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Kevin C.
Kevin C., Subaru Master Tech.
Category: Subaru
Satisfied Customers: 1727
Experience:  34 years of Subaru service experience, ASE Master Technician.
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Sure, today my imprza 2.5 2002 coded p0304 & p0303....and

Customer Question

Sure, today my imprza 2.5 2002 coded p0304 & p0303....and p1443..... Still looking for the cause
Submitted: 1 year ago.
Category: Subaru
Expert:  camerontech replied 1 year ago.

hi there,

im cam, a licenced diagnostic tech.

these codes are for two different systems.

we'll start with the p1443...

this is for the evap system.

i have replace quite a few canister vent solenoids for this. this valve should hold vacuum when energized and fully release when commanded.

a smoke machine is the proper way to check evap system problems but manually checking valves is just as effective when one is not available.

the p0304 and p0303 codes are misfire codes for those 2 cylinders.

you have done the correct repairs to try and fix these codes.

I noticed that you replaced the coil pack. I would recommend swapping the 2 coil packs and clearing the codes and seeing if the same codes come back, as ive seen bad coil pack right out of the box. if there are different cylinder codes than the new coil pack was defective.

if its the same codes, than a compression test in all cylinders are necassary to eliminate engine mechanical problem.

compare cylinders 1 and 2 to 3 and 4. also attach a vacuum gauge while the engine is running and watch for the gauge rocking back and forth. this will also show a valve problem. But i would start with the coil swap first to eliminate a bad coil.

let me know what you find out


Customer: replied 1 year ago.
For the p1443 I've purchase the selenoid already ans will install tomorrow morning.For the missfire codes :The coil makes fire allright.
Wires are new oem subaru purchased in october with ngk plugs.
Compression is ok on all cylinder. (Motor has just been rebuilt @ 260000km)Could it be front 02 sensor...even if have nothing to do with fire
Customer: replied 1 year ago.
Or a bad spark plug..
Expert:  camerontech replied 1 year ago.

to eliminate secondary ignition (spark plugs, coils), this is where i would swap plugs and coils. a coil will have no problem firing a spark tester, but can sometimes break down when trying to fire the spark plug under compression as it demands more power to do so.

it could be a bad plug, and you will see this if you swap cylinders and clear codes and watch for diffferent misfire codes.

i have seen many techs saying coils are ok, but when i do a scope test on it, it fails.

the airflow fuel ratio sensor can do it too, but generally will store codes for the cylinders and also.

Customer: replied 1 year ago.
Ok will try this tomorrow morning and let you know if it fixes the problem...
Customer: replied 1 year ago.
One last thing....when asking to swap the 2 coil pack mean the actual with the old one ?!? There is only one coil pack on this car
Expert:  camerontech replied 1 year ago.

yes, sorry on the coil info.

was there any other misfire codes present the first time?

Customer: replied 1 year ago.
In the beginning no missfire at all (or no odb readings i should say)
Car only lack of power...always around 2500 rpm.....highway or city......was trying to accelerate but nothing happens when pushing on gas..... No speed incrase and rpm stay the same even if a give all possible gas. After a few minutes everything was ok again.
Missfire codes started this morning.
Does'nt seems to mather if engine is hot or not ....
Expert:  camerontech replied 1 year ago.


the misfire codes could be false due to lack of engine response.

replace that valve tommorow, clear the codes and if it does it again, record the codes and see if the same codes come back.

at this point im thinking, crank or cam sensore issue, fuel pump , throttle position or that air fuel ratio sensor that we were taking about.

all these components will have to be monistored for this issue.

pain in the butt, i know, but necassary

Customer: replied 1 year ago.
Ok will do tomorrow firts thing in the morning when i arrive from work.
Will let you know, thnx
Expert:  camerontech replied 1 year ago.

im leaning more towards that air fuel ratio sensor, as ive done a bunch of them too. these will definatley cause misfire codes and lack of power

Customer: replied 1 year ago.
Onle last thing.....motor is shaking like it wants to get out of it's compartment.....really look like it's missfiring
Expert:  camerontech replied 1 year ago.

kind of a silly question, but you are sure that you have the right firing order and ignition wires on right place?

Customer: replied 1 year ago.
Where is this sensor located, how can i test or is it possible to clean....
Customer: replied 1 year ago.
firing order and wires are in the right place.
Everything where running fine since october
Expert:  camerontech replied 1 year ago.

the air fuel ration sensor is the front o2 sensor on these. a defective one will cause a lean condition causing misfire codes.

I have included a complete locator attachment for you with the reply.

the engine shaking is a bit of a concern.

I would really recheck a few things...

1.the compression check must be recents

2. make sure you did not install a defective coil pack right out of the box, like i said i have had bad coils for this vehicle before right out of the box. if unsure, exchange for a known good one.

3. make sure the cat is not plugged, if the misfire is bad enough it can cause a cat to melt pretty quick.

you can always remove the o2 sensor to let it breath and see if the running problems disappear . if you have a backpressure sensor, the pressure should not exceed 1.5psi.

4. make sure you have injector pulse on all cyclinders too

Customer: replied 1 year ago.
Ok will check and replace coil pack tomorrow ( still have the old one, known good)...will let you know in the morning
Customer: replied 1 year ago.
Thnx for the schema btw , really cool !
Customer: replied 1 year ago.
Well after test : coil ok, plugs and wire ok, sensor changed.......this morning it runs even worse......would not even take chance to hit the street....