BMW 420d
Modular four-cylinder diesel, successor to the N47. Timing chain now at the front of the engine — the N47's biggest problem is eliminated. EGR and DPF remain wear items.
Reliable Diesel
Modern four-cylinder diesel — frugal and long-lived, but not a driver's engine.
Engine Weaknesses 6
Early B47 engines (2014–2016) are prone to premature chain elongation. The tensioner can no longer compensate. Rattling on cold start is the typical warning sign before chain snap and engine damage.
Symptoms: Metallic rattling or clattering on cold start; subsides after warm-up; in advanced cases audible during driving; MIL may illuminate
Overheating and leaking EGR coolers can vaporise glycol and trigger self-ignition. BMW recalled affected vehicles. If coolant enters the combustion chamber, hydraulic lock and engine damage are possible.
Symptoms: Coolant level drops without visible leak; whitish smoke from exhaust; rough idle; MIL; limp mode; rarely smoke from engine bay
With frequent short-trip use the DPF never reaches sufficient regeneration temperature. Soot accumulation blocks the filter and the engine enters limp mode. Long-term, oil level rises due to fuel intrusion.
Symptoms: MIL with message 'Particulate filter fault'; severe power loss; increased fuel consumption; black exhaust smoke on cold start; oil level rising
Common-rail injectors operate at up to 2,500 bar and wear at higher mileages. Knocking noise on cold start and misfires on individual cylinders. Replacement is complex and expensive.
Symptoms: Knocking noise on cold start; fault code 'injection quantity out of tolerance' or 'cylinder X misfire'; stumbling; increased consumption
From about 150,000 km onwards, failures of the electronic thermostat and electric water pump become more frequent. Overheating without early warning is possible. Replacing both parts simultaneously is recommended.
Symptoms: Coolant temperature rises unusually quickly or too high; heater blows cold; fault codes relating to the water pump; rarely a sudden failure
Primarily urban short-trip use causes the EGR valve to coke up. Power loss and increased emissions.
Symptoms: Power loss, rough running, engine check light, black smoke
Vehicle Weaknesses 5
The four-cylinder N20 petrol engines (420i, 428i) suffer from premature timing chain stretch. Neglecting the issue risks engine damage.
The N47 diesel (418d, 420d up to March 2015) is notorious for timing chain failure. Chain snaps and causes engine damage.
In the F36 Gran Coupé, water enters the boot through faulty clips on the tailgate trim and damages the Harman Kardon amplifier.
The xDrive transfer case is sensitive to mixed tyres and infrequent oil changes.
The A/C condenser is vulnerable to stone chips and loses refrigerant through small cracks.
Reports & Tests
112 owner complaints filed with NHTSA (2013–2020). Most reported: Electrical (51), Engine & Cooling (35), Engine (20).