P0173 on BMW: Causes, Symptoms and Fix Cost

P0173 means the ECU has pushed long-term fuel trim on Bank 2 to the limit of its correction range and still cannot maintain the target air-fuel ratio. Because P0173 only affects one bank, it usually points to something specific to Bank 2 (a leak, a degraded sensor, or a dirty injector on that bank) rather than a whole-engine issue like a clogged fuel filter that would affect both banks.

Beyond the generic causes listed on the main P0173 page, these are the BMW-specific patterns we see most often:

Follow these steps to pinpoint the root cause of P0173:

1. Read live data: focus on short-term and long-term fuel trim for Bank 2. A long-term trim above +20% or below -20% confirms the bank has been hitting its limits
2. Compare Bank 2 trims to Bank 1. If only Bank 2 is out of range, the cause is bank-specific (one injector, one sensor, one manifold gasket section)
3. Smoke-test the intake on Bank 2 specifically. On V engines, intake manifold gaskets fail one side at a time as plastic shrinks unevenly
4. Swap the upstream sensors between banks and re-check live data. If the high trim follows the sensor, replace it; if it stays on the original bank, focus on mechanical causes
5. Inspect for exhaust leaks ahead of the Bank 2 upstream sensor. A small leak can pull fresh air into the exhaust stream and trick the sensor into reading lean

Estimated repair costs on a BMW (BMW parts and labour typically run 40% above the average for this code).

These codes commonly cluster with P0173 on BMW vehicles: