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Could a bad neutral switch kill my cat?


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8 replies to this topic

#1 skizix

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Posted 17 August 2006 - 11:27 AM

2002 outback wagon, 5MT, ej25 here.

Ok, so I finally got my biggest driveability issue solved. Turns out it was a bad neutral switch. Codes I've had, w/ CEL are: 1507 (idle speed control), and 420 (cat efficiency).

So, I realize that the purpose of the neutral switch is to let the ECU know if the tranny is in neutral or otherwise. But what is the point? And how does the ECU respond to a bad neutral swich? My main symptom was a very abrupt transition from accel-pedal down to foot-off, and vice-versa, no matter how gradually I let up/down on it -- basically: bucking when in slow-and-go traffic, or maneuvering at low speeds in parking lots. Way worse at some times than others. The problem, I think, when it was bad, was that rpm's plummeted way too fast when I let up on the pedal (which also led to having to blip the pedal to match revs, even when upshifting -- which I now have to un-learn). I also had intermittent rough acceleration (worse in very hot weather), and rapid on/off surging when trying to maintain a steady speed on a flat road (also intermittent). Idle was rock steady most of the time, but would occasionaly get wacky, and it stalled once pulling up to a red light, and almost stalled but bounced back a couple other times.

Things are much better now. But I've got a few questions re: this...

1) How could a bad neutral switch cause all of these problems?

2) Is there a possibility that the bad switch caused my mix to be too rich?

I ask these q's because I'm getting the 420 bad cat code. I've replaced the front O2 sensor, but not the rear one. But the guy who did the diagnosis on this said he could see the rear O2 sensor cycling rich/lean, and thus concluded that the sensor was good, and the cat bad -- not just trying to sell parts; in fact he advised me that I could drive with the bad cat as long as I wanted, I'd just have to get used to the CEL.

So, could the bad neutral switch have caused a rich running condition, thus frying my cat? The cat does not rattle at all when I bang on it. But it has been emitting the rotten egg smell occasionally, and also a metallic-y smell sometimes. So...

4) Is a smelly cat a bad cat? Or is it possible that even though rear O2 sensor is oscillating lean/rich, that the O2S is bad (but cat ok) and causing the code?

Finally: another mechanic suggested an "emissions system flush" procedure, where they run some solvents through the intake, and it supposedly cleans out the cat. Anybody heard of this and/or have an opinion?

#2 mtsmiths

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Posted 17 August 2006 - 12:11 PM

"emissions system flush" procedure, where they run some solvents through the intake, and it supposedly cleans out the cat. Anybody heard of this and/or have an opinion?



It's called a "wallet flush' in the trade.

If you're 'smelly cat' is rotten eggs smell (sulfer dioxide) it's prolly shot.

Try some Seafoam.

#3 OB99W

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Posted 17 August 2006 - 03:06 PM

2002 outback wagon, 5MT, ej25 here.

Ok, so I finally got my biggest driveability issue solved. Turns out it was a bad neutral switch. Codes I've had, w/ CEL are: 1507 (idle speed control), and 420 (cat efficiency).

Interesting...a bad neutral switch on an MT is supposed to set a P1592 code.

[...]I'm getting the 420 bad cat code. I've replaced the front O2 sensor, but not the rear one. But the guy who did the diagnosis on this said he could see the rear O2 sensor cycling rich/lean, and thus concluded that the sensor was good, and the cat bad -- not just trying to sell parts; in fact he advised me that I could drive with the bad cat as long as I wanted, I'd just have to get used to the CEL.

If the test is run when the cat is sufficiently hot and functioning correctly (and the mixture is reasonable), the rear O2 sensor shouldn't be cycling. If the mixture is going way too rich, even a good cat could get overwhelmed and there'd be some variation seen at the rear O2 sensor.

Did you replace the front O2 sensor with an OEM unit? Have you verified that there are no exhaust system leaks?

So, could the bad neutral switch have caused a rich running condition, thus frying my cat? The cat does not rattle at all when I bang on it. But it has been emitting the rotten egg smell occasionally, and also a metallic-y smell sometimes. So...

