What's the difference between P0137 and P0138?

They're the same sensor — the downstream O2 sensor on bank 1 — but opposite electrical symptoms. P0137 means the signal is stuck low (near 0V), which usually points to a short to ground, an open circuit, or an exhaust leak pulling in air. P0138 means the signal is stuck high (pinned near the rich end), which usually points to a short to voltage, coolant or silicone contamination, or a genuinely rich-running engine. The fix paths are different, so the distinction matters when you're diagnosing.

Can I keep driving with P0138?

In most cases yes — the downstream sensor doesn't drive fuel trim directly, so the car usually runs normally. The exception is when the root cause is a genuinely rich condition (a leaking injector or stuck purge valve), because sustained rich running can foul spark plugs and shorten catalyst life. You also won't pass an emissions test until it's cleared. Get the cause identified rather than just clearing the code.

Why would the sensor read high if the wiring is fine?

Two reasons. First, contamination — coolant from a small head-gasket leak or silicone from the wrong sealant coats the element and biases it rich. Second, an actually rich exhaust: a leaking injector, high fuel pressure, or an evap purge valve stuck open will give the downstream sensor a real high-oxygen-starved reading. Check fuel trims and look for coolant loss before condemning the sensor.

How much should P0138 cost to fix?

A downstream O2 sensor runs $150-400 installed. A wiring short repair is $50-500 depending on how much harness is damaged. If the underlying problem is a leaking injector or a coolant leak, the cost follows that repair and can be higher. The cheapest path starts with a wiring inspection — a chafed signal wire is a common and inexpensive cause.

OBD-II trouble code

P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

The downstream oxygen sensor on bank 1 is reporting a voltage that's stuck high — pinned near the rich end of the scale and staying there. Usually the sensor signal wire has shorted to a power source, the sensor is contaminated, or the engine genuinely is running rich.

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

What does P0138 mean?

P0138 is the high-voltage counterpart to P0137. Both involve the same part — the downstream oxygen sensor on bank 1 (Sensor 2), which sits after the catalytic converter — but they describe opposite electrical symptoms, and the causes barely overlap. P0137 sets when the signal is stuck low (near 0V). P0138 sets when the signal is stuck high, typically pinned above roughly 1.0V to 1.2V and refusing to come back down for longer than the PCM tolerates.

A zirconia oxygen sensor produces a higher voltage when it sees a rich (low-oxygen) exhaust stream and a lower voltage when it sees a lean (high-oxygen) one. A healthy downstream sensor on a car with a good catalyst should sit fairly steady in the middle of its range — often 0.4V to 0.7V — because the catalyst smooths out the rich/lean swings before exhaust ever reaches it. When the downstream reading climbs and stays near the top of the scale, the PCM has three suspects.

The most common is an electrical fault: the signal wire chafing against a hot exhaust component and shorting to a 12V or 5V source, which forces a false-high reading no matter what the exhaust is actually doing. The second is contamination — coolant from a small head-gasket or intake leak, or silicone from the wrong RTV sealant used during a past repair, will coat the sensing element and skew it rich. The third is a genuinely rich condition: leaking injectors, high fuel pressure, or a saturated evap purge dumping fuel vapor. Because the downstream sensor doesn't directly control fuel trim, P0138 rarely changes how the car drives, but it almost always blocks emissions readiness and shouldn't be ignored if the underlying cause is a rich-running engine that's slowly poisoning the catalyst.

Common causes

Signal wire shorted to voltage — chafed insulation contacting a power source or the 5V reference (the most common electrical cause of a stuck-high reading)
Coolant contamination of the sensor from a small head-gasket or intake-manifold leak
Silicone contamination from the wrong RTV sealant used in a prior repair
Genuinely rich exhaust — leaking fuel injector, excessive fuel pressure, or a stuck-open evap purge valve
Failed downstream O2 sensor reading high internally
Corroded or water-intruded sensor connector pulling the signal high
Damaged catalytic converter changing the exhaust chemistry the sensor sees
Aftermarket exhaust work that pinched or melted the sensor harness

Symptoms

Check Engine Light on, usually with no obvious change in how the car drives
Failed emissions test, or catalyst monitor stuck 'not ready'
Possible faint fuel smell or black-tinged exhaust tip if the engine is actually running rich
Slightly reduced fuel economy in the rich-condition cases
Occasionally a rough cold start if a leaking injector is the root cause
No symptoms at all in the pure wiring-short cases

Diagnostic steps

1.Read live data for the bank-1 sensor-2 voltage. A reading pinned above ~1.0V that won't drop confirms the code is reflecting a real stuck-high signal rather than an intermittent.
2.Inspect the sensor's signal wire along its full run, especially where it passes near the exhaust. A chafe point shorting to power is the single most common cause of a stuck-high downstream reading.
3.Check the connector for coolant, water, or corrosion. Green crust or moisture inside the connector can bridge the signal pin to voltage.
4.If the wiring is clean, check fuel trims and look for a rich-causing fault — a leaking injector, high fuel pressure, or a purge valve that won't close. Strongly negative long-term fuel trims point to a genuinely rich engine.
5.Look for coolant loss or a recent overheat. Coolant intrusion into the exhaust will both contaminate the sensor and skew it rich.
6.Swap-test the downstream sensor with the bank-2 unit if it's the same part number. If the high reading follows the sensor to bank 2 (setting P0158), the sensor itself is bad.

Repair cost

$150 – $600

Most P0138 repairs are a downstream O2 sensor replacement at $150-400 depending on access. A wiring short repair can be as little as $50 for a single chafed wire or $200-500 for a damaged harness section. If the root cause is a leaking injector or a coolant leak, costs run higher and follow those repairs. Diagnose before replacing — a clean wiring inspection often saves the sensor.

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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.