Fuel and air

P016B — Closed Loop Air/Fuel Control at Limit: System Too Rich

Severity: medium Drive with care, to a shop

What P016B means

The engine control unit keeps the air/fuel mixture on target using oxygen sensor feedback and fuel trim adjustments. P016B is set when the trim has reached its limit in the lean direction, yet the mixture is still too rich. The cause can be excess fuel, wrong air or oxygen sensor readings, or a failing sensor.

Symptoms

  • Check Engine light on
  • Worse fuel economy
  • Fuel smell from the exhaust or cabin
  • Rough idle
  • Hesitation or stumbling when accelerating
  • Black smoke from the tailpipe
  • Possible misfires

Possible causes

  1. Faulty or slow oxygen sensor (usually upstream)
  2. Dirty or inaccurate mass airflow (MAF) sensor
  3. Excess fuel pressure or a bad fuel pressure regulator
  4. Leaking or sticking fuel injectors
  5. Intake or exhaust leaks skewing sensor readings
  6. Damaged wiring or connector at a sensor
  7. Clogged air filter or a stuck-open purge valve
  8. ECU fault (rare)

How to diagnose

  1. Scan for codes and freeze frame data, and note any other mixture, MAF or misfire codes.
  2. Review short and long term fuel trims at idle and at about 2,500 rpm.
  3. Inspect the air filter and intake ducts, and check that MAF readings make sense for engine load.
  4. Test fuel pressure and the regulator, including the return line if fitted.
  5. Check oxygen sensor signal on a scan tool graph and inspect its wiring and connector.
  6. Test injectors for leaks and compare cylinder balance.
  7. Look for exhaust leaks ahead of the catalytic converter.
  8. Check the EVAP purge valve and smell the oil for fuel dilution.

Common fixes

  • Replace the faulty oxygen sensor
  • Clean or replace the MAF sensor
  • Replace the fuel pressure regulator or a fuel pump that causes excess pressure
  • Clean or replace leaking injectors
  • Repair intake or exhaust leaks
  • Repair damaged wiring or connectors

Questions and answers

What does P016B mean?

The ECU has used up its fuel trim range trying to lean the mixture, but the mixture is still too rich. The cause is either excess fuel or misleading sensor data.

Can I drive with P016B?

Short trips are generally okay, but don't put off the repair. A rich mixture hurts fuel economy and can damage the catalytic converter.

What should I check first?

Start with fuel trims, the MAF sensor, the air filter and fuel pressure. Then test the oxygen sensor and injectors.

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.