P0420 OBD-II code

[srs_49]

2002 OBW, H6, 3.0L, 96,000 miles.

 

Over the weekend I replaced the spark plugs (what a bear that was, but a story for another time). In putting the coil assemblies back on, I reversed 2 of them on the driver's side of the engine. A short test drive (2 miles) showed a problem, with the CEL blinking at me. Brought it back into the garage and flipped the coil assemblies back, and the problem seemed to clear up. Another test drive for a couple of miles was OK, as was my wife's 35 mile drive to work in the morning - car ran fine and no CEL.

 

On the way home however, maybe 5-10 miles into into the trip, the CEL comes on. I get the panic phone call at work. She says car is still running OK, and what should she do. I said drive it home if it's running OK, which is what she did.

 

When I got home, she mentioned a smell from the car. I put an OBD-II scanner on it, and it came back with P0420 (Bank 1 Catalytic Converter System Efficiency Below Threshold). In reading some of the threads here, it sounds like it could be the cat, but also maybe one of the O2 sensors. I did have to remove the connector to the O2 sensor on the driver's side when doing the plugs, but it's back on now.

 

My main question is whether driving the car for approx. 2 miles with 2 plug wires reversed could have damaged the cat. I have heard that a misfiring cylinder could cause damage by dumping unburned fuel into the exhaust, so I guess the reversed plus connections was effectively doing that to 2 cylinders.

 

Again, the car seems to run fine - no CEL at the moment. I will drive it tomorrow and see what happens.

 

Please, please, please, let it not be the cat!

[porcupine73]

For now I would just clear it and see if it comes back. Subaru's like to pop the P0420 sometimes. It is an emissions code.

 

Typically it is fixed with either the rear and/or front o2 sensor replacement. Your misfire issues may have triggered it. The cat can be damaged by excessive raw fuel dumping into the exhaust. Hopefully your episode wasn't enough to do that.

[srs_49]

Yes, I cleared out all the codes (the P0420 was the only one).

 

My concern about damage to the cat was trying to get a feel as to whether the initial 2 mile test drive with the crossed plug wires was enough to damage the cat.

[OB99W]

[...]My concern about damage to the cat was trying to get a feel as to whether the initial 2 mile test drive with the crossed plug wires was enough to damage the cat.

Yes, dumping enough raw fuel into the cat can cause a partial meltdown, especially if you were pushing the car hard to check performance at the time. However, sometimes a cat gets saturated without becoming permanently damaged; as long as everything's firing properly now, a highway trip that gets the cat hot (not due to raw fuel!) will sometimes correct the problem. You might try a 15-30 minute highway ride and see what happens.

 

At 96,000 miles, if the O2 sensors haven't been replaced before, they could be on the way out (especially the front one). The fuel dosing the two misfiring cylinders gave the cat could have pushed things over the edge as far as efficiency determination is concerned.

 

I'd try a highway trip, then consider replacing the front O2 sensor if the P0420 returns; use an OEM unit if you do. Even if the cat needs replacement, a new O2 sensor will help it live.

[srs_49]

Status update-

 

Monday evening - cleared out all of the OBD codes. Took car for short test drive, all seemed OK. Car ran fine and no CEL.

 

Tuesday - I took car to work (wife usually uses it). 7 miles one way distance, part local roads, part 60 mph highway. Again, car ran fine and CEL stayed off both ways.

 

Wednesday - Wife stayed home to run some local errands. CEL came on while on the local beltway, 65 mph, after 5 miles or so of driving. On return trip, CEL came on approx. 3 miles into trip (maybe because engine was already partly warmed up?). Since car is in driveway right now and I'm at work, have not been able to pull the OBD codes. Will do that this evening and see if it's the same P0420 code that started this thread or something else.

[ron917]

If it's the P0420 again, read this:

http://www.troublecodes.net/articles/catfailure/

[OB99W]

Further perspective:

http://www.motor.com/MAGAZINE/Pdf/102006_09.pdf

[srs_49]

Thanks for the two articles, OB99W and ron917. Pulled the codes last evening when I got home and was still P0420, 1 history (I assume), and 1 pending (the engine was running). I may try and borrow a 'scope from work and look at the outputs from the front and rear sensors. Lacking that will use my DVM, though I understand that it will provide a average of the readings and will be difficult if not impossible to see the switching on the front sensor.

