I have a check engine light on, and the code is reading a 12 and a 72. I know what the 12 means, but could not find any explanation for a reading of 72. Anyone know what this means?
- posted
17 years ago
I have a check engine light on, and the code is reading a 12 and a 72. I know what the 12 means, but could not find any explanation for a reading of 72. Anyone know what this means?
72 Catalyst efficiency below required level. (Same as code 64)
This means one of two things.
1) your catalytic converter is plugging/cracking/failing 2) the O2 sensor behind the cat is failingThis code is set when the second O2 sensor does not detect the cat convertor working after warmup.
This is not a critical code, but if the cat is failing (plugging) then your acceleration and fuel economy will suffer.
If the downstream O2 was failing you'd get a downstream O2 fault, not a cat efficiency fault.
I've seen O2 sensors go bad, but not bad enough to set a fault. Usually it's the forward one and the only thing it does is give you bad milage and black exhaust. Figured the rearward one could go equally bad and throw a code even if the cat was good.
the forward one and the only thing it does is give
Actually that just made me think... if the forward O2 sensor is weak, the rear O2 sensor may very well call out a bad cat because the engine will be running rich.
Too bad there isn't an easy way to test O2 sensors.
An OBD I jeep wouldn't even have a downstream O2 sensor. You just lost your money.
Wrong. A failed downstream sensor will not set a cat efficiency fault. In fact, the poorer the downstream sensor functions, the less likely a cat fault would result.
A 1997 vehicle would be OBD II, no matter what it was called, or who made it.
Nice assumption, but that's not the way it works.
the forward one and the only thing it does is give
Never seen an O2 fail causing a lean condition?
Very easy with a DSO.
Oddly, no. All the ones I've seen have failed rich.
I have one of those. :)
yes
I replace many more for causing a lean condition than the other way.
I'll try to rember to dig one out tomorrow at work. Basically it looks kinda like a sine wave, varying between 0 and 1 volt for a zirconia O2. It needs to be able to get down to at least .2v and up to at least .8v, and needs to be able to do it in less than 100ms.
You can't see if it has shifted, but you can tell if it's capable of switching fast enough to do it's job.
It could be a cascade type problem. Going off my own experience with the ZJs only O2 sensor failing (but never setting codes). I first see it as a 2mpg drop in economy and then as black soot at the exhaust. A new O2 sensor fixes everything for the next five or so years. Newer compooters may well be better at detecting a marginal sensor.
If the upstream sensor is off and allows the engine to run rich then the cat will be overburdened and the downstream will detect this as a loss of efficency. Replace the upstream, the engine then runs stoich, the cat can deal, and the downstream is back to happy again.
Oh really? Please tell us how you came to that conclusion.
You're both leaping to conclusions without even having a clue as to how the PCM determines the cat converter's efficiency.
And you would be wrong again.
Your conclusion that an OBD II 1997 Jeep wouldn't flash a code 72.
1997 Jeeps don't flash OBD II codes. They flash 2 digit numbers using the check engine light. If you scan one with an OBD II scanner, you will, however, get an OBD II code.That won't be necessary. Please keep impressing us with your lack of OBD II and 1997 Jeep knowledge?
The OP never mentioned OBD anything. YOU assumed OBD I because he mistakenly called his 97 Wrangler a 'YJ', even though the trouble code indicated a cat efficiency problem. Cat converters weren't even monitored for efficiency until OBD II was mandated.
See my other post regarding the difference between a flash code and an OBD II code.
You mean other than not having a clue as to how the system works? OK then, here you go.
First, testing a cat's efficiency means testing it's ability to store oxygen. In a high O2 environment(lean) it stores oxygen. In a low O2 environment(rich) it releases oxygen.
The PCM constantly drives the system between rich & lean. Ahead of the cat, the upstream O2 is constantly switching between high oxygen and low oxygen. After the cat, the downstream O2 sees a more even oxygen level because of the cat, so it won't switch as much.
The PCM determines the cat's ability to store and release oxygen by the ratio of switching between the upstream and downstream O2 sensors. As the cat ages and becomes less efficient, the downstream O2 sensor's switch rate starts to approach that of the upstream O2. When it gets close enough a cat efficiency fault is set.
As O2 sensors age, their ability to switch degrades. They slow down or stop switching altogether.
Now tell me, if a fast switching downstream oxygen sensor means the cat is degrading, how can a failed/slow downstream sensor cause a cat efficiency fault? It can't. The PCM thinks the cat is working or it determines the sensor has failed.
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