Miata CEL troublshooting

JamesShort · **Posted:** Tue May 04, 2010 10:06 pm

My friends 00 miata's coilpacks went and he put new CPs in. Now he's getting a 420 code. Said the car might have run for 5 minutes with the bad CPs. Could that have killed the cat that quickly? 420 seems to also say a primary O2 sensor might be going, but the fact the CPs just went seems to say the cat is gone.

Anyone with a 99/00 midpipe they want to get rid of?

Aaron Buckley · **Posted:** Tue May 04, 2010 10:53 pm

Pull the upstream o2 sensor, clean it and reinstall. Then clear the code. - AB

Sean O'Connell · **Posted:** Wed May 05, 2010 6:32 am

its not a cat issue. the secondary o2 sensor monitors cat efficiency.

JamesShort · **Posted:** Wed May 05, 2010 8:52 am

Interesting. He and I have read/heard that unburnt gas flowing through the cat when one of the two coil packs die (very common on 99/00) usually kills the cat pretty quickly. But at the same time cleaning the sec O2 sensor is free so we'll try that first.

Dustin Fredrickson · **Posted:** Wed May 05, 2010 10:43 am

I'm not sure I was smart enough to connect the 2 issues, but I had coil pack issues and upstream O2 issues (same CEL as I recall) at somewhat close timing on my 2000 Miata. I ended up replacing the O2 sensor with a used one (and of course the coil pack as well) and the issue disappeared.

JamesShort · **Posted:** Wed May 05, 2010 11:01 am

Dustin Fredrickson wrote:

I'm not sure I was smart enough to connect the 2 issues, but I had coil pack issues and upstream O2 issues (same CEL as I recall) at somewhat close timing on my 2000 Miata. I ended up replacing the O2 sensor with a used one (and of course the coil pack as well) and the issue disappeared.

So maybe the raw fuel hitting the O2 sensor caused it to malfunction.

So the interesting thing is that the primary O2 sensor does not monitor cat efficiency as mentioned so it'd be odd if a primary O2 sensor malfunction is tripping a 420 code.

EDIT:

Lots of miata.net-ers are claiming the miata ECU is dumb and that 420 codes are frequently the primary 02 sensor despite the sec o2 sensor doing the 'measuring' of cat efficiency.

Jason Mauldin · **Posted:** Wed May 05, 2010 12:55 pm

JamesShort wrote:

Lots of miata.net-ers are claiming the miata ECU is dumb and that 420 codes are frequently the primary 02 sensor despite the sec o2 sensor doing the 'measuring' of cat efficiency.

Their view is a bit limited.

Most ECUs use both the primary and secondary to monitor cat efficiency. The secondary is just compared to the primary. So, if there's an issue with either o2 sensor, there's the possibility of throwing that code. Since the primary sensor is closer to the fire, it is generally the one that goes bad first.

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

Correct ... OBD-2 systems measure cat function by comparing the signals from the upstream and downstream O2 sensors. If both signals match then the cat is having no effect and that's viewed as not good.

The OBD-1 systems (pre 1996) only had the first O2 sensor as that's the one that helps adjusts the air/fuel mix. The second O2 sensor was added in OBD-2 to check cat health.

Frank

JamesShort · **Posted:** Wed May 05, 2010 4:26 pm

Ok so now the only things it could be, given the discussion above (thanks), is either of the 02 sensors or the cat itself.

I read a thread that said when his coil packs went bad, he pulled his primary O2 sensor and it was covered in 'tar'. He supposition was when 2 of the cylinders were not burning fuel, the fuel, being a solvent, cleaned the carbon gunk off the pistons and exhaust valves in the non firing cylinders and just swept a wet carbony gunk into the exhaust stream. He clean the primary O2 sensor and all was well.

I'm gong to do a data read on the o2 sensor PIDs tonight and run the diagnostic test. Then I'm going to pull both sensors and clean them. That'll be free before we start throwing new sensors or a catalytic converter at the problem

.

Vincent Keene · **Posted:** Wed May 05, 2010 4:38 pm

Frank Catena wrote:

The second O2 sensor was added in OBD-2 to check cat health.

Exactly. When my cat got sick I decided to put it down.

I replaced it with a test pipe and 02 simulator and all was well in CEL land.

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

Take this with a grain of salt, but working cats get hot. If you have one of those infrared thermometers you could shoot it at your cat and see if it is indeed hotter than the exhaust pipe. If it is, you could deduce your cat is probably working ...

I tried to word this to set VK up for another cat story ... we'll see what he comes up with :-)

Frank

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Frank Catena wrote:

Quote:

If it is, you could deduce your cat is probably working ...
I tried to word this to set VK up for another cat story ... we'll see what he comes up with Smile

If Vincent tried the infrared indicators on his "cat" and found it "hot," he would have surmised that the cat had a fever, was deathly ill, and would have put it down anyway. Ever the lover of cats.
Charlie G

JamesShort · **Posted:** Wed May 05, 2010 8:21 pm

Both O2 sensors were dry and white...perfect. My buddy said he has been hearing a rattling that he thought was the heat shield on the cat rattling.....nope it's the cat itself....all sorts of shit in side the cat is rattling and moving. I think it's gone. He said the previous owner had a new cat put on. This looks like a generic 2.5" cat that was hack-job welded into a 2.25" exhaust system. I told him to have a new midpipe/cat delivered to my house and I'd put the whole thing in for him....that thing is gone.

Vincent Keene · **Posted:** Thu May 06, 2010 8:27 am

Frank Catena wrote:

Take this with a grain of salt, but working cats get hot. If you have one of those infrared thermometers you could shoot it at your cat and see if it is indeed hotter than the exhaust pipe. If it is, you could deduce your cat is probably working ...

If your cat is hot it could mean it's "in heat". Some cats use that excuse to be "working cats", which some would call just "catting around". Either way, it sounds like your cat has a problem and at the very least needs some type of therapy.

How's that Frank?

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

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Miata CEL troublshooting

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