Flashing green key light on Nissan Sentra

My guess is that the thief's chipped key cause the security system to block your key. Is the security light flashing?

Try a reset. There are a few ways listed in the link, do the battery disconnect one as the last resort.

https://www.google.com/search?q=2001+nissan+sentra+reset+security+system

If those don't work, you need an auto locksmith with a scan tool computer to pair you key and spare key. If you only have one key, get him to make you another.

https://www.google.com/search?q=auto+locksmith+near+me

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First do a hard reset by disconnecting both battery cables then touch and hold them together for 10 seconds, Now find the "original" key and it should work.

check that your spark plug wires are all secured tight. the far right cylinder is giving you the issue, cylinder #4. Is the engine light flashing while the car is running? if it isnt flashing, then its a past code and isnt misfiring anymore. if the engine light is flashing, you either have a bad plug wire, or a bad ignition coil/distributor.

There is no re-setting. You just change it out. Be sure and disconnect battery before unplugging the ecu. It's a simple swap and go.
If yours is like my '87 Sentra, there is no trouble code capability, but there are limited diagnostic functions. The red and green LED lights and the diagnostic switch (on/off), can tell you if problems are present in the system or not. You just have to follow the diagnostic procedure.
Here's how: Make sure diagnostic switch on ecu is off. Run engine at 2000 rpm for 5 minutes, at no load. Look at green led: if it flashes at least 5 times in 10 seconds (that is the oxygen sensor making it flash), turn car off. Turn diagnostic switch on. Turn key to on, do not start engine. Check led's: if red is off and green on steady-No problems present in system.
If the red light is on and green light is flashing, post back and I can furnish the rest of the diagnostic procedure-basically, you will be checking voltages and resistance of all solenoids and relays the ecu controls. The green led may flash in two different sequences (either longer flashes and short pauses, or longer pauses and short flashes) when in diag. mode (with the red led on), and that will lead you down two different diagnostic routes. Pretty simple, really, and pretty effective. Took me hours and hours before I realized I even had this diagnostic function.

Chances are you have problems with the ignition system.
First thing to do is get the computer scanned for trouble codes to find out why the check engine light is coming on.

The check engine light flashes is because all check engine light codes are caused by components that affect emissions. It's not actually an engine problem, it's a problem with either your O2 sensor, MAF sensor, fuel/air ratio, pyrometer, or exhaust.
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galletto 2

Its the brake system.
The flashing light tells you something is wrong with the cruise components. Could be the brake light switch, or the ABS if equipped.

O2 sensor/s faulty

Plug in OBD2 tool and check for error codes. Check code symptoms and reset where applicable.

Surely sounds like a low voltage issue. Relays commonly flash between open and closed when the power drops below 10 volts. Also other indicators in your descrp. explain the same. I would suggest giving the battery a good charge. Also if you can get your hands on a multi-meter and verify voltage that would be good. You can get one at walmart for around 10 bucks. Volts DC is what you want. Besides if you get it you'll def find other uses for it. Let me know if you need farther help