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Trouble Code: P0303-Cylinder 3 misfire detected, especially when car is started

Hello Pros,
I would really appreciate if someone can shed some light on the problem.

Car: 2002 Toyota Camry - 4 Cylinder
Odometer: 165000 miles

  • Couple of days back, when I started the car it had a rough start(rough idle).
  • Once on the move I did not feel any issue in driving.
  • On the way I stopped at the store and when I started the car again “Check engine light came on”.


  • Auto center OBD-II diagnosis came up as P0303-Cylinder 3 misfire detected.

  • Technician found burned cylinder 3 spark plug. Hence technician replaced all 4 plugs to iridium spark plugs and did 3-step fuel cleaning (1.Oil System 2. Fuel System Cleaner 3. Intake Cleaning).

  • Check engine light went away, drove 15-20 miles and next day when I started the car light came back on!

  • Filled up Supreme grade gas, to see if it helps :slight_smile:

  • Again took it back to the garage, OBD-II Code: P0303-Cylinder 3 misfire detected. And they replaced #3 coil.

  • Once again code went away. Drove couple of miles and once again when I shut off car and restarted, check engine light showed up again (OBD-II Code: P0303-Cylinder 3 misfire detected).

  • Also, technician mentioned that when check engine light was on, OBD-II tool identified cylinder 3 misfire happens only at the start, especially when engine is cold. When car is on the road, it does not register any misfire even when engine light is on!

  • I have noticed that since the problem started with rough starting idle, every time “Check engine light” came on when I started the car and drove around 100 ft.

  • There is no egg-smelling odor (to eliminate catalytic converter failure)

Couple of other things…

  • Since last summer while changing air filter, Vacuum filter valve’s (Toyota part: 90910-12199 [084600-5480 12V] 548 Denso-Japan) outlet tube broke from one of the vacuum tube. Which I tied back with wire. Don’t know whether this could cause any issue of P0303 after several months!
  • But since last month or so, when car is stopped and move by pressing gas pedal, I hear flow of coolant noise (behind the dash board-into heat chamber). Somewhere I read that due to vacuum leak or lack of coolant in radiator could cause these symptoms. The coolant reservoir and radiator is full. Temperature gauge is half way when car is running, there does not seem to have excess heat. Also, RPM gauge does not fluctuate at all.


  • Engine valve cover gasket was replaced last spring
  • Last spring I got the coolant flush service done

What should I be looking next?

  • Vacuum filter valve?
  • EGR valve?
  • PCV system?
  • Fuel injector?
  • Gas filter?
  • Blown head gasket?
  • Intake manifold?

UPDATE: Technician rechecked #3 spark plug and coil and light went away for at least 2 days and came back on. Also, noticed some oil leak and heat gauge at little over half mark when running.

Thank you so much for your response.


Why was the head gasket replaced last spring?

Did the head gasket fail on its own?

Or did the threads strip out of the block?

Did the engine severely overheat at that point in time?

The injector could be swapped with another cylinder and see if the problem follows. A leak down test and compression test should be done before wasting more money on parts.


Thank you for feedback.

Correction: It was not Head Gasket, but valve cover gasket.
Currently while running car, heat gauge is little over half-mark.


Thank you for your response.

Technician rechecked spark plug and coil and engine light went away for 2 days. They suggested to replace #3 injector as engine is misfiring on cold start. They also mentioned there is no compression issue or over heating. Don’t know whether they did any technical test on it or simply an observation.

Also, noticed some oil leak today, do not know where it is coming from. Heating gauge little over half mark while on the road.

Could it be Head cover gasket or intake gasket leak?


The dash coolant temp gauge at the 1/2 mark is at the correct spot after the engine warms up. That’s not likely a problem. For a proper “firing” there has to be several things in place and working inside the cylinder

  • Correct amount of air and fuel flowing into the cylinder
  • Correct compression of the mixture above
  • Healthy spark
  • Occurring at the right time
  • Resulting in a “bang”, which forces the crankshaft to accelerate
  • Which is detected by the engine computer via the crank position sensor
  • Quick and thorough venting of the exhaust gas products

So something’s amiss with all that. The big clue in your favor of a quick diagnosis is that it only affects one cylinder. That makes it unlikely to be associated with airflow into the intake manifold for example.

hmmm … given what your shop has already determined, and it sounds like they know what they’re doing, I think the first thing I’d do is make sure that cylinder’s spark looks good by visual inspection. Next, I’d test to see if the injector is getting electrically pulsed like it should. After that either I’d follow the idea above of swapping that injector with another one, or I’d do an injector balance test, to see if that injector wasn’t putting out the expected amount of gas per pulse, or too much. I wouldn’t purchase and install a new injector quite yet. I’d probably do a fuel trim test before installing a new injector. Injectors, especially Toyotas which are known to have a pretty much bullet proof fuel injection system, rarely fail. Especially true if the fuel filter has been properly serviced at the recommended intervals.

This didn’t happen soon after a fuel filter replacement did it?

What does your shop say about your diy’er “fix” on the vacuum line?

Especially true if the fuel filter has been properly serviced at the recommended intervals.
There is no service interval for the fuel filter, it is on the pump in the fuel tank on these Camry's.

Tech said there is a no compression issue OR overheating? One or the other? Or both?

In the common vernacular, if “it is not raining or snowing” then it is not raining and it is not snowing.


