P0173Generic code

P0173 Code: Fuel Trim Malfunction (Bank 2)

The P0173 code means the computer cannot keep the bank 2 mixture within its correction limits. It is the generic version of the lean and rich mixture codes.

Severity
Medium
Can I keep driving?
Yes, but with caution
Estimated cost
$20 to $600
Difficulty
Medium
System
Air and Fuel
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0173 code means

The computer corrects fuel injection with short term and long term fuel trims, using the oxygen sensor. If the correction needed on bank 2 goes beyond the allowed limits, or if the sensor does not respond to the corrections, it stores the P0173 code.

Some manufacturers (Ford, Chrysler, Hyundai) use this code instead of P0171 and P0172 (bank 1) or P0174 and P0175 (bank 2). The diagnosis is the same: find out whether the computer is adding or removing fuel and why.

The P0171 guide explains how to read the fuel trims step by step.

P0173 code symptoms

  • Check engine light on.
  • Unsteady idle or jerking.
  • High fuel use or lack of power.
  • Sometimes misfires.

Likely causes of the P0173 code

  1. Vacuum leak, if the trims are positive and get better as rpm goes up.
  2. Dirty MAF sensor.
  3. Fuel pressure too low or too high.
  4. Clogged or leaking injector.
  5. Exhaust leak ahead of the oxygen sensor.
  6. Slow oxygen sensor.

How to diagnose the P0173 code step by step

  1. Read the fuel trims at idle and at 2,500 rpm: positive means a lean mixture, negative means a rich mixture.
  2. Look for vacuum leaks with smoke or carburetor cleaner.
  3. Check the MAF: about 1 g/s per liter of engine displacement at idle.
  4. Measure the fuel pressure.
  5. Check the upstream sensor: it should swing fast.
  6. Compare with the other bank. If P0170 is also stored, the cause is common to the whole engine.

How to fix the P0173 code

  • Repair the vacuum or exhaust leak.
  • Clean the MAF with a cleaner made for it.
  • Correct the fuel pressure.
  • Clean or replace injectors.
  • Clear the codes (which resets the learned trims) and verify on the road.

Technical reference data

  • Normal total trim: between minus 10 percent and plus 10 percent.
  • Typical code limit: plus or minus 20 to 25 percent.
  • Approximate values only.

Brand-specific notes

Ford and Mazda use this code often.

P0173 on Ford

Pay special attention to the intake manifold gasket and the MAF sensor.

P0173 on Mazda

Mazda is another brand that uses this code often.

Frequently asked questions about P0173

Can I drive with a P0173 code?
With caution: a wrong mixture can cause misfires and damage the catalytic converter.
Is P0173 a lean or a rich mixture?
It can be either one. The fuel trims on your scan tool tell you which.
Does clearing the code fix it?
No. Clearing resets the learned trims, but the P0173 code will come back if the cause is still there.
How much does it cost to fix P0173?
Between $20 and $600: from a vacuum hose to a fuel pump.
What is the most common cause of P0173?
A vacuum leak, when the trims are positive. A dirty MAF sensor and low fuel pressure are the next suspects.

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