P0300 on a 2016 Mercedes Benz C Class Phev
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 2016 Mercedes Benz C Class Phev
Ranked from 317 NHTSA owner complaints filed for this vehicle.
- 1
PCV/crankcase vent regulator valve failure (P052E)
Medium severityThe overwhelming majority of engine complaints on this M274 population describe a recurring PCV valve fault that raises crankcase pressure and degrades idle/running quality; while it sets its own code, it is a widely reported driveability disruptor on this engine.
Reported in 18 NHTSA complaints - 2
Low compression from internal engine damage (worn rings/valves, broken piston)
High severitySeveral owners report catastrophic engine failure or a broken piston with the check engine light flashing (active misfire) at the moment of failure.
Reported in 3 NHTSA complaints - 3
Ignition coil failure
Medium severityAn owner reports faulty coils needing replacement (not covered by the existing C63 coil recall) - classic misfire root cause; confirm by swapping coils between cylinders and watching the misfire counter move.
Reported in 1 NHTSA complaintParts:ignition coil - 4
Camshaft/timing-chain component wear
Medium severityOne owner describes camshaft-related symptoms matching an unrelated recall, consistent with valve-timing drift causing rough running and misfire on this engine family.
Reported in 1 NHTSA complaint - 5
Worn spark plugs
Low severityFouled or worn-out gap plugs are a top standard cause of random/multiple-cylinder misfire on any gasoline engine.
Common causeParts:spark plug - 6
Vacuum or intake leak
Low severityUnmetered air entering after the MAF leans out individual cylinders enough to misfire, especially at idle.
Common cause - 7
Clogged or leaking fuel injector
Medium severityA partially clogged or dribbling injector starves or floods a cylinder, producing a repeatable misfire on that cylinder.
Common causeParts:fuel injector - 8
EGR valve fault
Low severityA stuck-open EGR valve dilutes the intake charge at idle/light load enough to cause a misfire.
Common causeParts: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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