1999 lincoln town car with engine code po354 . what is this

Replace all the spark plugs &
all the ignition coils

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

I would replace the oxygen sensor (or sensors).

Check this diagram for Town Car 4.6L (click images for zoom)...

Fig. Accessory drive belt routing-4.6L (VIN W) engine

Fig. Serpentine accessory drive belt routing-4.6L (VIN V) engine

Hope helps (remember to rate and comment this free answer).

Number 6 is located on the drivers side of the engine, 2nd one from the front. Passenger side is 1-2-3-4, drivers side 5-6-7-8.

There could be a hundred causes for a check engine light. First step is to hook up a computer scan tool and see what codes the computer has. The next step depends on the codes.

timing

Does the button on the passenger door work. They were prone to the drivers switches going bad. Usually the drivers window due to more use. They check engine light could be numerous things. Bad O2 sensor for example, Best to have it scanned. Could probably get the code at an AutoZone, Oriellys etc.

Check your gas cap. If someplace can test it you might luck out. This website advertises manuals and info online. The evaporator cannister is suppose to purify excess fuel vapors. But a system which is not sealed will code an error lamp.

brake light switch located on the steering column by brake peddal

change your o2 sensor