Failed smog check,no codes car runs fine. where do I start 98 neon dohc. if there was a problem with emissions would there be any codes? Idid find a bad coolant temp sensor. It would read that it was hot, when its not. Would this affect pcm fuel air timing raising emissions?

when you disconnected the battery you cleared all codes and your emissions[ smog/emissions need to run their cycles to be smog test ready as of now you will fail test ------https://smogcheckvacaville.com/general-motors-drive-cycle-for-smog-check/ ------------------https://repairpal.com/how-to-perform-a-basic-drive-cycle

This is common in the Nissan's could be the cam and crank sensor these go at around 70000 to 120000 should do both at once to be safe. that will fail an emissions test for sure. hope this helps

hard to say as to why it did not pass, but it sounds like you have a problem, most emissions are caused by too much hydrocarbins coming out the tail pipe, new cat converter,

Just because the car seems to run well, does not mean the emissions are within spec. For example, you could be running a little rich and the car will start and run great, but the exhaust is not within code.
What you can do is take it to a "smog shop", that is a repair shop with the proper equipment to diagnose the problem. If the testing is simply the reading of the on board computer codes, then most any place can read those, but you need an experinced mech to know what to do about it. May need an exhaust gas analyzer to physically sample and analyse the gasses. For the actual test, as long as the computer says all is fine, usually, you're good to go.
If the computer looses power any saved run data will be lost and must be re-accumulated to pass the smog test. A few hours to a day is usually enough run time to accumulate enough data.

Take it to autozone or another place like it and have them read the codes and tell us what they are.

Check the help link
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Vehicle smog test failed Due to NO MIL/check engine light?
http://schematicsdiagram.blogspot.com/2011/12/vehicle-smog-test-failed-due-to-no.html

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As you probably know by now there is no way to pass an emissions test as long as the check engine light is on....Your car is what they call an OBD2 system... You need to have the diagnostics codes pulled by a special tester specifically for OBD 2 systems... Once codes are pulled fix if necessary any problem and have the codes cleared... This will turn the check engine light off an enable you to pass smog providing your car is running properly...

p0410 code pos causes loose or broken hose on smog pump or bad check valve on exhaust man were smog hose atatches

when you drive it for a while does the temperature gauge seem to be pretty high or no?

CA you can take the car to a referree and get a one time waiver if you have spent a minimum of $450 on repairs not including smog fees. Gives you two years to get it fixed. Smog Referee Call Center
Toll Free: 1.800.622.7733
You can also sell the car to the state for "up to" $1,000