Long winded, but I thought this was interesting from AI. I uploaded that image and asked:
What can you determine from this O2 sensor reading data
The data shows live engine diagnostic readings for multiple oxygen (\(O_{2}\)) sensors and fuel trims, revealing improperly functioning downstream sensors and a likely rich or maxed-out condition on Sensor 2's bank.
Here is the breakdown of what is happening in the data stream.
1. Oxygen Sensor 1 & 5 (Upstream Sensors)
Reading: Voltage rapidly oscillates between ~0.1V and ~0.9V. Short-term fuel trim (STFT) sits stable at 0.000%.
Meaning: Normal operation. Upstream sensors (Bank 1 Sensor 1 and Bank 2 Sensor 1) are supposed to rapidly cycle up and down. This shows the engine control module (ECM) is successfully adjusting the air-fuel mixture between rich and lean.
2. Oxygen Sensor 2 & 6 (Downstream Sensors)
Reading: Voltage is abnormally high (1.190 V and 1.085 V) and shows extreme, rapid oscillation.
Meaning: Faulty behavior. Downstream sensors monitor catalytic converter efficiency and should stay relatively flat and steady around 0.45V to 0.7V.
The Problem:
An \(O_{2}\) sensor voltage should mathematically cap out around 0.9V or 1.0V. Readings above 1.0V typically indicate a short-circuit to power in the sensor wiring, a degraded sensor element, or a severe grounding issue.
The fact that they are oscillating wildly like upstream sensors indicates the catalytic converter is either completely depleted (failing to store oxygen) or the sensors themselves are completely failing.
3. Sensor 2 Short-Term Fuel Trim
Reading: Stagnant and flatlined at roughly +30%.
Meaning: Maxed out rich command. The computer is adding a massive amount of fuel to this bank, likely because it perceives a false lean condition, or the circuit is stuck.
Likely Root Causes
Bad Downstream \(O_{2}\) Sensors: The sensors themselves are likely old, contaminated, or internally shorted, producing voltages above the normal 1.0V threshold.
Wiring Harness Issues: A short circuit to the 12V heater wire or 5V reference wire can force the signal line to read over 1.0V.
Failed Catalytic Converter: Because the downstream waveforms are mirroring the upstream rapid switching, the catalytic converter is likely not scrubbing emissions properly.
To help narrow down the diagnostic steps, could you share the year, make, and model of the vehicle, and let me know if there are any specific Check Engine Light codes (P0xxx) present?
You said: 2014 GMC Sierra with 5.3 liter engine. Codes P0420 and P0430
The presence of P0420 and P0430 codes simultaneously on your 2014 GMC Sierra confirming that both the driver-side (Bank 1) and passenger-side (Bank 2) catalyst systems are performing below efficiency thresholds. [1, 2]
Because your scanner numbers the sensors as 1, 2, 5, and 6 rather than standard notation, the breakdown on your GM V8 engine layout is:
Sensor 1 = Upstream Bank 1 (Driver Side)
Sensor 5 = Upstream Bank 2 (Passenger Side)
Sensor 2 = Downstream Bank 1 (Driver Side)
Sensor 6 = Downstream Bank 2 (Passenger Side) [1, 2]
The Direct Link to Your Codes
The live data graphs explain exactly why your computer is setting the P0420 and P0430 codes: [1]
Waveform Mirroring: Look at the shape of the graph for Sensor 2 compared to Sensor 1 (and Sensor 6 compared to Sensor 5). The downstream sensors are mirroring the frantic, rapid switching of the upstream sensors. [1, 2]
Failed Oxygen Storage: When a catalytic converter is healthy, it stores oxygen, which causes the downstream sensor's voltage line to flatten out out into a stable, lazy line (typically around 0.6V–0.7V). Because your downstream lines look identical to your upstream lines, the computer sees that the catalytic converters have lost their ability to store oxygen and manage emissions, resulting in the efficiency codes. [1, 2]
Voltage Fault: The fact that the voltages are tracking over 1.0V (up to 1.190V) still points to a serious electrical, sensor degradation, or ground loop fault, which can skew the computer's trim adjustments. [1]
Platform-Specific Known Issues for the 2014 Sierra 5.3L
Before spending thousands on dual catalytic converters, check these three common failure points unique to this truck:
1. The Warped Air Box Issue (GM TSB #16-NA-111 / #18-NA-137)
This generation of GM trucks is highly notorious for the air filter housing warping and failing to seal properly. [1, 2]
The Cause: Dirt, sand, and unmetered debris bypass the filter entirely.
The Result: Abrasive particulate enters the combustion chamber, degrades the engine, fouls the O₂ sensor elements, and physically embeds into/clogs the catalytic converter faces on both banks, setting both P0420 and P0430 simultaneously.
Action: Pop your hood and examine the intake box edges to ensure it is sealing flush. [1, 2]
2. Broken Exhaust Manifold Bolts
The EcoTec3 5.3L engine frequently snaps the rear exhaust manifold bolts. [1]
The Result: This introduces fresh air into the exhaust tract right before the sensors. The upstream sensor reads this extra air as a "lean" condition, forcing the computer to dump excessive fuel (which matches the high fuel trim command seen on your Sensor 2 track). [1, 2]
Action: Listen for a rhythmic ticking noise on cold starts and physically look at the back of both exhaust manifolds to ensure the bolt heads are intact. [1]
3. Active Fuel Management (AFM) Oil Consumption
If your truck has high mileage, the cylinder-deactivation system (AFM) can cause the engine to consume engine oil. Burning oil creates chemical ash that coats and "poisons" the catalytic converter elements, destroying their efficiency over time. [1, 2]
Recommended Next Steps
Inspect the Air Filter Housing: Check for gaps or dirt tracks bypassing the filter. If it's dirty inside the tube, fix the housing before replacing any electronic components. [1]
Fix the Voltage / Trim Issue First: Resolve why Sensor 2 is pinned high and commanding massive fuel trims. If the fuel trims are dumping raw gas down the exhaust pipe, it will quickly melt or destroy a brand-new catalytic converter. [1, 2]
Perform an Exhaust Leak Test: Inspect the entire Y-pipe for cracks or leaks, especially near the flange gaskets or right after the downstream sensors. [1]
Are you experiencing any other symptoms, like a ticking sound on cold starts, reduced fuel economy, or engine oil consumption?
_________________________________________
You said: in town fuel economy is about 17-18 mpg. Oil Consumption is present.
_________________________________
An in-town fuel economy of 17–18 MPG is actually exceptionally good for a 2014 Sierra 5.3L V8 (the EPA city rating is around 16 MPG). This high fuel economy strongly suggests that your engine's combustion efficiency is still quite good, but the oil consumption is the smoking gun explaining your P0420 and P0430 codes.
Here is the connection between your oil consumption, the live data, and your codes.
