check engine light/ecu codes debate!!!!

[sil80forme]

ok well to start off. a friend of mine is haveing probs with his 240sx.. sooo i told him to check the codes on the ecu! well the response that i got was "they're aren't any codes to run". so i asked him to explain how he knew that. well he in return stated that the check engine light wasnt on. If the light isnt on ecu can not have codes.. So of course i know better and said " there doesnt have to be a check engine light to have a code come up." He in return said "omg... lol, who in the hell told you that".. so the debate begins..... who is correct myself or my friend????

[kevincao]

http://www.240sx.org/faq/articles/ecm_codes_doch.htm

Sentence before procedure number 1. I'm sure its in the FSM. Do some reading or search the forum. I'm sure someone had the same problem. Hope this helps:

OK. When you set the ecu to diagnostic mode does the check engine light blink then?
If not break out the test-light, multimeter or take it back to your mechanic.
You could also get an ECU from a friend, try it document everything give it back. You'll then know where you stand.
Also disconnecting the battery for a while (in between actions) will give you a good starting point, if you cant fix it.

[sil80forme]

but thats sayin that either can show u there is something wrong, either the ecu LED and or the check engine light. that does not speciy whether or not the check engine light has to be on for the ecu to have codes, which i dont think it does and other people have agreed with me it doesnt. i am tryin to clarify that even though the check engine light is not on, that the ecu can still have codes meaning something is wrong.

[projectRDM]

The ECU has a two trip fault cycle. It will pick up a code if a condition is apparent but not trip the CEL if it goes away within a certain time limit, the code is then stored for future indication if the condition is seen again. After 30 cycles it erases it completely.
So yes and no. There may be codes present in the ECU, but they're not a concurring problem if the CEL hasn't already been tripped to indicate them.
Also you may want to point out to him that a lot of engine problems won't throw a code at all since they may not relate to emissions output, so it's possible that there never was a code present.