I have a 2004 Elantra.
1. About 4.5 weeks ago the check engine light came on (Engine Code P1166).
2. About 3.5 weeks ago the symptoms started.
3. Symptoms: When I step on gas more than a little (about 1/4) the engine gets a throaty sound and no added power or acceleration. Difficulty climbing hills (revs high). Less often, the car idles roughly and has stalled a number of times in 2 weeks (usually after climbing a hill or stepping harder on the gas). Rough idle can be fixed by turning off car and restarting. Car can get up to interstate speeds slowly on flat ground.
4. 3.5 weeks ago found a nest that mice had made in air filter
compartment. Cleaned out.
5. 3.5 weeks ago I checked spark plugs and cleaned throttle intake and the mass air flow sensor. No improvement. The plugs were clean. A little carbon deposit on one eroded plug. Replaced that plug. I have 3 other new plugs.
6. 3.5 weeks ago I more thoroughly cleaned the mass air flow sensor. Check engine light went off briefly but then came back on. At that point I replaced the mass air flow sensor. No improvement.
7. 2 weeks ago the check engine light was off for most of the day and the car drove more normally around town but I did not try much hill climbing. The check engine light did come back on at the end of the day.
8. The next day I replaced the spark plug wires.
9. 1.5 weeks ago I took the car to the garage and they replaced the catalytic converter saying it was plugged. I thought the problem was fixed but the same symptoms came back in the afternoon on the 2nd day I drove it. The check engine light was back on shortly after I drove it home.