2002 Outback H6 P0402

[gretzky_91]

Hello everyone , 

 

I'm glad to join the group!

 

In September of 2016 I purchased a 2002 Subaru Outback LL Bean Edition with 262,xxx for 8 bones . The truck needed a power steering pump and hadn't cleared smog here in California with hideous tires. I replaced the power steering pump and was able to clear inspection.  In December the truck threw a P0340 and P0420 symptoms included sluggish acceleration , rough idling at stops and even stalled more times than id like to rember but never had to call a tow. I put my scanner on and found the Cam Sensor would be my next bet. I installed the cam sensor 2 weeks ago and cleared the light. I've felt better response but yesterday on my way to work the P0402 came back up any advice on how i can fix this to make sure the subie is good to go to Tahoe this weekend.

 

Best Regards , Ruben 

[idosubaru]

P0420 is benign you can drive the car 200,000 more miles and ignore that code. The ECU doesn't use the rear O2 sensor for any driving. It's simply there to trigger the check engine light.

 

Otherwise:

1. Make sure you have no exhaust leaks

2. Tune up items - plugs, PCV valve, favorite fuel cleaner.

3. Replace catalytic converter. Which is sad because chances are the converters that throw P0420s would pass actual emissions exhaust testing.

[gretzky_91]

P0420 is benign you can drive the car 200,000 more miles and ignore that code. The ECU doesn't use the rear O2 sensor for any driving. It's simply there to trigger the check engine light.

 

Otherwise:

1. Make sure you have no exhaust leaks

2. Tune up items - plugs, PCV valve, favorite fuel cleaner.

3. Replace catalytic converter. Which is sad because chances are the converters that throw P0420s would pass actual emissions exhaust testing.

@Grossgary thanks for your input. I will run a cleaner and get a tune up after I return from Tahoe. Ive seen a few videos on the PCV valve is the H6 in the same location as the h4s? I bought a downstream sensor but after reading on some of these groups it seems that could be a temporary fix. I also replaced the Fuel Filter the day i put the new cam sensor on it.

[1 Lucky Texan]

roughness at idle could be aftermarket front axles - annoying but benign. (if the engine feels smooth in P or N but rough stopped in drive, many people experience that with rebuilt axles. - a few people seem to think transmission mounts contribute to the problem)

 

if it is still sluggish - chances are w'ever is causing that, may cause the P0420. bad knock sensors, vacuum leaks, old plugs, maybe clogged PCV and certainly exhaust leaks 'could' cause that code - in addition to wring problems, bad rear sensor or a bad cat conv. NEVER jump to the converter as the first thing to try - especially on a poor-running car. Fix the running/idling issue first.

 

on a plug change - research that, double platinum NGK is what you want - vary from those at your own risk. NGK Iridium 'may' be OK. I found it easier to change them from under the car.

 

look at the 'sticky' threads here; http://www.subaruoutback.org/forums/110-gen-2-2000-2004/

Edited January 18, 2017 by 1 Lucky Texan

[idosubaru]

google the pcv H6 is on the drivers side valve cover area if memory is correct.  it's easy.

 

rear sensor is unlikely to fix it, but it could. 

it's possible for many many things to irritate the P0420.  

 

you can also install an extender on the rear O2 sensor to sort of trick it into making the light go out.  they're available on ebay for like $5.  remove sensor, install spacer into exhaust, then thread O2 sensor (new or old one) into the spacer.  this sometimes works and is worth a try on what is a benign issue. 

 

what's great is that they will pass emissions with that spacer installed.  so it'll pass with the spacer installed but otherwise requires a new converter to repair?  how does that work anyway?

 

and subaru converters were routinely lasting the life of the vehicle before rear O2 sensors were ever used and the P0420 code was even invented. maybe converters are lesser quality now or something, but it all seems like an unfortunate issue to me so i just work around or ignore it.  no way am i replacing a converter for a benign code the ECU doesn't even use for drivability. 

[Caboobaroo]

P0420 is generic on these. You need to be able to read the short term fuel trims on both sides.

 

Personally, I'd start with a set of new NGK plugs and a full tank of premium gas from a top tier station (Shell, Chevron, 76). If it comes back after that, then next step is to replace the front oxygen sensor that shows negative short term fuel trims. Personally, I would replace both front oxygen sensors while you're doing the plugs.

[gretzky_91]

google the pcv H6 is on the drivers side valve cover area if memory is correct.  it's easy.

 

rear sensor is unlikely to fix it, but it could. 

it's possible for many many things to irritate the P0420.  

 

you can also install an extender on the rear O2 sensor to sort of trick it into making the light go out.  they're available on ebay for like $5.  remove sensor, install spacer into exhaust, then thread O2 sensor (new or old one) into the spacer.  this sometimes works and is worth a try on what is a benign issue. 

 

what's great is that they will pass emissions with that spacer installed.  so it'll pass with the spacer installed but otherwise requires a new converter to repair?  how does that work anyway?

 

and subaru converters were routinely lasting the life of the vehicle before rear O2 sensors were ever used and the P0420 code was even invented. maybe converters are lesser quality now or something, but it all seems like an unfortunate issue to me so i just work around or ignore it.  no way am i replacing a converter for a benign code the ECU doesn't even use for drivability. 

Thanks the Subie made it to South Tahoe and back this weekend and has been driving great. i will be doing a Tuneup with New Plugs NGK of course new wires pcv valve and with that hope it cures all. Thanks for your help!

[gretzky_91]

P0420 is generic on these. You need to be able to read the short term fuel trims on both sides.

