P0300 on a 2010 Chevrolet Hhr

P0300 is a generic OBD2 powertrain code for random or multiple cylinder misfire. Unlike P0301 through P0308, it does not point to one specific cylinder, so the cause can be ignition, fuel, air, sensor, or mechanical.

Not your vehicle? See general P0300 info.

Most common causes on the 2010 Chevrolet Hhr

Ranked from 515 NHTSA owner complaints filed for this vehicle.

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1
Ignition switch (key cylinder) rotating out of RUN
High severity
GM ignition switch recall 14V171000/14V047000: key-ring weight or a bump can rotate the switch toward ACCESSORY, cutting spark/fuel; several owners describe a shake or stumble immediately before the engine dies.
Reported in 6 NHTSA complaints
2
Camshaft/variable valve timing solenoid fault
Medium severity
Stumbling and stalling engine complaints coincide with camshaft and timing-solenoid replacement on this same vehicle after an overheat/electrical event.
Reported in 3 NHTSA complaints
Parts:camshaft position sensor variable valve timing solenoid
3
Carbon-fouled throttle body / idle air passages
Medium severity
Dealers diagnosed a rumbling engine and stalling event as needing throttle body cleaning, though the symptom recurred within weeks.
Reported in 2 NHTSA complaints
Parts:throttle body
4
Corroded/leaking external fuel supply line
High severity
A fast-leaking rusted steel fuel line reduces delivered fuel rail pressure, especially under load, which can present as a lean misfire.
Reported in 1 NHTSA complaint
Parts:fuel line
5
Worn or fouled spark plugs
Medium severity
Classic random-misfire cause; check plug gap/condition and pull one from a suspect cylinder.
Common cause
Parts:spark plugs
6
Failing ignition coil(s)
Medium severity
Swap a suspect coil to a known-good cylinder and see if the misfire follows it.
Common cause
Parts:ignition coil
7
Clogged or leaking fuel injector(s)
Medium severity
Uneven spray pattern or a leaking injector causes a lean or fouled cylinder.
Common cause
Parts:fuel injector
8
Vacuum leak (intake/PCV hoses)
Medium severity
Unmetered air causes lean misfire, most noticeable at idle.
Common cause
9
Low cylinder compression (worn rings/valves)
High severity
A compression test isolates a mechanical misfire from an ignition/fuel one.
Common cause
10
EGR valve stuck open
Low severity
Excess exhaust gas at idle dilutes the charge and can trigger random misfire.
Common cause
Parts:EGR valve

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0300 on your car still needs shop confirmation before any repair.

Common symptoms

Flashing or steady check engine light
Rough idle
Shaking under acceleration
Loss of power
Fuel smell from exhaust

What to check before replacing parts

Scan for cylinder-specific misfire codes

Look for: A companion P0301–P0308 pins the misfire to one cylinder — start there. P0300 with no cylinder code points at something hitting several at once: fuel pressure, a large vacuum leak, or timing.

Inspect the spark plugs and coils

Look for: Pull the plugs: black/sooty = rich, oily = oil intrusion, white/blistered = lean or overheating, cracked porcelain or an out-of-spec gap = replace. Then swap a coil to another cylinder — if a cylinder-specific code follows the coil, the coil is bad.

Check fuel trims for a vacuum leak

Look for: High positive short- and long-term trims (worst at idle, improving as RPM rises) mean unmetered air is getting in. Smoke-test the intake and listen for hissing.

Verify fuel pressure

Look for: Compare running/cranking pressure to the vehicle spec. Low or dropping pressure starves multiple cylinders and reads as a random misfire — suspect the pump, filter, or regulator.

Compression / leak-down test if ignition and fuel check out

Look for: A cylinder 15%+ below the others, or leak-down hissing from the intake (intake valve), exhaust (exhaust valve), or oil filler (rings), means the misfire is mechanical — not a swap-in part.

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Matching symptoms

Car Runs Rough Flashing Check Engine Light