P0300 on a 2018 Ram 2500

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 2018 Ram 2500

Ranked from 2,507 NHTSA owner complaints filed for this vehicle.

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1
Intake grid heater failure shedding debris into a cylinder
High severity
Multiple owners report the 6.7L Cummins grid heater element/bolt breaking and entering the intake, causing mechanical cylinder damage and misfire/catastrophic failure.
Reported in 2 NHTSA complaints
Parts:intake air (grid) heater
2
Low cylinder compression from valve-train wear (lifter/camshaft)
Medium severity
One owner describes progressive engine ticking that felt like a misfire and inconsistent speed, diagnosed by a mechanic as failed lifters.
Reported in 1 NHTSA complaint
Parts:valve lifter
3
Faulty or clogged diesel fuel injector
High severity
Worn/leaking injectors give a poor spray pattern or misfuel a cylinder; common on high-mileage common-rail diesels and confirmed with injector balance/cylinder-cutout test.
Common cause
Parts:fuel injector
4
Weak lift pump or clogged fuel filter reducing rail pressure
Medium severity
Insufficient supply pressure to the injection pump/rail starves one or more injectors, especially under load; check fuel pressure at idle and under throttle.
Common cause
Parts:fuel lift pump fuel filter
5
Water or air contamination in the fuel system
Medium severity
Diesel fuel drawn with entrained air or water causes erratic injector firing and rough running until purged/bled.
Common cause
6
EGR valve stuck or carbon-fouled
Low severity
A sticking EGR valve upsets the air/fuel charge and combustion stability, contributing to random misfire.
Common cause
Parts:EGR valve
7
Grid heater relay/wiring fault causing hard cold starts and initial misfire
Low severity
A failed relay or connector prevents proper intake air pre-heat on cold starts, producing rough running until the engine warms.
Common cause
Parts:grid heater relay

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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