 

I am probably going to replace the front O2 sensor first anyway, since there's over 96,000 miles on the OBW.

 

Question - Is there 1 or 2 front sensors? Looks to me like there's 2 front sensors (this is an H6 3.0L), but all of the posts always seem to talk about the front sensor in the singular.

 

Thanks

[OB99W]

Thanks for the two articles, OB99W and ron917.[...]

You're welcome.

 

 

Question - Is there 1 or 2 front sensors? Looks to me like there's 2 front sensors (this is an H6 3.0L), but all of the posts always seem to talk about the front sensor in the singular.

When there are two banks with a catalytic converter off each one, there are two front sensors. We tend to speak of a single front sensor because on 4-cylinder models (the vast majority, for now ) exhaust is Y-connected before the cat.

[srs_49]

Quick update on the P0420 issue. It's Friday evening now. I've had the OBW for the past 2 days and there have been no CEL indications. I ordered a new front O2 sensor from Genuinesubaruparts.com - should be here in a couple of days.

 

Another question - if something is marginal or on the edge (such as an O2 sensor or the cat itself), would outside temperature make a difference? Reason I ask is that it was close to 100 degrees the days earlier this week when the CEL would turn on and the car would throw the P0420 code. The last two days, the temperature has only been in the mid 80's.

[porcupine73]

Did you order two front sensors then? Good call on using the oem part. I would think maybe extreme cold weather could cause a problem but hot weather should be better.

 

P0420 is probably one of those codes that requires three consecutive trips with fault so it might be happening one trip or two trips, then not the next, etc.

[srs_49]

Just ordered one, before I got OB99W's comment 3 posts earlier about there being 2 front O2 sensors on the H6. Will probably order a 2nd. Will use the first one to replace the one on the side that had the 2 reversed plug wires. Figure if either sensor was bad, it's that one.

 

Will keep this thread updated.

[srs_49]

As promised, here's the latest on my P0420 woes. Replaced the left front O2 sensor this past Saturday morning with a Subie OEM part and reset the OBD codes. CEL turned off. ( If you remember, my theory is that the left front sensor was damaged when I crossed two spark plus wires on the left side of the engine and dumped raw fuel into the exhaust). All day Saturday and Sunday things looked good - CEL stayed off. Then Sunday evening when driving down on the interstate, about 8 miles into the trip, the CEL comes on. Damn! Now what? Same ol' P0420 code.

 

I'll probably buy another front O2 sensor and replace the right front one, but I really don't have much hope that it will cure the problem. I think I damaged the cat with the unburned fuel, even though I only drove car maybe 2 miles under that condition. I will hold off with the cat, though, until it's time for the 2 year emissions test.

[Bserk]

Just browsing around tonight, thought I would throw in my 2 cents... Again, I apologize for my Nissan references, but thats where 99% of my experience comes from (30 years as a tech for a Nissan dealership)

 

We have had many instances of 95-03 Maximas, which use a coil-on-plug design (seperate coil for each cylinder) have one coil fail. It will set a primary ignition code, and eventually a misfire code. At that point, it the driver ignores the obvious 'skip', it WILL very quickly burn up the cat for whichever bank the failed coil is on. And then you have the dreaded PO420.. Im thinking switching 2 wires on one bank of the Subie in question would quickly destroy the cat as well. Hopefully, the more Subie experienced folks here will be right, and your cat will survive.

 

But if the cat is cooked, there is another solution. One Im curious to know if any Subaru folk here have considered. We have several young guys that own the 2.5 L Sentra SE-Rs. The main cat is integral with the exhaust manifold, so when these guys install headers, they are screwed as far as passing the state inspection (no main cat, instant PO420...no pass the emission tests) This group have found a 02 sensor simulator, which they wire into the rear 02 sensor circuit. It mimics the 'slow' voltage swing that a rear o2 sensor is monitoring to check for a healthy cat... I guess Im advocating 'cheating' the emission laws a bit, but on a older car I'd try it. 30 bucks or so for this simulator vs. hundreds for the cat... I know it works on the 2.5 Sentras, I see no reason it wouldnt work on a Subaru. I know they make them that are set up for dual sensors, for 2 bank engines. Just curious if anyone here has tried it, or wants to bash me for suggesting it...

