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BMW P0171 and P0174: system too lean
The engine computer is adding more fuel than it expects to, usually because unmetered air is getting in. Most often a vacuum or crankcase ventilation leak on a BMW.
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Codes on this page
| Code | Definition | Source |
|---|---|---|
| P0171 | System too lean (bank 1) | Generic OBD-II |
| P0174 | System too lean (bank 2) | Generic OBD-II |
| 29E0 | DME: fuel mixture control (BMW E-series) | BMW SI B07 03 14 |
| 29E1 | DME: fuel mixture control 2 | BMW SI B07 03 14 |
| 2882 | DME: mixture control bank 1 | BMW SI B07 03 14 |
| 2883 | DME: mixture control bank 2 | BMW SI B07 03 14 |
| 118001 | Mixture control: fuel-air mixture too lean (F-series) | SI B12 13 15 (PDF) |
| 118101 | Mixture control 2: fuel-air mixture too lean | SI B12 13 15 (PDF) |
P-codes are the generic OBD-II codes any reader shows. Six-character and four-character hex codes are BMW's own, read with a BMW-capable scanner; the wording is BMW's, from the source shown.
What it means
The oxygen sensors report a lean mixture, so the engine computer (DME) adds fuel through its fuel trims. When the correction passes its limit, it stores P0171 (bank 1) and, on six-cylinder engines with two oxygen-sensor banks or on V8s, P0174 (bank 2). BMW scan tools show their own mixture-control codes for the same condition. On inline sixes, "bank 1" is cylinders 1–3 and "bank 2" is cylinders 4–6. Both banks lean at once points to something shared, such as a crankcase ventilation (PCV) leak, the intake boot or a fuel supply problem; one bank alone points to something on that side.
Can I keep driving?
Usually yes, get it fixed soon. If the engine runs normally, it is generally safe to drive while you book a diagnosis. A lean engine can run rough, hesitate and, if left long enough, misfire. If the check engine light flashes or the car shakes, stop driving and have it checked.
Usual causes on BMWs
| Cause | Cars most affected | What you notice | Evidence | Repair prices (national average) |
|---|---|---|---|---|
| Valve cover / crankcase ventilation (PCV) leak | N52, N54, N55 (valve cover and PCV), B58 (valve cover with built-in PCV) | Lean codes on both banks, a whistle or hiss at idle, sometimes oil leaks from the same cover. | — | N55 valve cover / valve cover gasket (PCV) Indie $1,225–$1,547 Dealer $2,017–$2,557 Indie owners paid $930–$1,625 Dealer owners paid $1,450–$2,076 B58 valve cover / PCV failure Indie $1,248–$2,352 Dealer $3,128–$5,052 Dealer owners paid $3,049–$4,450 |
| Intake leaks: intake boot, charge pipe, gaskets | All, turbo engines especially | Lean codes plus boost faults on turbo engines; a smoke test finds the leak. | — | N55 charge pipe (intercooler to throttle) replacement Indie $578–$922 Dealer $1,287–$1,744 |
| Over-sensitive mixture check (software) | F80 M3, F82/F83 M4 (S55) built to 2/28/2015 | 118001 and/or 118101 with no change in how the engine runs. BMW's fix is reprogramming after correcting any mechanical fault found. | SI B12 13 15 (PDF) | — |
| Fuel supply or injectors | All | Lean under load rather than at idle; low fuel pressure codes may be stored too. | — | — |
| Valvetronic faults can trigger mixture codes as a side effect | N52 and other Valvetronic engines | An owner's N52K in limp mode had 29E0 and 29E1 alongside Valvetronic power supply faults; the mixture codes cleared once the Valvetronic motor and relay were replaced. | Owner report | — |
A bulletin isn't a recall and doesn't mean your car has the problem; open it for the exact models and build dates. Estimates are national averages built from BMW warranty labor time, regional shop rates and parts prices; open a repair for your ZIP. "Owners paid" is the middle half of sourced prices, shown where we have at least 3.
What to do
- Read the fuel trims (short and long term) at idle and at 2,500 rpm. Lean at idle that improves with revs points to a vacuum leak.
- Ask for a smoke test of the intake and crankcase ventilation before any parts are replaced.
- Fix any other stored codes first, especially misfire or Valvetronic faults.
Warnings it can bring up
Related codes
P0300–P0306 (misfire) P112F / 101F01 (throttle vs manifold pressure) 2A3F (Valvetronic power) All fault codes
Other BMW bulletins that mention it
- SI B13 04 20 Tank ventilation and mixture control faults stored in the DME G01, G02, G05, G07, G11 and others · May 2020
- SI B01 03 21 Oxygen sensor check engine light I01 · Jan 2021
- SI B01 20 21 I3 rex fuel injectors: ltd wty ext to 15 years/150,000 miles I01 · Nov 2021
- SI B01 03 24 G0x G12 2x 3x B46/58/S58 evap system purge (vent) valve ext ltd wty (elw) 15/150 G01, G02, G05, G06, G07 and others · Jun 2026
- SI B01 02 24 F2x/3x/48 G12 B46/48/58 evap system purge (vent) valve ext ltd wty (elw) 15/150 G11, G12, G20, F22, F23 and others · Jun 2026
- SI B01 13 25 I12/15 G12 F30 xb1 1h 2h evap system purge (vent) valve ext ltd wty (elw) 15/150 G12, F30, I12, I15 · May 2026
- SI B01 14 25 G01 G05 G20 G30 xb1 g h evap system purge (vent) valve ext ltd wty (elw) 15/150 G01, G05, F97, G30 · May 2026
- SI B01 05 24 I01 (i3 rex) evap system purge (vent) valve ext ltd wty (elw) 15 yrs/150,000 mls I01, G12, G30 · May 2026
24 of the BMW bulletins we hold mention it in their text; these are the closest matches.
Sources
- OBD-Codes.com: P0171 definition
- OBD-Codes.com: P0174 definition
- US EPA: What to do with the check engine light (EPA420-F-00-041)
- BMW SI B07 03 14: Diagnosing stored fault codes to help avoid repeat repair visits (NHTSA)
- E90Post owner thread: limp mode with 2A3F, 2A77, 29E0, 29E1 (N52K) and the fix
- NHTSA: BMW technical service bulletins and recalls by VIN
Bulletins are cited in the tables above. Explanations are ours; where a source is an owner report rather than BMW, it is labelled as one.