I'm the original owner of a 2000 Hyundai Elantra stationwagon and just had it inspected for renewal. The CEL is always on until my husband disconnects the battery and then that will last until there's over 60 miles on. When I have it checked, the code is always P1127, rich fuel. Would replacing the air filter cure the problem? Any suggestions would help. Thanks for your help.

 

P1127 is for long term Fuel Trim Error, engine running rich
Basically the CEL (check engine light) is saying the O2 sensor is seeing excessive fuel
Oh, it doesn't say whether it is the upstream or down stream sensor
There are two O2 sensors.

There are a number of reasons.
Bad O2 sensor and/or heater circuit
The wires on the O2 sensor is open or shorted or bad connection
Leaking pressure regulator or fuel injector
ECU

So, the first thing to check is the exhaust pipe to see if there is a leak
If the exhaust is not getting to the O2 sensor then CEL will turn on
Then check the wiring to the O2 sensor. Look for rubbing on metal/exhaust, fraying
Take a DVOM and check the resistance of the heater circuit (usually about 4 ohms)
You can look up this code on you tube for more information
If it looks okay then I would replace the O2 sensor.
If you live in the rust belt then bring it to a garage to change
It can be quite difficult to get out.

If that fails to fix it then need to check the pressure regulator and/or injectors

Good luck