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).
Medium severity. Can I keep driving?Yes, you can normally drive in moderation, but get it repaired soon: you will use more fuel and the catalytic converter can be damaged over time.
Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank. Sensor 1 is the oxygen sensor before the catalytic converter.
Symptoms
- Check engine light on
- Slightly higher consumption or rough idle possible
- Higher emissions, possible emissions test failure
- Often no noticeable symptoms
Most 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
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
- It affects emissions.
What each character of P015A means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 1: Air and fuel
- 5A: Specific fault in that system
Frequently asked questions
Can I keep driving with code P015A?
Yes, you can normally drive in moderation, but get it repaired soon: you will use more fuel and the catalytic converter can be damaged over time.
What causes code P015A?
The most common causes are: 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.
Related codes
- P015BO2 Sensor Delayed Response - Lean to Rich Bank 1 Sensor 1
- P014CO2 Sensor Slow Response - Rich to Lean Bank 1 Sensor 1
- P015CO2 Sensor Delayed Response - Rich to Lean Bank 2 Sensor 1
- P0133O2 Sensor Circuit Slow Response Bank 1 Sensor 1
- P2A00O2 Sensor Circuit Range/Performance Bank 1 Sensor 1
- P0131O2 Sensor Circuit Low Voltage Bank 1 Sensor 1
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
- en.wikipedia.org: Oxygen sensor
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.