P0300 on a 2021 Ram 1500 Classic

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 2021 Ram 1500 Classic

Ranked from 659 NHTSA owner complaints filed for this vehicle.

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1
Camshaft/lifter (valve-train) failure -- "Pentastar tick"
High severity
Multiple owners report rocker arms/lifters failing and cam lobes being ground flat, stopping valve action on individual cylinders and causing multi-cylinder misfire; a known issue on this engine family.
Reported in 3 NHTSA complaints
Parts:valve lifters camshaft
2
Coolant intrusion into a cylinder (cracked block/head)
High severity
A borescope inspection found a small crack bleeding coolant into a cylinder, fouling combustion and triggering a multi-cylinder misfire code; confirm with a compression/leak-down test and coolant loss with no external leak.
Reported in 1 NHTSA complaint
3
EGR cooler/bypass valve failure
Medium severity
A broken EGR cooler bypass control was confirmed by the dealer as an ongoing issue on these trucks; a stuck EGR path disrupts combustion and can trigger misfire.
Reported in 1 NHTSA complaint
Parts:EGR cooler EGR valve
4
High-pressure fuel pump failure disrupting fuel delivery
High severity
Recall-related HPFP failures caused stalling and rough running from inadequate fuel pressure; relevant on ecodiesel/recall-affected trucks.
Reported in 1 NHTSA complaint
Parts:high-pressure fuel pump
5
Worn or fouled spark plugs
Low severity
Degraded plugs are the single most common P0300 cause; inspect gap and electrode wear against the maintenance interval.
Common cause
Parts:spark plugs
6
Failing ignition coil(s)
Medium severity
A weak or dead coil produces an intermittent or cylinder-specific spark loss; swap coils between cylinders to confirm the misfire follows the coil.
Common cause
Parts:ignition coil
7
Vacuum or intake leak
Medium severity
Unmetered air leaning out one or more cylinders causes rough idle and misfire; check with a smoke test.
Common cause
8
Clogged or leaking fuel injector(s)
Medium severity
A restricted or leaking injector starves or floods a cylinder; check injector balance/flow rates.
Common cause
Parts:fuel injector
9
Low cylinder compression (worn rings/valves)
High severity
Mechanical wear reduces cylinder sealing enough to misfire under load; confirm with a compression test.
Common cause

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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