Powertrain, generic · Fuel and air metering

P015A O2 Sensor Delayed Response - Rich to Lean Bank 1 Sensor 1

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

The upstream oxygen sensor (sensor 1, before the catalytic converter) is the main control sensor for the air-fuel mixture. 'Delayed response' means the sensor starts reacting too late, or does not reach the target level at all, after the mixture has changed. This code covers the rich-to-lean direction, usually tested during deceleration fuel cut-off. GM uses this on switching (narrow-band) sensors: while the fuel cut-off for the downstream tests is running, it measures how long the upstream sensor takes to start dropping from above 680 mV and fails a filtered delay above about 0.58 s after one trip. Conditions include warm engine, 40–78 mph and a hot converter. CARB requires separate detection of slow and delayed faults in each direction (phased in from 2009 for downstream and 2011 for upstream sensors).

Likely causes

Upstream oxygen sensor aged or poisoned (silicone, oil, coolant, soot)
Exhaust leak ahead of the sensor (manifold, flange)
Mixture fault: intake air leak, injectors, fuel pressure, air flow sensor
Sensor heater weak, sensor too cold
Connector, wiring or control module software

Symptoms

  • Check engine light on
  • Slightly higher consumption or rough idle possible
  • Higher emissions, possible emissions test failure
  • Often no noticeable symptoms

Repair estimate

DifficultyEasyDIY possible
Parts and labour€80–300
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

oxygen sensor bank1 sensor1exhaust systemoxygen sensor wiring

Related codes

P015BP014CP015CP0133P2A00P0131

References: law.cornell.edu, gsi.ext.gm.com, en.wikipedia.org.