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P0173: Fuel Trim Malfunction (Bank 2)

Moderate SeveritySafe to drive (short-term)Fuel System

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.

Estimated repair costFrom €70 to €420+, varies by vehicle

What does P0173 mean?

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.

P0173 is a fuel system-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0173 and illuminates the check engine light.

What are the symptoms of P0173?

If your vehicle has triggered P0173, you may notice one or more of these symptoms:

  • Check engine light on
  • Rough or uneven idle, especially in Park or Neutral
  • Slight hesitation during acceleration
  • Reduced fuel economy on one bank
  • Possible faint fuel or sulfur smell from the exhaust

What causes P0173?

Here are the most common causes of P0173, ranked by how likely they are to be the culprit:

highVacuum leak on Bank 2 (intake manifold gasket, cracked hose)
highFaulty Bank 2 upstream O2 or air-fuel ratio sensor
mediumDirty or clogged injectors on Bank 2
mediumExhaust leak ahead of the Bank 2 upstream sensor
lowCarbon-fouled valves on Bank 2 (direct injection engines)

Is it safe to drive with P0173?

Generally yes, for short-term driving. The engine can typically still be driven, but a fuel trim out of range on one bank degrades fuel economy and risks catalytic converter damage over time. Fix within a couple of weeks.

How do you diagnose P0173?

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

How much does P0173 cost to fix?

Repair costs for P0173 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Fix vacuum leak on Bank 2 €10–€80 €60–€200 €70–€280 Moderate
Replace Bank 2 upstream O2 or air-fuel ratio sensor €60–€200 €60–€150 €120–€350 Moderate
Clean fuel injectors on Bank 2 €30–€120 €100–€300 €130–€420 Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0173 on specific vehicles

P0173 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

P0173 on BMW 3 Series

BMW 3 Series • 2006-2024

335i and 335d with the N54 inline-six are technically single-bank engines, but BMW V8 M3 (E92) and 6-cylinder V configurations historically logged P0173 with HPFP-related rough running. On modern N55/B58 inline applications P0173 is rare; on legacy V8 3 Series it typically traces to one side's pre-cat lambda sensor degrading first.

What causes P0173 on a BMW 3 Series?

BMW 3 SeriesUpstream lambda sensor degradation on legacy V8 3 Series
BMW 3 SeriesCarbon on Bank 2 intake valves on direct injection variants

P0173 repair cost for BMW 3 Series

Estimated costs for BMW 3 Series ownersadjusted higher than average due to BMW-specific parts pricing

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Fix vacuum leak on Bank 2 €14–€112 €84–€280 €98–€392 Moderate
Replace Bank 2 upstream O2 or air-fuel ratio sensor €84–€280 €84–€210 €168–€490 Moderate
Clean fuel injectors on Bank 2 €42–€168 €140–€420 €182–€588 Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0173 on Audi A6

Audi A6 • 2011-2024

C7 A6 with the EA837 3.0 TFSI V6 has well-documented Bank 2 fuel trim issues. The supercharger meters air for both banks but Bank 2 sits closer to the bulkhead, where heat soak accelerates O2 sensor aging. P0173 frequently appears before P0171/P0174 on this engine.

What causes P0173 on a Audi A6?

Audi A6Bank 2 upstream lambda sensor aging on 3.0 TFSI
Audi A6PCV diaphragm leak biasing toward Bank 2

P0173 repair cost for Audi A6

Estimated costs for Audi A6 ownersadjusted higher than average due to Audi-specific parts pricing

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Fix vacuum leak on Bank 2 €13–€104 €78–€260 €91–€364 Moderate
Replace Bank 2 upstream O2 or air-fuel ratio sensor €78–€260 €78–€195 €156–€455 Moderate
Clean fuel injectors on Bank 2 €39–€156 €130–€390 €169–€546 Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0173 on Mercedes E-Class

Mercedes-Benz E-Class • 2011-2024

W212 E-Class with the M276 V6 commonly logs P0173 from an air leak at the Bank 2 intake manifold runner gasket. Mercedes updated the runner gaskets mid-production. The job is labour-intensive because the intake plenum must be removed but the parts are inexpensive.

What causes P0173 on a Mercedes E-Class?

Mercedes E-ClassBank 2 intake runner gasket air leak on M276
Mercedes E-ClassBank 2 lambda sensor slow response after heat soak

P0173 repair cost for Mercedes E-Class

Estimated costs for Mercedes E-Class ownersadjusted higher than average due to Mercedes-Benz-specific parts pricing

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Fix vacuum leak on Bank 2 €14–€112 €84–€280 €98–€392 Moderate
Replace Bank 2 upstream O2 or air-fuel ratio sensor €84–€280 €84–€210 €168–€490 Moderate
Clean fuel injectors on Bank 2 €42–€168 €140–€420 €182–€588 Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0173 on Volkswagen Touareg

Volkswagen Touareg • 2010-2024

Touareg with the 3.0 TFSI petrol or 3.0 TDI diesel V6 commonly sees P0173 driven by Bank 2 lambda sensor aging or by minor air leaks at the PCV breather. Access on the Touareg is tight, and shop labour costs are higher than on smaller VW group V6 applications.

What causes P0173 on a Volkswagen Touareg?

Volkswagen TouaregBank 2 lambda sensor aging on 3.0 TFSI
Volkswagen TouaregPCV breather connection seep biasing toward Bank 2

P0173 repair cost for Volkswagen Touareg

Estimated costs for Volkswagen Touareg ownersadjusted higher than average due to Volkswagen-specific parts pricing

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Fix vacuum leak on Bank 2 €15–€120 €90–€300 €105–€420 Moderate
Replace Bank 2 upstream O2 or air-fuel ratio sensor €90–€300 €90–€225 €180–€525 Moderate
Clean fuel injectors on Bank 2 €45–€180 €150–€450 €195–€630 Professional

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

These codes are often seen alongside P0173 or indicate related issues:

P0170P0171P0172P0174P0175

Common questions about P0173

Which side is Bank 2 on my engine?

Bank 2 is the side that does not contain cylinder 1. On a V6 or V8 with cylinder 1 on the right (passenger side in most LHD layouts), Bank 2 is the left/driver side. Always confirm with a factory diagram for your specific engine; manufacturers do not always follow the same convention.

Why only one bank and not the other?

Single-bank fuel trim codes point to something localised: a leaking manifold gasket on that side, a degraded O2 sensor on that bank, or one injector flowing differently. Whole-engine problems like a weak fuel pump or a clogged fuel filter affect both banks and set P0170 or P0173 together with P0171.

Can a bad O2 sensor cause P0173?

Yes, particularly on the upstream sensor. A lazy or slow-responding sensor can mislead the ECU into adding or removing fuel until the trim hits the limit. Swapping sensors between banks is a fast way to confirm if the sensor is the culprit.

What's the difference between Bank 1 and Bank 2 fuel trim codes?

Bank 1 codes (P0171 lean, P0172 rich) cover the side with cylinder 1. Bank 2 codes (P0173 generic, P0174 lean, P0175 rich) cover the other side. The diagnosis is the same approach but you focus on parts and wiring belonging to the affected bank.

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