BMW 3 Series: N43 lean-burn petrol misfire — piezo injector and NOx sensor failure (318i/320i)
N43 lean-burn petrol misfire — piezo injector and NOx sensor failure (318i/320i) is a very common engine fault on the BMW 3 Series on the 2007–2012 cars, and we rate it major. Putting it right costs $900 to $4,000 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 88 documented faults on the BMW 3 Series.
- Vehicle
- BMW 3 Series
- Engine
- N43
- Years
- 2007–2012
- Fuel
- petrol
- System
- Engine
What you are looking at
Example of the part — not this vehicleA common-rail diesel injector. Contamination damages the nozzle at the tip.
What goes wrong
The N43 four-cylinder fitted to 318i and 320i models from 2007 runs a lean-burn direct-injection system with piezo injectors and a NOx sensor in the exhaust. The injectors leak internally and lose spray quality, and the NOx sensor fails, at which point the engine management abandons lean-burn and runs permanently rich. You get a lumpy idle, a warning light that often flashes, and much worse fuel economy. Injectors are replaced in sets and must be coded to the car, and a NOx sensor is expensive and frequently on back-order, so a misfiring N43 can cost more to put right than the car is worth. The engine was only sold in markets with low-sulphur petrol, so most N43 cars here are UK or European used imports.
What you would notice
- Lumpy, uneven idle that smooths out as the revs rise
- Engine warning light on, or flashing under load
- Hesitation and a flat spot when pulling away
- Petrol smell from the exhaust and noticeably worse fuel economy
- Rough running that comes and goes with engine temperature
How to check it
Where to look. Four injectors in the cylinder head beneath the plastic engine cover, and the NOx sensor screwed into the exhaust just behind the catalytic converter under the floor.
Start the car from stone cold and let it idle for two or three minutes with the bonnet up, because an N43 with failing injectors shakes and hunts at idle and often settles once warm — a pre-warmed engine hides the fault, so arrange to be there for the first start of the day.
Rest your hand on the gear lever and on the top of the dashboard at idle and feel for an uneven shake rather than a smooth buzz.
Watch the engine light on the drive out and again under hard acceleration in third and fourth gear.
Ask whether any injectors have been replaced, how many, and whether they were coded to the car.
Ask for the NOx sensor history too, because an owner who has already paid that bill is an owner who has maintained the car.
If a workshop inspection is possible: have a BMW specialist read the stored misfire counters per cylinder and the injector correction values, which show a failing injector long before it is audible.
What it costs to put right
$900 to $4,000 at New Zealand independent workshop rates. That is for this job on its own, not for the car.
Ask the seller
- “Have any injectors been replaced, and were they coded to the car?”
- “Has the NOx sensor been replaced, or is there a fault stored for it?”
- “Does the engine light ever come on, even briefly?”
Now check the BMW 3 Series you are actually looking at
Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.
Where this comes from
- BMWBlog — 'Spotlight On: BMW N43 Four-Cylinder Petrol 2007-2013'
- BimmerForums.co.uk thread — 'N43 misfires, hesitation, loss power'
- BimmerForums.co.uk thread — 'BMW E90 318i N43 Misfire'
- Bimmerprofs technical articles — 'The problems of the injectors and misfires' (N43/N53)
BMW 3 Series buyer questions
Is n43 lean-burn petrol misfire — piezo injector and nox sensor failure (318i/320i) serious on a BMW 3 Series?
We rate it major and very common on the BMW 3 Series. The N43 four-cylinder fitted to 318i and 320i models from 2007 runs a lean-burn direct-injection system with piezo injectors and a NOx sensor in the exhaust. The injectors leak internally and lose spray quality, and the NOx sensor fails, at which point the engine management abandons lean-burn and runs permanently rich. You get a lumpy idle, a warning light that often flashes, and much worse fuel economy. Injectors are replaced in sets and must be coded to the car, and a NOx sensor is expensive and frequently on back-order, so a misfiring N43 can cost more to put right than the car is worth. The engine was only sold in markets with low-sulphur petrol, so most N43 cars here are UK or European used imports.
How much does it cost to fix on a BMW 3 Series?
$900 to $4,000 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.
Which BMW 3 Series years are affected?
Our research records this fault on the 2007–2012 BMW 3 Series. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.
How do I check for it before I buy?
Start the car from stone cold and let it idle for two or three minutes with the bonnet up, because an N43 with failing injectors shakes and hunts at idle and often settles once warm — a pre-warmed engine hides the fault, so arrange to be there for the first start of the day. Rest your hand on the gear lever and on the top of the dashboard at idle and feel for an uneven shake rather than a smooth buzz. Watch the engine light on the drive out and again under hard acceleration in third and fourth gear. Ask whether any injectors have been replaced, how many, and whether they were coded to the car. Ask for the NOx sensor history too, because an owner who has already paid that bill is an owner who has maintained the car. If a workshop inspection is possible: have a BMW specialist read the stored misfire counters per cylinder and the injector correction values, which show a failing injector long before it is audible. Look here: Four injectors in the cylinder head beneath the plastic engine cover, and the NOx sensor screwed into the exhaust just behind the catalytic converter under the floor.