter so that the LPG techs can get a clear shot at the car.

Anyway what do you reckon? Could the failsafe mode be throwing them a curve ball? Any Suby techs out there that would know?

Thanks in advance!
 

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Premium Member
2008 XS 4EAT
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9,878 Posts
Sorry SloobyLoo, I don't have an answer to your problems, but you may want to update your Profile info with location and add your vehicle info into a Signature that will be seen every time you post. Just go into your User CP, Edit Profile add location, Edit Signature add vehicle info including LPG conversion. It took me a while to see by searching your posts that you're from Australia, Any luck with posting the question in there as well? LPG conversions seem to be more common over there and in Europe, so somebody may have an answer in those forums too. Any other conversion companies over there that may bring some insight into your problems. The trouble codes help, but I always thought the flashing CEL was indicating electrical issues related to ignition:shrug:. There are some links to other issues related to the P0420 code on Diagnose Check Engine Light Codes, Emissions And Oxygen Sensor Faults Let us know here if and when you have some success. Good Luck
 

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1998 Forester
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602 Posts
I thought the car automatically wrapped itself in bulletproof material and filled the passenger cabin with high density foam...just in case of attack.
 

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none none
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9,011 Posts
p0420 shouldn't make it go into failsafe mode as its just a cat inefficiency code (just the rear o2 sensor reading isn't within the expected values) this doesn't affect long term learning or anything. Front o2 sensor however affects the AF ratio and if failed and throwing a code will potentially go into failsafe.

Basically in failsafe it is in closed loop (?) or whatever it is normally at low rpms.
 

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Registered
2009 Forester X Premium
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54 Posts
Discussion Starter #5
I asked Subaru about that and the answer I got was it did go into failsafe on that code. As I have had it explained a P0420 code is thrown when the signal from the AF and O2 sensors sync up indicating that the cat is having no effect. Dead cat is the obvious cause but it is not impossible for it to be failing sensors (sluggish but not yet throwing a fault). Given that the prudent thing is probably not to assume it is a dead cat and go into failsafe mode.

However what has been described here, what I am experiencing and what I have been told by Subaru don't all line up.

1. The dash light flashes - supposed to indicate failsafe mode.
2. The cars behaviour changes, again suggesting failsafe mode.
3. The car is not limited in revs and drives quite well after the code is thrown - not what people here report when failsafe mode has been tripped.

So all I can think is that the failsafe mode has various routines that it goes into depending on the code. You are correct in that a P0420 should not be a big issue as far as the functioning of the engine goes. Subaru told me that on that code it goes into failsafe, ignores the long term fuel tables and goes back to default. My experience tends to support that! What I didn't ask was --> is that the only failsafe response or is there a number of failsafe responses.

I have a call into Subaru but so far they have not responded. I am hoping to talk to a tech that can explain exactly what the computer does. At this point it is more about understanding that than any issue with the LPG, until we have a firm handle on what the ECU is feeding back to us after a P0420 code has been thrown it potentially messes with any diagnostic attempts --- or not :confused:

Profile updated :biggrin: solly!
 

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Registered
2009 Forester X Premium
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54 Posts
Discussion Starter #6
Sorry SloobyLoo, I don't have an answer to your problems, but you may want to update your Profile info with location and add your vehicle info into a Signature that will be seen every time you post. Just go into your User CP, Edit Profile add location, Edit Signature add vehicle info including LPG conversion. It took me a while to see by searching your posts that you're from Australia, Any luck with posting the question in there as well? LPG conversions seem to be more common over there and in Europe, so somebody may have an answer in those forums too. Any other conversion companies over there that may bring some insight into your problems. The trouble codes help, but I always thought the flashing CEL was indicating electrical issues related to ignition:shrug:. There are some links to other issues related to the P0420 code on Diagnose Check Engine Light Codes, Emissions And Oxygen Sensor Faults Let us know here if and when you have some success. Good Luck
Thanks for the link! ATM I am most concerned about understanding the ECU and what it does, I don't think that the P0420 code is related to the gas as it has thrown on ULP as well, I think its just a compounding issue.

Profile updated, kinda, no where to say LPG!
 

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Premium Member
2008 XS 4EAT
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9,878 Posts
Profile updated, kinda, no where to say LPG![/QUOTE]

Go back into User CP, Edit Signature, add info, you might have to abbreviate some info as you are limited to characters
 

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Registered
2017 Limited
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1,018 Posts
SloobyLoo, you need to get a new tech. If this one you have says he has never heard of the fail safe/limp home mode, he is not much of a tech. Even if Subaru calls it something else, there isn't much explaining needed when the words are used. JMO
 
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