Q: rough idle, heavy missing, fault codes P0301 and P0456. lack of power, where do i start to diagnose this?

asked by on October 25, 2016

had been travelling a little bit today, everything was going fine, shut off the car to visit a business. came back out and it started to misfire really really bad, then my check engine light came on and I got the fault codes listed in the title. extreme loss of power, heavy shaking. cylinder specific fault code which is rare for the car so far since I've had it. the gas cap seems to be fine as well as the filler neck. Was just wondering what could cause these issues and where I would start diagnosing it.

My car has 150000 miles.
My car has an automatic transmission.

Hi there. From the description you have provided, for the P0301 rough idle and heavy misfire and P0456 evaporative small leak detected, the problems are unrelated. The P0301 is a #1 cylinder misfire code. It could be, a spark plug failure or an ignition coil has failed. It could be, a fuel injector failure or a cylinder compression problem, ie; valve train or piston issue. The P0456 is a small leak detected in the evaporative system. It could be, a faulty fuel cap. A visual inspection may appear okay but, does not seal the deal. It could be an evaporative vent solenoid not sealing or a faulty evaporative purge solenoid. With all of the possibilities that could be causing your vehicle’s misfire and evaporative small leak, I recommend having your vehicle’s Check Engine Light be diagnosed and repaired by a certified technician, such as one from YouMechanic.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing


Experienced Mechanic?

Earn up to $70/hr

What others are asking

Steering wheel shaking
The speed must be taken into consideration, in order to properly diagnose a shaky steering wheel during driving. A shaking or vibrating wheel at low speed may point to a problem with tires, or with bent/broken suspension components. On the...
Camshaft position sensor needs replacing
The most common cause of that vehicle not starting is the battery. The battery should be load tested to rule out that problem, then the next most common problem is the fuel pump. I would have a technician from YourMechanic...
AC problem on a '97 Isuzu Rodeo
That thing is almost 20 years old. It's 19 years old and it probably just needs refrigerant. You'll need to evacuate all the old refrigerant out, so you've got to have a recycling machine. Then you would put a factory...
Why is my car dying as im driving down the road.
If you are getting the car to die going down the road, then see if you are losing spark (https://www.yourmechanic.com/article/how-to-read-your-spark-plugs-by-olivia-marsh) to the spark plugs. If so, then check the distributor to see if you are getting oil in it or...
Revs up and out of control
Hello. The throttle cable (https://www.yourmechanic.com/services/throttle-control-cable-replacement) could be sticking causing the RPMs to increase excessively if the throttle cable is holding the throttle open when it shouldn’t be. There also could be a bad throttle position sensor (https://www.yourmechanic.com/services/throttle-position-sensor-replacement) or idle control...
Car stalls, hesitates, misfires - 1998 Oldsmobile Delta 88
There are several potential problem areas (besides fuel supply which you seem to have addressed), which could cause lean running and the symptoms you describe. A dirty throttle body (https://www.yourmechanic.com/services/clean-throttle-body), failing mass air flow sensor (https://www.yourmechanic.com/services/mass-airflow-sensor-replacement), vacuum leak (https://www.yourmechanic.com/article/is-it-safe-to-drive-with-a-vacuum-leak), or...
It feels like my car is about to break down, like I am very carefully carrying the engine home so it doesn't fall out.
Hi there. I would typically start by checking to see if the check engine light is on and then I would scan the computer for codes. This can give some insight as to what is going on. If there are...
I have a 2007 ford f150 I need an alternator without taking it out how do I know if it's a D plug in or an oval
If you send your VIN number, I can look up the exact Ford part number and visually look at the plug. Ford part number GLV-8704-RM appears to have an oval plug. The GL-974-RM appears to have a D plug. If...
Car slows down while accelerating
You should first replace the fuel filter and road test to see if engine runs better. If problem continues then test and replace the fuel pump.

Related articles

A Buyer’s Guide to the 2012 Ford Expedition
The The 2012 Ford Expedition has ample room for passengers and cargo, while still being able to tow anywhere from 8,900 to 9,200 pounds. The Expedition has two-tone exterior packages making it stand out against its competitors. Along with that,...
P2184 OBD-II Trouble Code: Engine Coolant Temperature Sensor 2 Circuit Low
P2184 P2184 code definition P2184 is an OBD-II generic code for the engine control module (ECM) detecting the engine coolant temperature sensor (ECT) #2 is indicating low range and does not correlate with ECT sensor #1. What causes the P2184...
P0663 OBD-II Trouble Code: Intake Manifold Tuning Valve Control Circuit/Open Bank 2
P0663 means that a fault has been detected by the PCM in the intake manifold tuning valve control circuit due to a faulty driver in the PCM.