P0173 Fuel trim at its limit on bank 2

Fuel trim malfunction (bank 2)

Severity not verified — if the car runs badly, have it inspected

The sources describe poor economy, rough running and hesitation, but none rates how safe it is to keep driving, so no class is claimed.

It gets more serious if:

  • Inspect soon: Get it inspected soon if the engine stalls, will not start, or runs rough enough to hesitate badly. TroubleCodes.net lists hard starting, misfire and shutdown after start among the symptoms, and a Ford bulletin ties the code to no-start on some Lincoln LS cars.

Source: how we rate severity

P0173 in short

P0173 means the computer has run out of room to correct the fuel mixture on bank 2. The code itself does not say rich or lean, so read the fuel trim numbers first. A dirty air mass sensor, an air leak or a fuel pressure problem are common causes. Expect poor economy and rough running.

The computer constantly nudges the amount of fuel up or down to keep the mixture right, using the oxygen sensors on each bank. That correction is called fuel trim. P0173 is set when bank 2 needs more correction than the computer is allowed to give, around a quarter of the total.

The code does not tell you if the mixture is too rich or too lean. A dealer reading on one Mercedes C230 owner report described the paired P0170 and P0173 as the mixture being too rich at idle, so the scan tool numbers matter more than the code title.

Makers have found different causes. Mercedes has a bulletin that replaces the air mass sensor for P2004, P0170 and P0173, and Ford says P0172 to P0175 on 2018-2020 F-150 5.0L trucks can come from incorrect flex fuel learning, refueling habits or a purge valve stuck open.

Most likely causes

  1. Dirty or oil-contaminated air mass sensor

    Common general cause
    Why it ranks here
    TroubleCodes.net lists an oil-contaminated MAF sensor first, and a Mercedes bulletin replaces the air mass sensor for P0170 and P0173 together.
    What would change this
    Rises if the sensor is dirty or its reading is off at idle against the expected airflow. Falls if airflow readings look right.
    How to test the mass airflow (maf) sensor

  2. Air leak into the intake or an exhaust leak near the sensor

    Common general cause
    Why it ranks here
    TroubleCodes.net lists vacuum and exhaust leaks, which push the mixture away from target and use up the trim range.
    What would change this
    Rises if a smoke test shows a leak or trim is much worse at idle than at higher speed. Falls if the intake and exhaust hold smoke.
    How to test the vacuum hoses and intake leaks

  3. Fuel pressure too high or too low, or a restricted line or filter

    Common general cause
    Why it ranks here
    TroubleCodes.net lists pressure faults, damaged fuel lines, clogged filters and a defective pressure regulator, and its steps include testing pressure and the regulator.
    What would change this
    Rises if measured pressure is outside the maker spec. Falls if pressure is in spec.
    How to test the fuel pump

  4. Contaminated or faulty oxygen sensor

    Possible — needs testing
    Why it ranks here
    TroubleCodes.net lists oil-fouled and defective O2 sensors and silicone additives. A bad sensor can read wrong and push the trim, so it ranks below the direct causes.
    What would change this
    Rises if the bank 2 sensor reads stuck or slow and other causes are cleared. Falls if its signal swings normally.
    How to test the oxygen (o2) sensor

  5. Flex fuel learning or purge valve stuck open (2018-2020 F-150 5.0L)

    Supported by vehicle data
    Why it ranks here
    Ford SSM 48573 says P0172 to P0175 on these trucks may be due to incorrect flex fuel learning or a stuck-open purge valve. It applies to that truck only.
    What would change this
    Rises on a covered F-150 where the diagnostic manual test points to flex fuel learning or the purge valve. Falls on any other vehicle.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read the stored codes, freeze frame and live fuel trim for bank 2, and note whether trim is strongly positive (adding fuel) or negative (removing fuel).

    Low riskTool: OBD-II scan tool

    • IfTrim is strongly positive on bank 2

      It meansThe computer is adding fuel because the mixture is lean. Look for air leaks, low fuel pressure or airflow reading errors.

      Go to step 2

    • IfTrim is strongly negative on bank 2

      It meansThe computer is removing fuel because the mixture is rich. Look at the airflow sensor, fuel pressure and purge valve.

      Go to step 2

    • IfBank 1 trim is just as far off

      It meansLook at what both banks share, such as the air mass sensor or fuel pressure.

