OBD2 s14 California smog results High Hydrocarbons caused by pre-cat or main cat?!

[LJBS13]

hello I am looking for some input from you guys.
Vehicle is a 1996 240sx all stock. I recently did a tune up with spark plugs, oil change, air filter, valve cover gaskets, and a few other leaks. the car had a check engine light throwing codes for knock sensor and front o2 sensor. I replaced both cleared the codes and took it to test my smog numbers. the car was slightly above the limit for hydrocarbons at idle and took longer than usual to drop numbers when taken to 15mph. smog tech told me it was my cat. well the car is obd2 so I have 2 cats, a pre catalytic convertor and main catalytic converter.
how do I know which to replace? there is no longer a check engine light. anyone in this position? I need to get this smog done asap any input is appreciated!

[coupesallday!]

Change both?

[San Diego Kouki]

I had the same problem with CEL for the cat, i only changed the cat and not the pre cat and the light remained. Luckily I don't have to smog but I say change the pre cat first since it's probably cheaper and hope that it's what you need...?

[racepar1]

The cat on the downpipe is the MAIN cat, NOT the pre-cat. The cat below that looks like a normal cat is the POST-CAT not the main cat. If you take it off you'll see the honeycomb section on the post cat is MUCH thinner than it would be on a standard cat. Also, look at the positioning of the O2 sensors. The downstream o2's ONLY reason for existence is to monitor the function of the cat. The downstream o2 is just below the main cat, BEFORE the post-cat. I have seen a shitload of S14's where people gut out the main cat, thinking it's just a pre-cat. In reality you guys should be buying test pipes and removing the post-cat...

[LJBS13]

The cat on the downpipe is the MAIN cat, NOT the pre-cat. The cat below that looks like a normal cat is the POST-CAT not the main cat. If you take it off you'll see the honeycomb section on the post cat is MUCH thinner than it would be on a standard cat. Also, look at the positioning of the O2 sensors. The downstream o2's ONLY reason for existence is to monitor the function of the cat. The downstream o2 is just below the main cat, BEFORE the post-cat. I have seen a shitload of S14's where people gut out the main cat, thinking it's just a pre-cat. In reality you guys should be buying test pipes and removing the post-cat...
that makes sense, thank you for the clarification. now if the downstream o2's primarily focus is to monitor the main cats function would the car throw a code if the cat was in bad condition? I just don't want to throw money at the wrong cat because the post cat alone is 220$ for a quality brand and the local exhaust shop said he doesn't even carry the main cat.

[racepar1]

that makes sense, thank you for the clarification. now if the downstream o2's primarily focus is to monitor the main cats function would the car throw a code if the cat was in bad condition? I just don't want to throw money at the wrong cat because the post cat alone is 220$ for a quality brand and the local exhaust shop said he doesn't even carry the main cat.


Just to start, there is not limit on hydrocarbons at idle. Only the 15mph and 25mph tests. What did you fail the ACTUAL tests for? How many miles are on the car?

The OE Nissan cats last for a super long time so I would like to know the mileage of the car we're talking about. The main cat for your car might be dealer only in CA. In which case you're talking very big money for a replacement.

The cat being bad can set a code (P0420), but it doesn't always do that especially on old OBD2 cars.

If you replaced the post-cat it would probably pass as well. The post-cat is just there to clean-up what the main cat can't handle.

[LJBS13]

Just to start, there is not limit on hydrocarbons at idle. Only the 15mph and 25mph tests. What did you fail the ACTUAL tests for? How many miles are on the car?

The OE Nissan cats last for a super long time so I would like to know the mileage of the car we're talking about. The main cat for your car might be dealer only in CA. In which case you're talking very big money for a replacement.

The cat being bad can set a code (P0420), but it doesn't always do that especially on old OBD2 cars.

If you replaced the post-cat it would probably pass as well. The post-cat is just there to clean-up what the main cat can't handle.

The car has 190,000 miles on the original motor.
I have a friend that owns a smog shop, so he pre tested my car before actually putting it in the system so i would know if it would pass a star smog test. I failed the 15mph test for high hydrocarbons. right after that he said check ur plugs and you need a new cat. I passed smog last year but soon after, my cel went on for front o2 sensor and knock sensor. i waited a long time, over 7 months driving with the cel on because the car felt fine. I replaced the sensors and plugs before smog, I cleared the codes and this is where im at now. I removed the post cat to see inside because i was reading how they are not really a cat but hollow inside, and in my case my post "cat" is completely hollow inside.

[lambkfob]

i was reading how they are not really a cat but hollow inside and my post "cat" is completely hollow inside.http://www.oksky.us/imiss/images/25.gif

[idlafie]

LJBS13,

Click on the following link to help you troubleshoot your high hydrocarbon readings

What causes high HC. What causes high Hydrocarbon - SmogTips.com. Failed HC Emissions. Why did my car fail the emission test for high HC emissions. What is HC. My vehicle failed the emission test for high Hydrocarbons. Why my car produced high HC. Hi (http://www.smogtips.com/failed-high-hydrocarbons-HC.cfm)

You might want to swap out your distributor cap & rotor if you already haven't. Also check your spark plug wires to make sure there are no shorts in the wiring. The above link also gives other areas you might want to check out also.

As for your catalytic converter, before you go out & spend money, try using the following tip. Drop your catalytic converter in a bucket of Dawn dish washing detergent and water. ...see attached link

Fixing Bad Catalytic Converters With Inefficiency Code P0420 - YouTube (http://www.youtube.com/watch?v=5icTmYItwiE)

The two above links should help in lowering your HC readings so that you can pass smog.

GOOD LUCK!!..hope it helps

ID