Fuel and air

P0173 — System Too Lean, Bank 2 (Fuel Trim Malfunction)

Severity: medium Drive with care, to a shop

What P0173 means

P0173 means the engine control module is making large fuel adjustments on Bank 2 and still can't bring the air-fuel mixture back to normal. The ECM normally keeps the mixture near ideal using O2 sensor feedback and fuel trim. When trim hits its limit, this code is set. It usually points to a mixture that is too lean, with too much air or too little fuel.

Symptoms

  • Check engine light on
  • Rough or unstable idle
  • Hesitation or stumble when accelerating
  • Reduced fuel economy
  • Fuel smell from the exhaust
  • Rough running on V-type engines
  • P0174 or P0171 may also appear
  • Possible misfires

Possible causes

  1. Vacuum leak or unmetered air on the Bank 2 intake side (hoses, intake gaskets)
  2. Dirty or faulty mass airflow (MAF) sensor
  3. Faulty Bank 2 upstream oxygen sensor
  4. Fuel delivery problems: weak pump, bad pressure regulator, clogged injectors
  5. Exhaust leak ahead of the Bank 2 O2 sensor
  6. Secondary air injection system fault
  7. Damaged wiring or poor connections at the sensors

How to diagnose

  1. Scan for codes and freeze-frame data, and note any P0171 or P0174
  2. Read short-term and long-term fuel trims at idle and at about 2,500 rpm
  3. Inspect intake hoses, vacuum lines and manifold gaskets for leaks
  4. Check MAF readings and sensor condition, and clean it if dirty
  5. Look for exhaust leaks upstream of the Bank 2 O2 sensor
  6. Measure fuel pressure and compare to specification
  7. Check the Bank 2 O2 sensor signal on a scan tool graph
  8. Test injectors: resistance, cylinder balance, and clean if needed

Common fixes

  • Replace cracked hoses and intake gaskets to remove the air leak
  • Clean or replace the MAF sensor
  • Replace the Bank 2 O2 sensor
  • Repair or replace the fuel pump, pressure regulator or injectors
  • Fix exhaust leaks and repair damaged wiring

Questions and answers

What does P0173 mean?

The engine computer can't correct the air-fuel mixture on Bank 2. Fuel trims have reached their limit, most often due to a vacuum leak, a MAF sensor problem or a fuel delivery issue.

Can I drive with a P0173 code?

For a short time, yes, but get it diagnosed soon. A lean mixture can cause misfires and damage the catalytic converter.

Where should I start diagnosing P0173?

Start by checking the intake for air leaks and reading fuel trim data. Then test the MAF sensor, fuel pressure and the O2 sensor.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.