Chevrolet Tahoe iwth 250K miles, started shaking a little, P0300 code

Chevrolet Tahoe with 250K miles, 5.3L V8 engine 2WD. It’s my wife’s daily driver. Started shaking today, CEL came on. I checked CEL with my generic scanner and P0300 came up as the only code. Tahoe received new spark plugs and wires installed by a fulltime mechanic less than a year ago, July 2017. Chevrolet recommended plugs and wires used. Any ideas for why the code and why the rough running and shaking appreciated. Thanks in advance.

Maybe one of more coils is going bad. Usually they don’t trigger a random misfire code like a P0300 but it could happen. I’d pull all the plugs back out and have a look at them and run a compression test to make sure all is well with the engine. If the compression test looks good but one or 2 of the pugs looks sooted up, I’d think those cylinders are misfiring.

A scanning OBD2 reader can also be handy to see what is going on when the car is misfiring in real time with real data capture. Something the cheapie readers don’t do as well, or at all.

Or given that the engine has 250K, it may just deserve a nice new set of coils to eliminate that from the list of possibles.

What year is it?

Google is your friend:

Possible causes

•Faulty spark plug (s)
•Faulty ignition coil (s)
•Clogged or faulty fuel injector (s)
•Intake air leak
•Fuel injectors harness is open or shorted
•Fuel Injectors circuit poor electrical connection
•Ignition coils harness is open or shorted
•Ignition coils circuit poor electrical connection
•Insufficient cylinders compression
•Incorrect fuel pressure

Tech notes

The P0300 code means that a cylinder(s) is misfiring or is randomly misfiring. Start by checking for intake leaks, intake gaskets are a common caused of multi-cylinder misfiring. If no leak is found the next step is to replace the spark plugs. If the problem persists more tests needs to be done to diagnose the problem, see “Possible Causes”.

There are factory service bulletins for the following Chevrolet models:

2000 Chevrolet S-10
2000-2008 Chevrolet Camaro
2000-2008 Chevrolet Lumina
2000-2008 Chevrolet Cavalier
2000-2003 Chevrolet Impala
2000-2003 Chevrolet Malibu
2000-2003 Chevrolet Monte Carlo
2000-2003 Chevrolet Silverado
2000-2003 Chevrolet Suburban
2000-2003 Chevrolet Tahoe
2000-2003 Chevrolet Venture
2001-2008 Chevrolet Express
2002-2008 Chevrolet Astro
2000-2003 Chevrolet Avalanche
2000-2003 Chevrolet Trailblazer
2003-2008 Chevrolet Blazer
2003-2008 Chevrolet S-10
2003-2008 Chevrolet Trailblazer EXT
2000-2008 Chevrolet Factory Service Bulletin OBDII Code P0300

2014 Chevrolet Silverado 1500
2014-2016 Chevrolet Corvette
2015-2016 Chevrolet Silverado, Suburban, Tahoe
2016 Chevrolet Camaro

Sounds like you have a simple misfire. Chevrolets of that era will not provide you with a code for the specific cylinder that is misfiring. They just throw a P0300–Random Cylinder Misfire. You need to get out of the code menu of your scanner and get to the live data that shows misfires per cylinder in real time and in history. A generic DIY code reader may not have that ability.

If I were to guess, at that mileage I’d suspect an ignition coil failing. But that’s just a shot in the dark.