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E36 (1991 - 1999)
The E36 chassis 3-Series BMW was a huge hit among driving enthusiasts from the first moment the car hit the pavement. The E36 won numerous awards over the years it was produced and is still a favorite of many BMW enthusiasts to this day! -- View the E36 Wiki

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Old 12-11-2007, 04:25 PM
jfreg jfreg is offline
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Location: Silver Spring, Maryland (near Washington DC)
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Mein Auto: 1997 328i Convertible
Check Engine, OBD II code 420, Catalyst System

Check engine light in my '97 328i came on the other day. Code PO 420, "catalyst system below efficiency," came up on my tester. I've read that O2 sensor problems or leaks in the exhaust could cause that code to come up, even when catalytic converter is OK. Besides looking for leaks, and maybe inspecting the O2 sensors for major corrosion, what can be done to pinpoint the problem? The car has 102K miles, so I'd imagine the sensors are about due for replacement, but I just found out there are four of the buggers, and I hate to throw that much money at the problem without being sure of a solution. Anyone solved a PO 420 problem with O2 sensors?
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Old 12-11-2007, 10:12 PM
MartinBlank MartinBlank is offline
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Location: Portland, Ore. USA
Join Date: Oct 2007
Posts: 30
Mein Auto: 1997 328is (coupe, 5sp)
I'm by no means an expert, but I'll share my story if it helps.

I had my catalytic converter fail on me about two years ago, giving me the check engine light. Replaced it for $1550.

A few months later my secondary air pump caused another check engine light. I've been told that a failing secondary air pump can foul the catalyst, and was probably the root cause of the problem.

So, if you replace the catalyst, maybe you should replace the secondary air pump ($268) or any other components that might foul the new catalyst.

Can any of the regulars confirm what I'm saying? Or do I have it all wrong?
A BMW? In Detroit? That's sacrilege!
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Old 12-12-2007, 12:18 AM
Ethirtysicks Ethirtysicks is offline
Location: North Jersey
Join Date: Nov 2007
Posts: 95
Mein Auto: 97 328is
The secondary air system pumps air into the exhaust on cold starts to get the cats hot quickly, so your o2 sensors can come online faster. it would'nt screw your cat up-you should get secondary air faults though. what you can do is jack the car up and crawl under there. if you can, use a rubber mallet (your hand will work too) and bang on the cats. if they make noise like rocks rattling around in there your F&%$ED!
and you need new break out the anal lube.
otherwise check for vacuum leaks-they will make the car run lean and burn up the cat....
good luck buddy!

Last edited by Ethirtysicks; 12-12-2007 at 12:23 AM. Reason: add somthing
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Old 12-12-2007, 11:34 PM
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drivinfaster drivinfaster is offline
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Location: in the sticks you piney
Join Date: Nov 2007
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Mein Auto: rescued bmw's
thre are a few ways to confirm a cat is bad. all of them require a skilled tech and a 4 gas analyzer. do not rely on infra red thermo scanners to check cat temps. buncha crap test. hitting it will only tell you if the 'strate is broken or not. if it is, then DUH, you need one. but don't let a heat shield or loose exhaust clamp or bracket fool you.

first, loosen the exhaust check nuts on the manifolds prior to starting and replace snugly. then tell the tech about them (a lot of non bmw people don't know what these are...), as he will need to get a pre cat sample, and tailpipe sample. when the vehicle is warm, read the gasses. the O2 pre cat, CO, & HC should be noted. then compare with the post cat readings. if all the gasses go down, then the cat checks out ok (at this point). if the gasses are the same, ESPECIALLY the O2, the cat fails. the cat should retain O2. a good tech will "load" the cat up with a few good revs, and recheck the "O2 retention" test. if the results are the same, bad cat. if the O2 drops down to almost 0, no higher than 0.5% (rule of thumb) then it may have been "full". hence the reason for the test. compare the CO2, this should be constant. (14.7 is optimum, but anything 14 or higher is good).

next, with a fully warmed and operating cat (normal op temp, cooling fan kicks on, etc) have the ignition system disabled. (how to do this should be figured out prior to the actual test...but just unplug the coils) and crank the engine. the fuel system should be working and fuel should be entering the cylinders. only crank for about 5-8 seconds and watch the exhaust readings. the HC should be going up up up and away, while the CO2 should be going up veerrrrrrrry sssssssllllloooooowwwwllllly. O2 may drop slighlty as CO2 is rising, which is great if it does, but the thing you want to see here is CO2. this comes from combustion, and since the ignition was disabled, fuel doesn't burn itself, it must be the cat doing something. the higher the number, the better the cat. the faster the number rises, the better the cat. if no CO2, you need a cat. period amen. if under 2%, better budget for a cat.

by the way, the O2 retention test and CO2 conversion test are good on ANY make and model vehicle that has a cat, bar none.

and what "kills" a cat, misfiring, running too rich too long. they don't "wear out", no moving parts. what makes them rattle? melt down. from what? working too hard. whorking too hard at what? see "what kills a cat"....

hope y'all like the info. any ????? just pm me.

drivinfaster (cleaner, too)
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Old 12-13-2007, 12:09 AM
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ffej ffej is offline
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"Code 420"? Your exhaust system has been hitting the bong

Apart from that, drivinfaster is pretty much spot on

Building a ZHP wagon... click the pic!!!
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For Sale - HUGE part out of 2005 ZHP sedan
Originally Posted by KrisL View Post
My input is that ffej knows what he's talking about .
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Old 12-14-2007, 05:37 AM
rrbgeb rrbgeb is offline
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Location: DFW, TX
Join Date: Dec 2007
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Mein Auto: 98 328i
Newbie to the forum, have a similar issue, and from searching this seems to be the most relevant thread.

Dealer quote of $2960 to replace converter in a 1998 328i. 122K. At 121K I had an incident with fouled plugs that left me dead in the water from a driveway start / stop incident. Got a feeling this fuel dump may have been the death of the converter. O2 sensors were new at 90K.

I have a hard time spending 3K on a car worth $6-7 at best but would like to drive another year or two. The car is currently running fine otherwise.

Converter prices (I'd prefer bolt on solutions) are all over the place from around $400 to over $1200 from my searches. Are the DEC units okay? What am I leaving on the table for a $400 bolt on?

Any sources for the parts would be great.

Should I replace the O2 sensors behind the converter?

Do I need any tools to clear the check engine light?

Did I miss anything?

Thanks in advance, Richard
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Old 12-14-2007, 07:20 AM
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jasonf860 jasonf860 is offline
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Mein Auto: 94 325i, 98 Protege
I've heard DEC units are good, but do not last as long as OEM ones. You could also buy a set of aftermarket "universal" cats and have a muffler shop cut and weld. As a last option, there are always used OBDII midpipes for sale in the classifieds on this and other forums. They are a popular upgrade for OBDI cars as they are true dual all the way through. The range in used price from $150-$300 depending on mileage etc.
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