      Go to step 2

  2. Inspect the air mass sensor and its connector for oil or dirt and compare its reading with expected airflow at idle.

    Low riskTool: scan tool, MAF cleaner

  3. Have the intake and exhaust smoke-tested for leaks, and compare trim at idle with trim at higher engine speed.

    Best left to a shopTool: smoke machine, scan tool

    Warning:Leak testing is done with the engine running or the intake pressurised, close to hot and moving parts. Let a shop do it.

  4. Test fuel pressure against the maker spec, and on a flex fuel Ford check the purge valve and flex fuel learning as the diagnostic manual describes.

    Best left to a shopTool: fuel pressure gauge

    Warning:Fuel under pressure can spray and ignite. Relieve pressure and work away from sparks or hot parts. Let a shop do it.

Symptoms

  • Check engine light on
  • Worse fuel economy
  • Rough idle or hesitation
  • Hard start in some cases
  • Engine stalls or shuts down soon after starting Gets more serious: stop and have it checked.

Compare with: Rough idle with the check engine light on

How to fix

  1. With the engine off, check that the air filter box and the hose to the throttle body are tight, and look at the air mass sensor connector for looseness.

Parts and cost context

  • Mass airflow (MAF) sensor:A dirty sensor can often be cleaned with a cleaner made for it; Mercedes replaces the sensor in its bulletin. Confirm with trim data before buying. Labor varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Ford

  • 2018–2020 · F-150

    Supported by vehicle data

    Ford SSM 48573 links P0173 on the 5.0L F-150 to incorrect flex fuel learning or a purge valve stuck open. Whether it is still current is not verified.

  • SSM 48573Status unverified

    5.0L with DTCs P0172, P0173, P0174 and/or P0175

    2018–2020f-150 · issued · NHTSA 10171964

    Says the codes may come from incorrect flex fuel learning, refueling habits or a stuck-open purge valve, and sends the technician to a diagnostic manual pinpoint test.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Mercedes-Benz

  • 2001 · M-Class

    Supported by vehicle data

    A Mercedes bulletin replaces the air mass sensor for P2004, P0170 and P0173 on the M-Class (and C230 and others). Whether it is still current is not verified.

  • 070735Status unverified

    Air mass sensor replacement with P2004, P0170 and P0173

    2001m-class · issued · NHTSA 10002959

    Replace the air mass sensor when the check engine light is on with these codes.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • 070734Status unverified

    Air mass sensor replacement with P2004, P0170 and P0173

    2001c-class · issued · NHTSA 10002960

    Same air mass sensor replacement for the C230 and other chassis listed in NHTSA record 10002960.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Lincoln

  • 2003–2005 · LS

    Supported by vehicle data

    Ford bulletin 0565 covers no start, rough idle or MIL on with P0173 among several lean and misfire codes on the LS. The summary does not give the repair.

  • 0565Status unverified

    No start, rough idle or MIL with P0171, P0174, P0173 and others

    2003–2005ls · issued · NHTSA 10017903

    Lincoln LS bulletin for these codes; the NHTSA summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0170Bank 1 fuel trim has gone out of range

    Other bank Supported by vehicle data

    P0170 is the same trim fault on bank 1. A Mercedes bulletin lists P0170 and P0173 together with P2004 and points to the air mass sensor, which both banks share.

  • P0174Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

    Same system Supported by vehicle data

    P0174 is the lean-condition code for bank 2. A Lincoln LS bulletin lists P0171, P0174 and P0173 together, so check for a shared lean cause.

  • P0175Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

    Same system Supported by vehicle data

    P0175 is the rich-condition code for bank 2. Ford SSM 48573 lists P0172 to P0175 together on the F-150 5.0L.

All Fuel trim (air-fuel mixture) codes

Sources and review

  1. P0173 trouble code page — TroubleCodes.net
  2. On-Board Diagnostic Trouble Codes (generic code title list) — Delaware Division of Motor Vehicles
  3. Ford bulletin 0565: no start, rough idle or MIL with P0171, P0174, P0173 and others (Lincoln LS) — Ford Motor Company ,
  4. Mercedes-Benz bulletin 070735: air mass sensor replacement with P2004, P0170 and P0173 — Mercedes-Benz ,
  5. Mercedes-Benz bulletin 070734: air mass sensor replacement with P2004, P0170 and P0173 — Mercedes-Benz ,
  6. Ford SSM 48573: 2018-2020 F-150 5.0L with DTCs P0172, P0173, P0174 and/or P0175 — Ford Motor Company ,
  7. NHTSA complaint ODI 10440915 — NHTSA Office of Defects Investigation
Last reviewed
Written by
Saurabh K.
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