 

Personally, I'd start with a set of new NGK plugs and a full tank of premium gas from a top tier station (Shell, Chevron, 76). If it comes back after that, then next step is to replace the front oxygen sensor that shows negative short term fuel trims. Personally, I would replace both front oxygen sensors while you're doing the plugs.

Is there an affordable scanner besides the stuff at Autozone Walmart etc even if its a few bills which would be best to further diagnose the car instead of just getting generic P codes?? Thanks for your help brother will keep in mind.

[1 Lucky Texan]

if you have a smartphone, investigate Torque and other apps that use an ELM327 BT adapter.

Edited January 25, 2017 by 1 Lucky Texan

[micheall]

Is there an affordable scanner besides the stuff at Autozone Walmart etc even if its a few bills which would be best to further diagnose the car instead of just getting generic P codes?? Thanks for your help brother will keep in mind.

 

I bought this and leave it on 24/7 and use the torque app with it:

https://www.amazon.com/gp/product/B005NLQAHS/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

[idosubaru]

. If it comes back after that, then next step is to replace the front oxygen sensor that shows negative short term fuel trims. Personally, I would replace both front oxygen sensors while you're doing the plugs.

 

Excellent, thanks for that nugget. 

 

 

Thanks the Subie made it to South Tahoe and back this weekend and has been driving great. i will be doing a Tuneup with New Plugs NGK of course new wires pcv valve and with that hope it cures all. Thanks for your help!

 

Of course it did, I told you it would but it didn't really have any other choice.  The P0420 algorithm and rear sensor in that car isn't even used by the ECU for driving/performance/engine management.  You could literally cut off the rear O2 sensor and the car would run absolutely perfectly like a dream, it would just say "check engine" and give you that benign code. 

 

When I said you could drive it 200,000 miles with that code on - I wasn't making that up, it's not a guess, it's just a completely benign code for that ECU.

 

On other vehicles and even other Subaru's that might not be true but it is on this one. 

. If it comes back after that, then next step is to replace the front oxygen sensor that shows negative short term fuel trims. Personally, I would replace both front oxygen sensors while you're doing the plugs.

[keith3267]

The P0420 is not a benign code.  It indicates that the cat isn't working so in California, you will not pass the smog check with that light on.  There are a number of things that can cause the P0420 code besides the cat, the rear O2 sensor is the most common.  The rear O2 sensor does affect the operation of the engine in that it sets the course f/a ratio so the front sensor can fine tune it.  The computer does have a workaround if the rear sensor isn't working.  Once the front sensor takes over the fine tuning, the rear sensor only monitors cat efficiency after that.

 

The fact that you lost power is a big clue here.  If you still experience loss of power, your cat may be plugged up.  There is a way to check the system for excessive back pressure.  I'd get that checked and if it is too high, then you will need a new cat.

[GreaseMonkey03]

I'm slightly confused. Is it a 420 or a 402? You mentioned both. If it's the egr, that could explain some power loss. If it's a 420 it's not benign, it's generic. Could be a couple of different things. Usually, but not always, if it's one of the O2 sensors at fault, you will have an additional code to the 420 like a B1S2 for example.

 

You can somewhat check the flow of the cat two different ways. Using a temp sensor to check the temps at the immediate front and back of the cat with the engine at idle or firmly placing your hand over the tailpipe, preferably NOT after it's been running for a long time. If the temp at the back of the cat is excessively hot or there is a large difference from the front, the cat is most likely clogged.

A clogged cat can cause a fire among other issues, so it's not something you want to ignore.

 

If it's a 402, check vacuum lines to egr and pull the egr and clean thoroughly to see if that helps.

[idosubaru]

The P0420 is not a benign code.

Mechanically speaking it is 100% benign. You can cut the rear O2 sensor off the car and run it indefinitely with a P0420 code with zero ill effects.

To that extent it's benign. But i understand what you're saying....

 

Emissions does complicate things, but he cleared the code and the P0420 didn't come back for two weeks. Plenty time for all the monitors to set and pass emissions. Clear the code, then check again (with the CEL off) - if there's no pending code - then simply clear it and go have emissions done while the light is out.

People do that all the time.

 

Then you'll have time to plan what you want to do in the future and don't have to feel railroaded into a massive repair costs right now.

 

There's also a $5 spacer. Remove rear sensor, install spark plug non-fouler, reinstall rear sensor into spark plug non-fouler. Code goes away.

 

But it is annoying if you want to retain cruise and it's reappearing all the time. And a proper tune up, fuel system cleaning, checking fuel trims/ O2 sensors, would be wise and hopefully mitigate things.

 

I generally plan on owning my cars 300,000 miles so I'll just replace O2 sensors around 175k just because I highly doubt they'll make 300,000 miles, so I might as well replace them early and curb any potential issues and get the most out of my O2 sensor costs. They can be tested to.

 

A mechanic (Caboobaru above) mentions testing fuel trims and comparing left and right sides and looking for issues there regarding the front O2 sensors. I'd probably start there.

Edited February 12, 2017 by grossgary

[1 Lucky Texan]

I don't think the rear sensor is used for f:a adjustment until the canbus stuff came along - maybe 05-ish ?

 

anyway, even if it's an exhaust leak or rodent-chewed wires, States/areas with emissions testing are not gonna let it slide.

[idosubaru]

 

I don't think the rear sensor is used for f:a adjustment until the canbus stuff came along - maybe 05-ish?.

Yes for outbacks and legacy. Not sure if imprezas and foresters followed suit the same year?