Cat Question

[chazglenn3]

I have been getting the PC0420 code occasionally..."Catalyst Efficiency Low". Clearing the codes makes it go away for a while. I think it might be cheap Costco gas, but the FSM says bad gas will sometimes make a marginal converter look bad. So, I guess I should start thinking about replacing the cat. Is this another case of stick with OEM, or ar aftermarket units okay to use.

Charles

[Bruce Nunnally]

There are regular replacement cats and high-flow replacement cats. I would lean toward a regular cat as I am unclear if the so-called hi-flow models are actually higher flow, as well as being as good at conversion, besides being more expensive.

Within the regular cat category, aftermarket and less expensive equivalent seems fine.

[OynxSTS]

I have been getting the PC0420 code occasionally..."Catalyst Efficiency Low". Clearing the codes makes it go away for a while. I think it might be cheap Costco gas, but the FSM says bad gas will sometimes make a marginal converter look bad. So, I guess I should start thinking about replacing the cat. Is this another case of stick with OEM, or ar aftermarket units okay to use.

Charles

Looks like you need a new Cat... OEM ones are not cheap... I have had success with Aftermarket units and they are about 1/10 of the cost.

The easiest way to test the cat is a full throttle 2-3 shift... with a plugged cat it will miss like crazy and the car will drive like a slug.

[chazglenn3]

The converter definatly isn't plugged...just not converting like it should is how I read it. There seems to be no restriction or lack of performance.

Charles

[Ed Hall]

The converter definatly isn't plugged...just not converting like it should is how I read it. There seems to be no restriction or lack of performance.

Charles

Then I wouldn't worry about it unless of course it doesn't pass smog.

[Logan]

If....the car is under 80K miles..... It would be covered under the Federal OBDII 8 year 80,000 mile warranty.

The 8y/80k warranty is only for the CAT and the computer. No sensors included...

[chazglenn3]

No such luck, Logan...just about to turn 120K. The light came on again this morning on the way to work. I'll call around today to see what kind of prices I get from the local muffler shops.

Charles

[Cadillac Jim]

I recently replaced my mufflers with Borla high-performance stainless steel mufflers, which give my car a nice tiger-purr V8 exhaust note. Here's some ambiguous information:

When I go to a redline shift at WOT, I can hear just a trace of cat restriction above 5,000 rpm. My cat is original and I have 120,000 miles on it, but I'm sure that I would get a trace more performance above 5,000 rpm with either a new cat or a low-restriction cat. When I was getting my exhaust fixed, I asked the muffler shop if I would benefit from a low-restriction cat and he said no, 300 hp and 280 cid isn't big enough to benefit from a low-restriction cat. After looking at the car, he pronounced my existing cat OK and we left it alone, so he is definitely honest.

[chazglenn3]

Thanks, Jim. I won't waste any extra money on a high-flow cat. So far, it looks like $179 is the best price I have found.

Charles

[Cadillac Jim]

That's not a bad price. I got a list price of about $250 for mine, and was happy to leave it alone. I notice that your car is a Sterling; are you in Australia? Is the $179 in Australian dollars?

[chazglenn3]

No...I am in Tacoma, WA. The price is in US greenbacks. The color of the car is Sterling (silver). Perhaps I'll tweak my profile a bit...

Charles

[clarkz71]

It could also be a defective "post converter" o2 sensor. If you have a FSM go through the diagnostic test to verify that it's working properly. Your car should have 2 pre cat sensors and 2 post cat sensors. I think 2000 models only have 1 post cat sensor.

[Cadillac Jim]

My 1997 Eldorado has only one post-cat sensor. It's a heated oxygen sensor (HO2S).

[chazglenn3]

Shouldn't the oxygen sensor, if bad, set a DTC of it's own?

[Cadillac Jim]

The oxygen sensors in the exhaust manifold see pulsations as each cylinder fires, so if they stop functioning the PCM can tell. The one after the cat doesn't see pulsations so you might get a bad cat code without an oxygen sensor failure being detected.