4) Is a smelly cat a bad cat? Or is it possible that even though rear O2 sensor is oscillating lean/rich, that the O2S is bad (but cat ok) and causing the code?

The smells you describe are typical of a cat that's being asked to do more than it is capable of; one possible cause is a mixture that is too rich. I don't know if a neutral switch malfunction could be behind that in this case, but if it somehow allowed unburned gas to get to the cat, that could have damaged it.

If your mechanic has an exhaust gas analyzer, it can often tell a lot more than can be determined by looking at OBD-II codes and even real-time scans.

Finally: another mechanic suggested an "emissions system flush" procedure, where they run some solvents through the intake, and it supposedly cleans out the cat. Anybody heard of this and/or have an opinion?

A good hard run on the highway often will do as much to "clean" out the cat as anything else, assuming things are otherwise normal.

#4 skizix

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Posted 17 August 2006 - 03:18 PM

"Wallet flush" -- that is funny. Kinda what I thought. The shop that fixed the neutral switch is the most honest/best shop I've ever found. The emmision-system-flush guy is an unknown quantity.

Anyway, are you saying that a cat that is ok will **never** emit the bad-eggy smell? It rarely smells, like only after powering up a long climb, and even then, not always (the smell is not strong, but is unmistakable). Could an intermittent too-rich condition make it stink occasionally, and perhaps throw the 420 code, while the cat is actually still ok?

The reason I ask is: the neutral switch most definitely fixed the rpms-dropping-too fast, and big jerk on letting up the accel pedal. Runs way, way better. However, I'm still having some rough acceleration (and I bet that'll be worse if hot weather pushes back over 100 degrees). Also, idle is mostly good, but today I watched it move significantly, inexplicably, whilst at a red light. Usually it's steady, but sometimes it's funky...beginning to think I might need a new IAC (I did clean it recently, and it didn't look dirty when I removed it, so I don't think that's it).

So, if the IAC is hosed...that could certainly cause a rich mix intermittently. I realize that could've cooked the cat, but could it temporarily cause the cat to **look** (to the ECU) and/or smell bad, even if it's ok? As you can tell...not wanting to buy a new cat if I can help it.

The guy who said I didn't need to worry too much about having a bad cat also said an aftermarket cat might not prevent a 420 code, as they are usually too small to perform up to the sube ECU's high standard. But I'm seeing aftermarket ones for < $100. Tempting. Can anyone recommend a good brand? Is Bosal any good? Do they make one that fits an 02 OBW?

#5 skizix

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Posted 17 August 2006 - 03:25 PM

Interesting...a bad neutral switch on an MT is supposed to set a P1592 code.

If the test is run when the cat is sufficiently hot and functioning correctly (and the mixture is reasonable), the rear O2 sensor shouldn't be cycling. If the mixture is going way too rich, even a good cat could get overwhelmed and there'd be some variation seen at the rear O2 sensor.

Did you replace the front O2 sensor with an OEM unit? Have you verified that there are no exhaust system leaks?

The smells you describe are typical of a cat that's being asked to do more than it is capable of; one possible cause is a mixture that is too rich. I don't know if a neutral switch malfunction could be behind that in this case, but if it somehow allowed unburned gas to get to the cat, that could have damaged it.

If your mechanic has an exhaust gas analyzer, it can often tell a lot more than can be determined by looking at OBD-II codes and even real-time scans.

A good hard run on the highway often will do as much to "clean" out the cat as anything else, assuming things are otherwise normal.



Hmm...I was typing as you posted. Yes front new O2S is OEM. And I do give it a good hard run on the highway every day :headbang:.

I guess you answered my question about an ok cat stinking/throwing a code. I may in fact have a way-too-rich mix intermittently -- driving at low speeds, engine occasionally feels like it misses (no miss code though).

Verified no exhaust leaks? No. I need to get the car on stands, run it, and feel for an exhaust leak. Possible. Would that really mess up the O2S's/ECU?

