I have a 2004 Nissan Sentra 1.8L. When I first start the car in the morning when the engine is cold. The idle is rough like the engine is missing or something. Sometimes I have to depress the gas padal so that the engine will not turn off. After a few minutes, the RPM will be around 2.5 RPM and I have to let the engine warm a little and the miss idiling will go away. I took it to the car repair shop and they looked up the codes, Cooling fan Temp swith, thermostat, cooling temp sensor, crank sensor and the cam positioning sensor. Nothing worked. Other codes found were: P1278 air fuel sensor, p0420 catytic covertor. Please left me know if you have any suggestions.

Error Code P1278 -
Symptoms- Engine Light ON (or Service Engine Soon Warning Light)
- Possible rough engine idle
Possible causes- Air fuel ratio (A/F) Harness or connectors is open or shorted
- Air fuel ratio (A/F) sensor 1 Bank 1
- Air fuel ratio (A/F) sensor 1 heater Bank 1
- Fuel pressure
- Fuel injector
- Intake air leaks
- Exhaust Gas leaks
- PVC - Air Flow Meter

A code P0420 may mean that one or more of the following has happened:

•Leaded fuel was used where unleaded was called for
•An oxygen sensor is not reading (functioning) properly
•The engine coolant temperature sensor is not working properly
•Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
•Retarded spark timing
•The oxygen sensors in front and behind the converter are reporting too similar of readings


I have not had any issue of this sort before and I am not extremely mechanical but I would point fingers at your O2 sensors being bad

The Nissan delear discovered that the timing chain has stretched and has to be replaced. They said that it is a dual timing chain. Please explain. Does that mean that 2 timing chains need to be replaced? They say I'm looking at about 1200 dollars. Can I get it done cheaper?

If chains break, could it damage my valves? Or do the chains break on these cars?

Maybe your mechanic was talking about dual row chain.
But your chain is not actually a dual row timing chain, although it looks that way.
Some timing chains have two rows for the gears like this:


But your Sentra's 1.8 is just one very large gear like this:


Either way, both systems use one large chain, so no, you do not have to replace 2 chains.

If a chain breaks, it will damage your valves along with your pistons.
So far I've never heard of a case of timing chains breaking under normal operation.

I highly doubt your timing chain has stretched enough to cause any issues, I'd focus on the error codes.