Just replaced all oxygen sensor and coil pack and spark plugs and my 2000 deville is still stalling periodically I checked codes and got po 184, po 140, and po 155 and egr system what else can it after i replaced sensors

No, not the oxygen sensor. A couple of things you need to make clear. What light flickers, then stays on? That can't be a sensor light, it would have to be the "check engine light" or an engine temperature light (for like when your cluster has no temperature gauge, but just the "red light").
At any rate, the sensor you changed is not the engine temp. sending unit for the instrument panel. I believe on your '89 there would be two sensors for temp. One is the CTS for the engine computer, and the other is a sending unit for the dash, probably a 1 wire connector. It could be at fault, but it wouldn't cause stalling, I do not think anyway.
If it stalls and won't restart until cooled, this is a symptom of loss of ignition (spark) caused by either a faulty coil, or faulty Ignition Control Module (the coil controller), or last a faulty Crank Position Sensor (this sensor used by computer to determine ignition spark timing). You could test if ignition is lost, by checking for spark right after the engine warms and then stalls. If you have no spark to a spark plug right after it stalls, this is the reason for stalling. Usually it is the crank sensor, rather than the ignition control module or an ignition coil. Your car has 3 coils anyway in a "coil pack", so if one coil went bad, you'd still have two good coils, but engine would only be firing on four of the six cylinders-could easily stall a car.
You should check for trouble codes whenever you have a problem. A code might have been set. Easy to check for codes on your car, just ask if you don't know how-only takes a jumper wire or even a paper clip works. good luck.

P1401 Differential Press Feedback EGR Circuit High Input
Your EGR port is plugged. Replace your EGR valve, and clean the ports
A code P0302 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer
• A code P0305 may mean that one or more of the following has happened:
  
  • Faulty spark plug or wire
  • Faulty coil (pack)
  • Faulty oxygen sensor(s)
  • Faulty fuel injector
  • Burned exhaust valve
  • Faulty catalytic converter(s)
  • Running out of fuel
  • Poor compression
  • Defective computer
  • A code P0301 may mean that one or more of the following has happened:
    
    • Faulty spark plug or wire
    • Faulty coil (pack)
    • Faulty oxygen sensor(s)
    • Faulty fuel injector
    • Burned exhaust valve
    • Faulty catalytic converter(s)
    • Running out of fuel
    • Poor compression
    • Defective computer
    • A code P0302 may mean that one or more of the following has happened:
      
      • Faulty spark plug or wire
      • Faulty coil (pack)
      • Faulty oxygen sensor(s)
      • Faulty fuel injector
      • Burned exhaust valve
      • Faulty catalytic converter(s)
      • Running out of fuel
      • Poor compression
      • Defective computer
      • p1409
      • EGR Vacuum System Leak - Read Our Article on EGR Valve Codes For Help With This Oldsmobile Check Engine Light Code

Replace all the spark plugs &
all the ignition coils

Replace the oxygen sensors,
if over 100,000 miles on them

PO 720 is an output speed sensor circuit malfunction

That issue will have to be diagnosed & may be the shifting
problem

PO 304 may have been cyl #4 or not & you may have
others going bad as well

You never do just one spark plug

Only proper testing will solve your issues
no one on the phone or internet can diagnose
the actual vehicle,you need to have it

A P0304 and/or P0306 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #4 and/or #6.

Symptoms may include:
* the engine may be harder to start
* the engine may stumble / stumble, and/or hesitate
* other symptoms may also be present

Causes: A code may mean that one or more of the following has happened:
* Faulty spark plug or wire
* Faulty coil (pack)
* Faulty oxygen sensor(s)
* Faulty fuel injector
* Burned exhaust valve
* Faulty catalytic converter(s)
* Running out of fuel
* Poor compression
* Defective computer

Possible Solutions:
* If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
* If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.


Hope helps.

A code P0303 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

A P0301 code means that the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #1.

Symptoms may include:
· the engine may be harder to start
· the engine may stumble / stumble, and/or hesitate
· other symptoms may also be present

A code P0301 may mean that one or more of the following has happened:

· Faulty spark plug or wire
· Faulty coil (pack)
· Faulty oxygen sensor(s)
· Faulty fuel injector
· Burned exhaust valve
· Faulty catalytic converter(s)
· Running out of fuel
· Poor compression
· Defective computer

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

if its running rough you should have codes stored, have you checked for them yet ??

ck if you have spark at the coil for that cylinder. check the plug wire closely. if it has spark at the coil, then replace all the spark plug wires.

Information courtesy of http://obd-codes.com

A code P0304 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor (s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer

Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Hope this helps with your problem. Best of luck.
Greg