I second knfenimore’s ideas. Start by moving the questionable and one other injector. Probably a pretty easy thing to do. If one injector is the problem, it can be cleaned or replaced. My hunch is one injector is leaking due to debris or deposits that the fuel system cleaning did not resolve.

Thank you all for your valuable feedback. Really appreciated.


  • All spark plugs are replaced with Iridium and #3 coil has been changed.

  • As per yours, @knfenimore and @shanonia’s suggestion, I told them to swap the injector with another one. But they said labor would be the same (they quoted me for #3 injector replacement around $400+, parts: $250, labor: $150+). Even though they charged me for diagnostic service and other parts and services, as per them they are not full service shop so could not do any other tests like leak down/ injector balance/ compression tests. So it seems whatever they told me initially about no compression issue or over-heating issue are all visual inspections.

  • I asked about my DIY vacuum filter valve fix, they even did not know what that part it is. But they said misfiring only happening at the cold start on cylinder#3 hence this is a non-factor.

  • Technician leaning towards #3 injector replacement.

  • I also asked about coolant flowing sound, they said it’s common. But my hunch is since coolant flush service last spring I had issues with air-condition in summer and now heat issue in winter (both started working after initial hiccups) and even though coolant reservoir and radiator are full, it might need burping (to remove air bubbles).

@GeorgeSanJose, @PvtPublic, @shanonia

  • I never replaced fuel filter. And prior to “check engine light” problem, twice I filled up the gas at different station and it’s possible that gas might be a problem.

Since yesterday light came back on and I got the same P0303-Cylinder 3 misfire detected code. Tech might have reset the code and light went away after that. I am sure it would come back if I start driving without warming up the car.

More ever since last 2-3 days I noticed some oil leak on left side of the engine (it does not seem to coming from “Valve cover gasket”). It seems somewhere in middle-left section of the engine.

It seems I need to find another full-service shop and get it diagnosed for compression/injector balance tests. Also get vacuum filter valve and gas fuel filter replaced. And hope it’s not the head cover gasket problem.

Thank you.

Not a full service shop? hmm … it seems from what you say this is a pretty good shop for routine maintenance and common-problem diagnoses, but for this particular problem you’d probably serve yourself better time and money-wise to seek out a well recommended shop with the needed skills and test equipment. At least for the diagnosis.

I concur that the vacuum filter probably isn’t the issue here, since the problem is confined to one single cylinder. And I don’t see how air in the cooling system would cause this either, but there shouldn’t be air in the cooling system since it can cause overheating, so if you think there are some air bubbles remaining, now is a great time to get rid of it. I’ve always been of the opinion that it is more efficient if all known common engine issues are addressed before trying to diagnose and fix tricky problems like this one.

You need a REAL mechanic. Check the “Mechanics Files” link above for a good mechanic in your area. These clowns are wasting you money and time.

1 Like

Let me throw a wild guess here.

You said you had the valvecover gasket replaced. These engines use spark plug tube oil rings that are integrated into the valvecover gasket (see PN 11213 detail I the parts list). The proper torque values for the valvecover bolts are low, 10 ft/lb for the center bolts and 8 ft/lb for the peripheral bolts. The head is cast magnesium (yeah, it surprised me when I looked it up too).

If the gasket seating groove was improperly cleaned, the “O-ring” portion around the #3 plug not properly seated, and/or the bolts not properly and/or unevenly torqued (I’d bet the overwhelming majority of mechanics don’t bother with a torque wrench) it’s entirely possible that oil is leaking past the “O-ring” onto the plug base and getting drawn into the cylinder when the parts are still cold and fouling the sparkplug. That would also answer the “black sparkplug” comment. The valvecover, being magnesium, may have even been forcibly warped out of flat by overtorqued bolts.

The next time the #3 plug is removed, take a photo. If my wild guess is correct, it’ll be obvious on the plug.

Here’s the image I referenced:

I’d suggest a simple compression test considering the high miles. You state these guys are not equipped to run compression and leakdown tests, etc but they state there is no issue with compression. Not.

What can happen with low compression is that over time it can cause the spark plug to foul and misfire. With a compression test you will know for sure what the mechanical health of the engine top end is like and that can influence how much money you’re willing to spend on the car or whether you will keep it for the much longer term.

As Mountainbike mentions, the visual appearance of the plug can very well mean something.

As to compression, ideally what you want is 175 PSI per cylinder and up considering the miles.
All cylinder readings should be close to each other.

Cast magnesium? Darn; get the hot dogs and marshmallows out… :slight_smile:

@GeorgeSanJose, @knfenimore
Agreed, but they are one of the national auto repair chain. So I trusted them to have all kinds of equipments for testing. But it seems I have to look up “Mechanic Files” for recommendations to find next one.

@“the same mountainbike”, @ok4450
Valve cover gasket was replaced in spring 2015 at the same repair shop, and until this month there were no issues. First time when they opened up plugs, they found #3 spark plug burned out. But since then they replaced all of them and last 2-3 times they rechecked all the spark plugs and did not found any oil/coolant or fouled spark plugs.

As you guys suggested, I am going to test it for compression/ leak down tests in next couple of days.

Thanks for all your help.

The best way to find a good inde shop in your area is to ask friends, co-workers, fellow church-goers etc who they use. From among that list, choose one that specializes in your make.