Also, been thinking vacuum leak (i.e. on the intake side). But I'm thinking that would always cause a too-lean condition, right? If that is a possibility...how to find it? All hoses look good/connected. How would one assess, say, the intake manifold gasket, etc. w/o removing the manifold, etc.?

#6 skizix

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Posted 17 August 2006 - 03:32 PM

Oh, and...if it were an exhaust leak, wouldn't the problem be less intermittent? Wouldn't an exhaust leak be pretty steady, if perhaps gradually getting worse?

BTW, no code thrown for the neutral switch, but real-time scan verified it was hosed, and replacing it had a definite, positive effect. No question. Looking like not the the only thing screwed up though, obviously.

Any chance the bad cat (if in fact it is) itself would cause driveability issues? (it does not rattle when malletted though, and does not seem to be obstructed, at least significantly).

#7 mtsmiths

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Posted 17 August 2006 - 03:51 PM

how to find it? All hoses look good/connected. How would one assess, say, the intake manifold gasket, etc. w/o removing the manifold, etc.?


The time honored (and endorsed by Tom'n'Ray) method is to get a spray can of propane, or a propane propelled product, and spray it around the hoses, gaskets, etc. When the engine speeds up from ingesting the propane, you've found the area of the leak.

The exhaust leaks, you use liquid detergent or soap (or kids bubbles?) in a spray bottle and squirt it around and look for bubbles. Gotta work fast tho, once the exhaust heats up, the stuff evaporates on contact.

#8 OB99W

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Posted 17 August 2006 - 04:51 PM

[...]Verified no exhaust leaks? No. I need to get the car on stands, run it, and feel for an exhaust leak. Possible. Would that really mess up the O2S's/ECU?

A leak in the system including (but not beyond) the cat can cause the O2 sensors to supply the ECU with false data; naturally, that can "mess up" things.

Also, been thinking vacuum leak (i.e. on the intake side). But I'm thinking that would always cause a too-lean condition, right? If that is a possibility...how to find it? All hoses look good/connected. How would one assess, say, the intake manifold gasket, etc. w/o removing the manifold, etc.?

If the vacuum leak is minor, the ECU can correct because the front O2 sensor will provide the necessary feedback. With a severe enough vacuum leak, the mixture can be so lean that you'd get misfire (which in the exhaust is very rich :) ), but that should set a code.

Oh, and...if it were an exhaust leak, wouldn't the problem be less intermittent? Wouldn't an exhaust leak be pretty steady, if perhaps gradually getting worse?

Can't say for sure -- sometimes a leak at a joint will change with road bumps and chassis flexure. When checking, if possible have someone else hold a wadded-up old towel or something similar against the tailpipe opening; that'll increase pressure somewhat in the exhaust system and make any leaks a bit easier to find. Obviously, take the necessary precautions against heat and fumes. If you have an exhaust leak and particularly if running rich, you might be able to see some carbon/soot deposits at the site of the leak.

BTW, no code thrown for the neutral switch, but real-time scan verified it was hosed, and replacing it had a definite, positive effect. No question. Looking like not the the only thing screwed up though, obviously.

It's unfortunate, but that's why I advocate for real-time scans as opposed to just reading codes in many instances.

Any chance the bad cat (if in fact it is) itself would cause driveability issues? (it does not rattle when malletted though, and does not seem to be obstructed, at least significantly).

An unobstructed cat, even if otherwise malfunctioning, is mainly an emissions issue, and shouldn't impact drivability. (Nevertheless, I'd like to think that people care enough about the environment to change a bad cat.)

#9 skizix

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Posted 17 August 2006 - 05:10 PM

Yes, I will certainly swap out the cat for clean running...**if** I gain confidence that it's bad, and then **if** I can first sort out what killed it (i.e. what's making me run rich).

Any recommendations on a good aftermarket (cheaper than OE) cat, i.e. one that will satisfy the ECU/not throw a 420, and does not require mods (i.e. welding it in)?




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