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99 BMW 528I Fault Codes P1250, P1188, P1189: Diagnosis & Fixes for O2 Sensor and Fuel Trim Issues

Model: 99 BMW 528I Fault Code: P0133 Posted: 2008-08-15 18:20

I own a 1999 BMW 526I with persistent fault codes P1250, P1188, and P1189. Both upstream oxygen sensors have been replaced, but the codes remain active. The short-term fuel trim is at -6%, long-term is at +10%, engine coolant temperature reads 187°F, and the vehicle operates in closed-loop mode. I'm unable to find reliable information on what causes P1250 specifically. Any insights into potential root causes or overlooked diagnostics would be greatly appreciated.

Related fault codes
P0133P1188P1189P1250
Comments (3)
Anonymous 2011-02-17 07:53

I had the same issue on my 1999 BMW 528i with codes P1250, P1188, and P1189. I cleaned the Mass Air Flow (MAF) sensor and re-scanned—no improvement. I then checked for intake air leaks, which reduced a whistling noise during acceleration but didn't clear the codes. After replacing the fan clutch (bearing failure was confirmed), all fault codes cleared completely.

Anonymous 2011-02-17 07:58

Additional codes I encountered on my 1999 BMW 528i include P1188, P1189, P1421, P1250, P1423, P0101, and P0170. After replacing the fan clutch and adding transmission fluid following a P0740 code, my vehicle ran smoothly again. This combination of fixes resolved most of the issues on this model.

Anonymous 2011-02-17 15:54

P1188 — Fuel Trim Bank 1: Range/Performance Issue P1189 — Fuel Trim Bank 2: Range/Performance Issue P1250 — Unknown cause, possibly related to engine temperature or ECU timing; may indicate a sensor or control module issue. P1421 — Secondary Air Injection System (AIR) Bank 1 has insufficient flow detected P1423 — Secondary Air Injection (AIR) System Bank 1 insufficient flow detected P0101 — Mass or Volume Air Flow Sensor Range/Performance Issue (possible intake leak or MAF sensor fault) P0170 — Fuel Trim Bank 1 issue P0173 — Fuel Trim Bank 2 issue My experience suggests an air leak is likely the root cause. When the engine is running, gently pull up on the oil dipstick—any misfiring indicates a leak. If no change occurs (smooth idle), the problem lies in the intake system. A common failure point is the oil separator valve and breather pipe assembly under the intake manifold. If disassembled carefully, a broken internal membrane can be found. Replacing this valve and its associated pipes resolves the issue—these components are prone to cracking during removal. The intake manifold must also be fully stripped for proper inspection.