OBD-II trouble code
P0158: O2 Sensor Circuit High Voltage (Bank 2, Sensor 2)
The downstream oxygen sensor on bank 2 is reading persistently high — a steady rich-looking voltage that won't drop. As the bank-2 mirror of P0138, the usual causes are a genuinely rich condition, a sensor or wiring fault (a short to voltage or a contaminated sensor), or coolant/fuel contamination. Because it's a post-catalyst sensor, driveability is often unaffected, but a real rich condition behind it can foul plugs and overheat the converter.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0158 mean?
The downstream (sensor 2) oxygen sensor sits behind the catalytic converter and monitors how well the converter is working; behind a catalyst that is storing oxygen the trace is steadier than the upstream sensor, with no universal mid-range voltage. P0158 sets when the bank-2 downstream sensor's voltage stays abnormally high — pinned toward the rich end and not coming down. High oxygen sensor voltage corresponds to a rich, oxygen-poor reading, so the PCM is seeing either genuinely rich exhaust at that point or a sensor/circuit reporting high when it shouldn't. 'Bank 2' is the cylinder bank without cylinder number one, and 'sensor 2' is the post-catalyst position.
The causes divide into real rich conditions and sensor-circuit faults. A genuinely rich-running bank 2 — from leaking injectors, high fuel pressure, a contaminated upstream sensor driving the mixture rich, or other rich-side faults — can hold the downstream voltage high, and the rich code for this bank is P0175. P0172 is bank 1. On the circuit side, a short to voltage in the signal wire, a sensor contaminated by coolant or fuel that biases it high, internal sensor failure, or a wiring fault can each produce a stuck-high reading. Coolant intrusion from an internal leak is a notable cause, though for a reason worth stating precisely: contamination degrades the sensing element and makes its output untrustworthy rather than reliably biasing it in one particular direction, so the reason to care about coolant on a sensor is the internal leak it implies, not a tidy story about why the voltage is high. Since the rear sensor isn't the primary fuel-control input, the concern is less immediate driveability and more whether a real rich condition is harming the engine and catalyst.
For the driver, P0158 often appears as just a check engine light, but if a real rich condition is behind it you may notice a fuel-smell, fouled plugs, reduced economy, or rough running. It will fail an emissions test. Diagnosis checks the bank-2 downstream sensor live data, reviews fuel trims for a real rich condition (and companion codes like P0172), inspects the signal wiring for a short to voltage, and looks for contamination before replacing the sensor — because swapping the sensor won't fix an underlying rich condition.
Common causes
- Genuinely rich-running bank 2 (leaking injectors, high fuel pressure)
- Contaminated upstream sensor driving the mixture rich
- Short to voltage in the downstream sensor signal wire
- Sensor contaminated by coolant or fuel, biasing it high
- Internal downstream sensor failure stuck at high voltage
- Coolant intrusion from an internal engine leak
- Heater-circuit voltage leaking into the signal pin through a corroded or wet connector
Symptoms
- Check engine light with P0158 stored
- Often little or no change in driveability
- Possible fuel smell, fouled plugs, or rough running if truly rich
- Reduced fuel economy when a real rich condition is present
- Failed emissions test
- Steady high bank-2 downstream reading, sometimes with the bank 2 rich code P0175
Diagnostic steps
- 1.View the bank-2 downstream O2 sensor live data and confirm the voltage stays abnormally high.
- 2.Check bank 2 fuel trim. The rich code on this bank is P0175. P0172 names bank 1 and does not locate a bank 2 mixture fault.
- 3.Inspect the signal wiring for a short to voltage that would pin the reading high.
- 4.Work safely before anything comes apart: the post-catalyst sensor threads into pipework next to the converter, which stays hot long after shutdown, so let the exhaust cool, and if the car has to come off the ground put it on level ground on proper stands rather than a jack alone.
- 5.Look for sensor contamination from coolant or fuel, which also signals a deeper problem if coolant is present — and note that a contaminated sensor's reading can't be trusted in either direction, so treat the contamination as the finding rather than as an explanation of the voltage.
- 6.Verify the upstream sensor and fuel system aren't driving the mixture rich.
- 7.Replace the downstream sensor only after a real rich condition, wiring shorts, and contamination are ruled out.
Repair cost
$150 – $600
The low end of the printed bracket is a signal wire shorted to voltage. The high end is the bank 2 downstream sensor, or the rich-mixture repair on that bank (P0175, not P0172) when the trims say the mixture is actually rich. Read the bracket as a planning span, not a quoted price. Coolant in the exhaust is a larger job and can run past the printed high.
Estimate your repair
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Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.