P015B is a generic trouble code, which means it has the same meaning any vehicle. In the case of the Cadillac STS, It indicates that the Oxygen (O2) sensor on bank 1 is not cycling from lean to rich as fast as it should be.
P015B is usually caused by a bad oxygen sensor, but there can also be other causes.
P015B Definition: O2 Sensor – Delayed Response Lean to Rich (Bank 1 Sensor 1)
Here’s the definition of P015B for the Cadillac STS, broken down into its three main parts (it has the same basic definition for the Cadillac STS as it would any other vehicle).
Your STS’s oxygen sensors are responsible for metering the air as it exits the engine and travels through the exhaust system. The powertrain control module (PCM) uses this data to optimize the air-fuel ratio.
Delayed Response Lean to Rich
The oxygen sensor sends data to the PCM as a voltage value. When the PCM richens the air/fuel ratio, the oxygen sensor in question is not reporting the change fast enough. When this happens, P015b is stored in it’s memory, and the check engine light will come on.
Bank 1 Sensor 1
- Bank 1: The side of your STS’s engine with the first cylinder. With most engines, it’s the cylinder that’s furthest away from the flywheel/flexplate. If there’s only one cylinder head, you don’t need to worry about this. The bank 2 version of this code is P015D.
- Sensor 1: S1 is the first sensor in line. It’ll usually be on the exhaust manifold or directly after it. Sensor one is the most important sensor for exhaust metering.
Cadillac STS: P015B Symptoms
There are usually symptoms related to P015B. Here are the most common ones:
- Low engine power, stuttering, or misfiring
- Check engine light
- Decreased fuel economy
- The smell of gas
Cadillac STS P015B Causes
Here’s what you can do to diagnose P015B at home. We’ll start with what’s easy to check and work our way up from there.
Wiring Harness Damage
O2 sensors are notorious for having wiring harness issues, this is due to their proximity to the exhaust system. The hot pipes can make the wiring brittle.
Inspect your STS’s B1S1 wiring harness for burnt, cracked, or otherwise damaged wiring. If it’s damaged, repair it, and that should clear P015B.
When diagnosing P015B, there can be two scenarios. The first is that the oxygen sensor is bad, either due to an internal issue or a wiring problem. The second is that the O2 sensor is working fine and that the data it’s recording is accurate.
Exhaust issues are the way scenario two happens. If your STS’s catalytic converter is clogged, the exhaust gases can be delayed getting to the tailpipe. This increase in exhaust pressure can lead to a delayed change in recorded values from the oxygen sensor, which will be recorded as P015B.
An exhaust leak can cause the same exact scenario, but you would be able to hear it leaking.
The rest of this P015B diagnosis section will focus on determining if the B1S1 O2 Sensor is bad or not.
Bad Oxygen Sensor
Bad Oxygen Sensor – Swap Test (Easy Check on V6 and V8’s)
If your Cadillac STS happens to be a V6 or V8 model, you can do the “swap test.” This involves clearing the codes and seeing if the code “jumps” to bank 2.
Here’s how the swap test is done:
- Clear the DTC codes with your scanner.
- Swap the Bank 2 Sensor 1 O2 sensor with the Bank 1 Sensor 1. It’ll be on or right after the exhaust manifold.
- Run the engine until the check engine light comes back on.
- If the code changes to P015D on the Bank 2 side, that’s proof enough you need to replace the oxygen sensor you swapped.
- If the code remains P015B, you’ll need to continue pursuing your diagnosis, knowing that both O2 sensors are working fine.
Bad Oxygen Sensor – Capture the Short/Long Term Fuel Trim Values
If you happen to have access to a scan tool, you can use it to capture your STS’s short and long-term fuel trim values. You’ll need to ensure that the engine is warmed up before doing this.
Doing this will help you determine if the O2 sensor is operating within spec without looking under the hood. Compare the values that you captured with your tool with the values the manufacturer specifies.
While this isn’t a skill most DIY mechanics have, the concept is simple, and you can learn enough to use fuel trim to your advantage by watching this 11-minute video (it’s worth it).
P015B in the Cadillac STS is most likely caused by a bad O2 sensor.