Usually a bad cat code is a bad cat, because the oxygen sensor after the cat doesn't go bad often. But, it's easier and cheaper to fix than the cat, and the code may just mean a bad HO2S, so it's worthwhile to check the wiring and the HO2S before changing the cat.

I had mine looked at while my car was in the shop for mufflers, and the exhaust man told me that cats don't go bad from miles or time, but from "engine problems." He said that putting a lot of coolant through the exhaust could cause the cat to overheat, for example. Really bad oil-burning (a quart every 100 miles, for example) can similarly overheat a cat.

My personal opinion is that the most common cause of cat failure is road damage impact that cracks the ceramic. Next would be jack or lift damage during servicing by Neanderthals. Mine has a dent in the heat shield.

[clarkz71]

This is from the OBD II web page.

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for

An oxygen sensor is not reading (functioning) properly

The engine coolant temperature sensor is not working properly

Damaged exhaust manifold / catalytic converter / exhaust pipe

Retarded spark timing

Now you might have a bad converter but you should still go by the FSM diagnostic test. I don't have a 97 FSM but I'm sure under "P0420" it doesn't say replace converter and nothing else.

As far as the two o2 sensors on the manifolds go, no they don't see exhaust pulses. The only thing they "see" is oxygen, or a lack of. The PCM doesn't see a degradation of function, only a complete failure. So you don't always get a code.

My 1997 Eldorado has only one post-cat sensor. It's a heated oxygen sensor (HO2S).

Your correct, there are two sensors to measure catalytic converter efficiancy. One in front of the converter and ond after.

[Patrick7997]

From My 1997 Eldorado Deville Seville Factory manual:

--------------------

DTC P0420 - TWC System Low Efficiency

Cutting out the blah blah, this efficiency is calculated using data collected from pre-cat and post cat oxygen senors...

Steps:

1. Was Powertrain On-Board Diagnostic (OBD) System Check performed??

(assume thats okay)

2. Check codes. Any other codes set??? If so, go to the codes that are set

(I'm assuming this is your only code)

3.

1. Check exahust system for leaks, damage, losse or missing hardware

2. Make repiars as needed.

If you found something / repaired something in step 3, go to step 5. If that's okay, it's on to step 4.

4.

1. Verify that the correct cat is installed

2. Check the Cat for damage, dents, severe discoloration, holes, etc...

3. Ensure that Oxygen Sensors are properly installed and that Oxygen Sensors, wiring, and connections are undamaged and properly retained

4. Make repairs as needed.

5. Select DTC Status, Operate vehicle under test conditions...

If you don't get a code, it's fixed. If you get a code again...

6. Replace the Cat.

The Step 5 test conditions:

This test is run once per ignition cycle and only after the Cat has warmed up to operating temperature..

Test conditions: Whole pile of codes not set.

Cat at operating temperature

ECT at 167F or more

IAT is -18F or higher

Air flow between 15 and 35 grams per second

Average MAP signal less than 59.3 kPa

Difference between current MAP signal and average MAP signal less than 8 kPa

Vehicle Speed between 20 and 75 MPH

Throttle Position greater than 2.2 degrees

Engine Speed 3000 RPM or less

Closed loop fuel control enabled

Front and rear oxygen sensors ready

Commanded air fuel ration 14.7 to 1

short term fuel trim not limited.

--------------------

As I read this, I think this is telling you to replace the Oxygen Sensors before you replace the CAT....

Hope this helps & good luck....

[Cadillac Jim]

The HO2S sensors, once warm, generate their own voltage from 0 to about 1v. A waveform of an operating HO2S in a closed-loop DFI system is on this page:

http://lightning.prohosting.com/~v8mate/oxy4.html

(scroll down about a screen). The cycling is at about 30 Hz, which would correspond to about 7.5 RPM if it did see exhaust pulses, so I stand corrected.

You can check your oxygen sensors, if you can see if they generate voltage within the proper range. The engine should be warm or at least running for a few minutes. There is a connector at each oxygen sensor that may provide access, or you can go to the PCM connector.

The oxygen sensors before and after the cat are different from the ones on the exhaust manifold. The one after the cat should read 0.6 volts or more.