How to Diagnose a Bad Catalytic Converter

[Dr_Snooz]

My truck just failed smog on HCs and NOx. I suspect a bad catalytic converter, but cats are expensive. I want to be sure it's bad before shelling out the dough to replace it. If this were an OBD2 truck, it would be telling me that the cat is bad. However, like our 3g's, it's OBD0 so I have to be more clever. There are a number of different ways to diagnose a bad cat and I'll be sharing a few here as I test my truck.

1. Consider the car's history. Cats fail for a lot of different reasons. Any time the car is not operating optimally, it can wreck your cat.

• Any situation that results in a rich tune can cause the cat to overheat. The overheating will cause the catalyst media to melt, resulting in exhaust restrictions and/or a failed smog test.
• Any situation that results in the wrong kinds of gases traveling through the cat. If you are burning oil, the soot can coat the catalyst media, resulting in a non-functioning cat. A coolant leak from a bad head gasket can also destroy your cat. Using non-O2 sensor safe gasket sealer can also wreck it.
• If you are heavily loading the drivetrain, eg: towing a heavy load in hot weather, it can adversely affect the cat. In my case, I know that the previous owner had used the truck to carry a camper (very heavy) and ignored a bad fan clutch (chronic overheating). Thus, the chances are good that he burned up the cat.
• If you hear clacking, rattling or other noises coming from the cat at idle, chances are good that it's bad.
• Poor car maintenance habits such as ignoring oil changes and running crummy fluids and parts, can also adversely affect your cat. If it's been decades since your last tune up, you MUST correct that before trying to diagnose anything else. On my BMW, the previous owner had used cheap spark plugs (Champion) when the specs call for Bosch Platinum+2. Both cats went bad. Here in California, cheap universal cats were illegal at the time, so I was looking at cats from the dealer for $800 each. It doesn't make a lot of sense to save a few bucks on plugs and then pay $1,600 on new cats.
• Past smog test results. It's always a good idea to monitor the numbers on your smog tests. The previous owner of my truck gave me the results of his last smog check at purchase. I noticed that the HC numbers had only narrowly passed and that gave me some advanced warning that a cat purchase would be in my near future. If you see your HC numbers creeping up over time, you can be fairly certain that the cat is wearing out.

2. Physical damage – obviously, if the cat has a giant dent in it, or a rusty hole, it's time to replace it. My cat looks pretty good from that standpoint.



3. Discoloration – an overheated cat can turn characteristic shades of bronze or blue, much like headers turn blue from heat. Inspect the converter case (inside the shroud) to see if it has turned colors from overheating. My cat is an even dirt color so I'm not seeing anything problematic here either.

4. Hammer – many times, the catalyst media in a bad cat will collapse and break apart. This can cause rattling, clacking or other noises at idle. Bang on the cat (gently!) with a rubber mallet or deadblow hammer to see if you can reproduce any rattling noises.



I heard a few grains of sand rattle, but nothing more, so I'll move on to the next test.

5. Backpressure – a good cat will impede exhaust gas flow minimally. If you see pressure in the exhaust pipe, before the cat, greater than 5psi, it's a sign of blockage. You can test exhaust pressure by removing the O2 sensor and inserting a tester like this.



You can also make your own tool from an old O2 sensor, a pressure gauge and some careful drilling.

Your right foot is also a good tester. If your car seems to lack the gitup that it had in the past, but is otherwise running just fine, chances are you have an exhaust restriction. My truck seems to have plenty of power (it's a 7.4L). Moreover, I lack the funds to buy a tester and the initiative to make one, so I won't be doing this test.

6. Thermometer – a properly running catalytic converter will have an operating temperature of roughly 300-500* at the inlet welds. The media will heat the exhaust gases another ~150* by the rear welds. Use an infrared thermometer to take the temperature of the cat at both the front and rear welds (not on the heat shield).

Temp at the inlet is 329*, so that is within range.



Temp at the outlet is quite a lot cooler. Worse, the outlet temp varies widely from a low of 240* to a high of nearly 450*. The outlet temp is well out of range and highly erratic. This is a bad cat.



Here in California, our biggest challenge in keeping old cars on the road is not rust or severe weather, it's California's notorious smog checks. There are very few shops that don't do the money dance when an old car fails smog, so it's worth learning to diagnose this stuff yourself.

Good luck!

References:
Walker Catalytic Converter Flip Chart - http://www.rockauto.com/info/Walker/ConvTipChart.pdf
Eric the Car Guy - https://www.youtube.com/v/9VZ5K8n5jj0

[Rendon LX-i]

Should not see any pressure close. To 0 as possible . 2-5psi is bad when it comes to cat flow.

Remember cats don't commit suicide they get murdered. A. Lean or Rich condition or bad head gaskets (letting coolant into cumbustion chamber ) can be a lot of things.

Simple pressure test before the cat tells you everything an the temp of the cat

[ecogabriel]

High HC may mean the truck is running rich, and that would in turn overheat and melt the CAT - you already covered that point though.

When my 5G Civic failed emissions it was on NOx; HC and CO were well under normal limits. The cat was melted so it was replaced together with the O2 sensor.

Is there any way you can replace the cat without going to a shop?

Nice posting by the way - like the temperature observation. Good luck

[Dr_Snooz]

Is there any way you can replace the cat without going to a shop?

It should be welded in. If you can't weld, then most auto parts stores sell exhaust pipe clamps and couplers that you can use. On our Hondas, the OE cats bolt in. Until a muffler shop gets ahold of them...

[88Accord-DX]

Good write up on the subject. Most catalytic converters I've seen go bad are from slow responding oxygen sensors,with a MAF & lack of changing the spark plugs. Once in a while, a few bad fuel injectors can take one out in time.

[ecogabriel]

Here in California, our biggest challenge in keeping old cars on the road is not rust or severe weather, it's California's notorious smog checks. There are very few shops that don't do the money dance when an old car fails smog, so it's worth learning to diagnose this stuff yourself.

It seems that the biggest challenge is finding a mechanic more interested in fixing the car instead of fixing HIS wallet (at your expense).
If you could find what was wrong with the emissions test, why a mechanic supposedly trained to do that cannot?
When my Civic failed NOx I had already done some research and I suspected the cat; the guy that did the testing took the sheet and concluded - from the readings - that the engine was running fine (both CO and HC were well below limits) and since that car does not have an EGR system all the NOx reduction comes from the cat. I did the replacement myself (it is bolted on like in the 3G); it has passed smog like a champ every time.
I like that you researched the topic and took a fact-based approach; I'm getting myself a IR thermometer based on what you did (it may help for other diagnostics as well).

[Chrismfc]

I feel bad for you guys. In Oklahoma we have it cut out and put a straight pipe in place for fifty bucks. Then take the cat to recycling place and make seventy bucks from the hole problem. We got rid of emissions laws I think in 97. It's gotta suck.

[Dr_Snooz]

Postscript: I replaced the cat the other day and passed the check. The HC numbers were right on the average and the NOx numbers were way below average.

[2geeSEi]

I have a Magnaflow direct fit Cat on the way for my 2g. I has the original cat, and 2 years ago it barely passed on HC, and NOX had gone up significantly from previous times. I had it off when I was working on my exhaust, and the catalyst looked fine, but when I checked it with the laser thermometer, I was barely getting 40 deg F more at the outlet after freeway driving.