Worn or failed upstream O2 sensor, bank 1 sensor 1 (both sources). Burnt, broken or disconnected wiring or connectors (both sources); OBD-Codes.com says to check harnesses routed near hot exhaust parts and sharp exhaust-shield edges. Exhaust leaks (both sources). Defective catalytic converter (OBD-Codes.com only). Engine computer (PCM) failure (RepairPal).

Repair burnt, broken or disconnected O2 sensor wiring or connectors (both sources). Repair exhaust leaks near the sensor (both sources). Replace the bank 1 upstream O2 sensor once wiring and leaks check out (RepairPal lists oxygen sensor replacement as the common repair). Replace the catalytic converter only if testing shows it has failed; OBD-Codes.com warns that low-grade replacement or remanufactured converters are prone to repeat failure. Repair or reprogram the engine computer only after everything else checks out.

P015B🌫 Emissions / oxygen sensorsDrive with caution, fix soon

P015B: O2 Sensor Delayed Response, Lean to Rich, Bank 1 Sensor 1 (Meaning, Causes, Fixes)

Q: What does P015B mean?

P015B means the engine computer saw the upstream oxygen sensor on bank 1 (the one in front of the catalytic converter, on the side with cylinder 1) respond too slowly when the exhaust switched from lean to rich. Common causes are a worn O2 sensor, damaged wiring or connectors, exhaust leaks and, per OBD-Codes.com, a bad catalytic converter. The upstream oxygen sensor tells the engine computer how much oxygen is in the exhaust, so it can keep the air-fuel mixture right. A healthy sensor's voltage swings quickly between low (lean) and high (rich). OBD-Codes.com explains that P015B is stored when the bank 1, sensor 1 O2 sensor or its circuit fails to cycle as quickly or regularly as expected over a set time and under set conditions, specifically when going from lean to rich. It describes a healthy warmed-up upstream sensor fluctuating regularly between about 0.1 and 0.9 volts. Its sibling P015A is the same delay in the rich-to-lean direction, and P015C/P015D cover bank 2. RepairPal describes P015B more broadly as the engine computer detecting a failure of the sensor or of the information it receives from it, and its page doesn't name the bank or sensor position. The code is generic, so it can appear on any OBD-II vehicle; OBD-Codes.com forum threads show it on Chevrolet, GMC and Nissan models among others.

Q: Can I keep driving with P015B?

Drive with caution, fix soon. Get it checked soon. The sources disagree on how serious it is: RepairPal rates P015B Moderate, meaning it may not threaten safety right away but should be diagnosed within a reasonable time, while OBD-Codes.com says it should be treated as severe because the sensor has stayed slow or unresponsive for an extended time. The car usually still drives, but fuel economy and performance can suffer, and a lasting fuel-mixture problem can affect the catalytic converter. Let the exhaust cool fully before touching the sensor or its wiring.

Reviewed 2026-10-23 · Based on 2 cited source(s)

What P015B means

The upstream oxygen sensor tells the engine computer how much oxygen is in the exhaust, so it can keep the air-fuel mixture right. A healthy sensor's voltage swings quickly between low (lean) and high (rich). OBD-Codes.com explains that P015B is stored when the bank 1, sensor 1 O2 sensor or its circuit fails to cycle as quickly or regularly as expected over a set time and under set conditions, specifically when going from lean to rich. It describes a healthy warmed-up upstream sensor fluctuating regularly between about 0.1 and 0.9 volts. Its sibling P015A is the same delay in the rich-to-lean direction, and P015C/P015D cover bank 2. RepairPal describes P015B more broadly as the engine computer detecting a failure of the sensor or of the information it receives from it, and its page doesn't name the bank or sensor position. The code is generic, so it can appear on any OBD-II vehicle; OBD-Codes.com forum threads show it on Chevrolet, GMC and Nissan models among others.

Severity: can I keep driving?

Drive with caution, fix soon

Get it checked soon. The sources disagree on how serious it is: RepairPal rates P015B Moderate, meaning it may not threaten safety right away but should be diagnosed within a reasonable time, while OBD-Codes.com says it should be treated as severe because the sensor has stayed slow or unresponsive for an extended time. The car usually still drives, but fuel economy and performance can suffer, and a lasting fuel-mixture problem can affect the catalytic converter. Let the exhaust cool fully before touching the sensor or its wiring.

Common symptoms

Check engine light on
Lower fuel economy (both sources)
Decreased engine performance (both sources)
Other related trouble codes stored at the same time (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Worn or failed upstream O2 sensor, bank 1 sensor 1 (both sources)
Burnt, broken or disconnected wiring or connectors (both sources); OBD-Codes.com says to check harnesses routed near hot exhaust parts and sharp exhaust-shield edges
Exhaust leaks (both sources)
Defective catalytic converter (OBD-Codes.com only)
Engine computer (PCM) failure (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Fix any misfire, throttle position sensor, MAP sensor or mass airflow sensor codes first. OBD-Codes.com says those must be diagnosed and repaired before P015B, since an engine that isn't running well can set it.
Check for technical service bulletins (TSBs) for your vehicle that match the code.
With the exhaust cool, inspect the bank 1 upstream O2 sensor wiring and connector for melting, chafing or looseness, and look and listen for exhaust leaks ahead of the sensor.
Record freeze frame data with a scan tool, clear the code and see whether it returns.
If it returns, warm the engine up and watch the upstream O2 sensor voltage on the scan tool's live data at idle. OBD-Codes.com says it should swing regularly between about 0.1 and 0.9 volts; slower swings point to the sensor or its circuit.
Use your service manual for sensor resistance, voltage and ground checks with a multimeter. OBD-Codes.com advises disconnecting related control modules before resistance tests to avoid damaging them.

Typical fixes

Repair burnt, broken or disconnected O2 sensor wiring or connectors (both sources)
Repair exhaust leaks near the sensor (both sources)
Replace the bank 1 upstream O2 sensor once wiring and leaks check out (RepairPal lists oxygen sensor replacement as the common repair)
Replace the catalytic converter only if testing shows it has failed; OBD-Codes.com warns that low-grade replacement or remanufactured converters are prone to repeat failure
Repair or reprogram the engine computer only after everything else checks out

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The code comes back after a new sensor (an OBD-Codes.com forum post describes a car where it returned after both O2 sensors were replaced)
Misfire, MAF, MAP or throttle codes are also stored
The sensor is seized in the exhaust or hard to reach
You need live-data or wiring tests you can't do at home

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P015B?

Usually yes for a short time, but get it checked. RepairPal rates it Moderate, while OBD-Codes.com says to treat it as severe. Both list lower fuel economy and reduced performance.

Does P015B mean I need a new O2 sensor?

Often, but not always. Both sources list the sensor, wiring and exhaust leaks as causes, and OBD-Codes.com adds the catalytic converter. Check wiring, leaks and other engine codes first.

What's the difference between P015A and P015B?

Both are delayed-response codes for the bank 1 upstream O2 sensor. Per OBD-Codes.com, P015A is a slow switch from rich to lean, and P015B is a slow switch from lean to rich.

How much does it cost to fix P015B?

Neither source gives a repair cost for P015B. It can range from a wiring repair to an O2 sensor, so get it diagnosed first.

Sources

Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.