What does a mapsensor do

Could be a few things could have a vacuum leak could need a tune up could be a bad injector Is the check engine light on?

code p 1399 refers to random misfire ===causes --fuel /ignition--MAP sensor---IAC valve ( idle air control valve )
misfires are more from faulty injectors and low fuel pressure than from electronic problems
unless you have been burning lots of oil , the ports will be clean so have the injectors serviced

manifold absolute pressure sensor. its on fuel injected vehicles with computers. the computer changes air intake, timing as well as fuel pressure based off of map sensor. so you don't have to adjust anything at higher or lower altitudes and runs at maximum efficiency

Check that you have the proper air pressure in your tires. Change the air filter. If you're constantly hauling a lot of people or stuff, the extra weight will lower fuel economy.
Next go with a tune-up- new sparkplugs and wires.
You may also want to look for a small fuel leak. This could be particularly dangerous if you have one. If it is a fuel leak, don't drive the vehicle until it's repaired.

leaks.! why not fix that, first thing???????
no engine option stated.
but if the engine is MAF based, it will go lean (air/fuel mix) with ANY un-metered air leaks. (illegal)

that flash CEL means you will burn the CAT up ,if you keep driving
the operator guide tells you that, read it.

when all cars glow the CEL the mech, uses a scan tool
to scan PCM to see all the errors
im sure you have P0300 (misfire DTC errors) USA cars.
but there may be others, why not scan it and look????????????

running lean and flogging the engine can burn valves too.

1: check fuses
2: scan it,
3: report.
or its 2 days hard work , doing it the hard way.

I would start with a tune-up: new plugs and wires, new distributor cap/rotor button, new air filter, maybe a new fuel filter. If you get gas at the same location all the time, try a different one for a while too.
These would be the easiest to do first to eliminate some common issues.

triple posted, why not just run 1 post. and add to it?>?????
nobody wants to answer 3 posts for one car,
stick with one, please.
the answer to all posts (and has differ cars, why is that)
is , USE A SCAN TOOL
you need to post country the car is in,or we can not help
exact car, and country. or all answers will be wrong.
type it out.
the the CEL glows, all cars need to be scanned.
that is how OBD works.
all cars 1988 to now.
and for sure cars with SES, CEL or PGM-FI (honda) lamps glowing

define alot, that word means nothing
post range per gallon, MPG.
or post Liters per 100km
or >?
only data makes senses.
by pick the most expensive part on car to change out. on a guess.
post your MPG.

here are all thing for low mpg.

start at measurement errors. or stolen fuel.

http://www.fixkick.com/MPG.html

• A clogged fuel filter or problems with the electrical system powering the fuel pump can result in symptoms similar to a failed fuel pump. Have the shop verify that the pump is the problem before replacing.
  

Best Practices

• The fuel filter should always be replaced when replacing the fuel pump.
  

Common Symptoms

• A failing fuel pump will not create enough pressure in the fuel system, potentially resulting in a loss of engine power or a "no start" condition. You may also have a lean running condition that can set a diagnostic trouble code and turn on the Check Engine Light.
  
• Fuel pumps require replacement if they leak or begin making too much noise, which is a sign of malfunction. the fuel pump relay is located under hood inside a box with alot of other fuses in it and relay it will be maked fuel pump relay
  

check fuel filter if it runs good and only acts up on sudden increase in speed then I would say fuel related for sure,have fuel pressure at rail and see what it is.