[gbrand]

text fom previous thread on subject-have you checked to see if your vehicle is covered under warrenty or similar mentioned below:

 

 

 

 

mtsmiths

 

Registered User

Posts: 1

(12/6/01 9:29:06 am)

Reply

Persistant 'Check Engine' light The check engine light came on in the '00 Lagacy I just bought for the Pretty One. Just after it rolled 100,000 mi the check engine light came on, she read the manual and found the she had failed to tighten the gas cap, which will send a fault signal. We disconnected the batterey overnight and the light went out. It came back a day or so later, so we took it to the local Subaru dealer and had the fault read. They say it's a faulty catalytic converter signal, but can't find anything wrong with the cat. Dealer says that even tho the cat seems fine, if the signal comes back the only way to get rid of it is to replace the cat to the tune of $600+. It's come ba-aaaack.

 

 

Since I know the cat is OK I really don't want to pay $600 to turn off an idiot light. On the other hand - Now we won't know if anything else goes wrong that we *should* be paying attention to, since the light is always on. I'll hit the Subaru board, but anyone here got any ideas?

 

THANX

gbrand

Registered User

Posts: 177

(12/6/01 10:14:01 am)

Reply | Edit Re: Persistant 'Check Engine' light From what I understand the way the ECu sees the cat bad is that the second Ox sensor , behind the cat, mointors thecat, not used for controlling engine I am told. So, Oxygen content in air past cat should be less than at upstream sensor(in manifold?) as CAt should be using Oxygen to burn up partially burned CO and HC. O2 sensors put out a voltage that gets higher as the oxygen concentration gets less, so typically read 0 Vols in air, and 100-200 millivots in normal operation. My truck downstreamsensor was reading well over 300 millivolts, saw at emission test there was 0% oxygen in exhaust. SO, if measure output of sensor and compare to up stream side, if same the cat ain't working, if higherthe cat is. Or downstream sensor is bad. Check after engine has warmed up, but don't replace blindly. The three or 4 wires are heater voltage, heater gnd(will be to case if 3 wire sensor), signal outut, signal gnd. So measure between signal output and signal gnd and see what you get. Of course diconnecting and measuring with engine running will set code for O2 sensor, so clear it later or someone will sell you an O2 sensor you don't need mtsmiths

Registered User

Posts: 9

(12/11/01 3:15:47 pm)

Reply Check engine Light Follow-up Thanks for all the comments, especially to gbrand. I got the car registered with Subaru national, who promptly informed me that the car had not responded to recall WXW80 error reporting by front 02 sensor - resulting in *you guessed it* check engine light.

 

I called the dealer that had diagnosed a bad cat and asked, "Don't you diagnose a bad cat by comparing 02 sensor read-outs?" he said "Yes". "If the car had a bad front sensor could that lead you to deduce a bad cat?" he said "Yes". "Did you verify recall WXW80?" ... slight pause he says "Is that the front 02 sensor recall notice?" *I* said "Yes", he said "No"; and scheduled the recall replacement.

 

The moral of the story is ... the Subaru dealer was fully prepared to sell me an unneeded $800.00 cat converter, without checking a known recall, and of course during that repair they would have replaced both sensors, so I would never have known.

 

I don't know if they are crooks or idiots, but I sure don't trust them either way. I sure wish my indy mechanic that keeps our '87 alive could work on this NewGen Soobie.

[srs_49]

Bserk-

 

Thanks for your comments. And no need to apologize for your Nissan references. I still have a '92 SE-R that's still a lot a fun to drive.

 

That's interesting about the rear O2 sensor simulator. Seems like it would be relatively easy to cook something up like that, circuit-wise. I may try that, just as a troubleshooting aide, of course.