P0138
- Powertrain
- Generic (SAE)
- Emissions
- Safety
Diagnose soon
The downstream sensor's main job is monitoring the catalyst, not controlling fuel delivery, so a fault here is primarily an emissions compliance problem — the catalyst efficiency monitor will not complete and the vehicle will not pass an inspection. It is not a mechanical or safety hazard. It does not sit at low because some calibrations apply a slow fuel trim correction based on the downstream sensor, and a signal falsely pinned rich can push the ECU to lean the mixture out. It also earns attention because the same chafed harness or water intrusion causing it can affect adjacent circuits, and because one of the possible causes — a genuinely rich exhaust — damages the catalytic converter if left running.
- Safe to drive
- Usually safe to keep driving — this sensor monitors the catalyst rather than controlling fuelling. It will fail an emissions test and may slightly worsen fuel economy. Stop driving and get it looked at promptly if you also smell fuel, see black smoke, or the engine runs rough, since a genuinely rich exhaust will damage the catalytic converter.
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The oxygen sensor behind the catalytic converter on bank 1 is reporting a voltage higher than that type of sensor can physically produce on its own. That usually means something is feeding voltage into its signal wire — a chafed wire, a wet or corroded connector — rather than the sensor genuinely seeing a rich exhaust.
A conventional zirconia oxygen sensor generates its own voltage, swinging roughly between 0.1 volts lean and 0.9 volts rich. It is a galvanic cell; it has no source of external energy and cannot output much above about 1.0 volt. P0138 sets when the module reads the bank 1 sensor 2 signal above a high threshold — commonly around 1.0 to 1.2 volts, though the exact figure is vehicle-specific — and holds there. That is the key insight in this code: a reading above what the sensor can generate points at voltage entering the circuit from elsewhere far more often than at an exhaust condition. Sensor 2 is the downstream, post-catalyst sensor, used mainly to monitor catalyst efficiency rather than to control fuelling.
How the car detected itThe module samples the downstream sensor's signal voltage and compares it against a calibrated upper limit. If the voltage stays above that limit continuously for a defined period, under conditions where the sensor should be active and cycling, the fault is stored. Most calibrations require the sensor heater to have run and the engine to be in closed loop before the test runs, and many require the condition to repeat across two drive cycles before confirming the code and illuminating the lamp.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Usually safe to keep driving — this sensor monitors the catalyst rather than controlling fuelling. It will fail an emissions test and may slightly worsen fuel economy. Stop driving and get it looked at promptly if you also smell fuel, see black smoke, or the engine runs rough, since a genuinely rich exhaust will damage the catalytic converter.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- The check engine light stays on and the catalyst monitor stays incomplete, so the vehicle fails emissions testing and the light masks any future code you would want to know about. If the calibration uses the downstream sensor for a fuel trim bias, an artificially rich reading nudges the mixture lean, which costs a little economy and driveability. If the cause is a harness chafing against the exhaust or the subframe, that damage keeps progressing and can eventually take out neighbouring circuits. And if the reading genuinely reflects a very rich exhaust, the underlying richness is what will damage the catalytic converter over time.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Increased fuel consumption
- Possible rough idle or hesitation
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Signal wire shorted to supply voltage | Very common | — |
| 2 | Downstream sensor failed | Common | — |
| 3 | Connector pins corroded - high contact resistance | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
The sensor's live voltage with the key on and the engine off
With the engine off, a healthy sensor circuit should sit near the module's bias voltage, typically around 0.45 volts on most systems. A reading pinned above 1 volt with the engine not running is proof the fault is electrical, not exhaust-related. This single check saves most of the diagnosis.
The connector and the harness where it runs near the exhaust and underbody
Water intrusion after deep puddles or a pressure wash, and chafe against the exhaust or subframe, are the two dominant real-world causes. Both are visible and both cost a fraction of a sensor.
Whether anyone has recently fitted an aftermarket or universal sensor
Universal splice-in sensors get wired wrong regularly, and crossing the signal wire with a heater feed produces exactly this code. Also check that upstream and downstream connectors have not been swapped.
Fuel trims and the upstream sensor's behaviour
It tells you whether the exhaust is actually rich. Large negative trims and a lazy or pinned upstream sensor would suggest a genuine rich condition; normal trims with a healthy upstream sensor point firmly at the downstream circuit.
Freeze frame conditions and any other stored codes
P0138 alongside misfire, fuel system, or heater circuit codes changes the priority order. Knowing whether it set on a cold start, at idle, or at cruise narrows the search considerably.
Read freeze frame and all stored codes
Tools Scan tool with freeze frame
Record engine speed, load, coolant temperature, and fuel trim values at the moment the code set. Note any companion codes — particularly upstream sensor codes, heater circuit codes, misfire codes, or a rich fuel system code, which would suggest a genuine exhaust condition rather than a circuit fault.
ExpectedA repeatable condition, and either the presence or the clear absence of rich-condition companion codes
Check the sensor reading with the engine off
Tools Scan tool with live data
Turn the key on without starting and read the bank 1 sensor 2 voltage on the scan tool. A cold, inactive sensor should report close to the module's bias voltage, around 0.45 volts on most systems. A value above 1 volt at this point proves voltage is entering the circuit from an external source.
ExpectedApproximately 0.45 volts, or whatever bias the manufacturer specifies
Disconnect the sensor and re-read the value
Tools Scan tool with live data
This is the step that splits the diagnosis in two. With the sensor unplugged and the key on, read the same PID. If it drops to the bias voltage, the fault lies in the sensor or its own connector. If it stays pinned high with the sensor disconnected, the fault is in the harness between the connector and the module, or in the module itself.
ExpectedThe reading falls to the module's bias voltage once the sensor is unplugged
Inspect the connector and the full harness route
Tools Inspection light, lift, contact cleaner, terminal tools
Open the connector and look for water, green corrosion, oil, melted insulation, and spread or pushed-back terminals. Follow the harness along its whole route looking for contact with the exhaust, heat shields, the subframe, or driveline components. Underbody oxygen sensor harnesses take a great deal of abuse.
ExpectedDry clean terminals and an intact harness with no chafe or heat damage
Test for a short between the signal and the heater or supply circuits
Tools Multimeter, factory wiring diagram
With the sensor unplugged and the key off, measure resistance between the signal wire and each of the heater circuit wires, and between the signal wire and any nearby supply circuits in the same harness. These should read effectively open. A low resistance here identifies the exact fault mechanism behind a signal pinned above the sensor's own output capability.
ExpectedAn open circuit between the signal wire and every power or heater circuit
Verify the signal return and grounds
Tools Multimeter, back-probe leads, factory wiring diagram
Check the sensor's signal return or low-reference circuit for continuity and for excessive resistance back to the module. A lost or high-resistance signal return biases the reading upward and can produce this code with a perfectly healthy sensor. Check the associated engine and module grounds at the same time.
ExpectedLow resistance on the signal return with clean module grounds
Confirm the exhaust is not actually running rich
Tools Scan tool with live data, spark plug inspection
Only once the circuit is proven clean: watch the upstream sensor cycling, check short and long term fuel trims across idle and cruise, and look for black smoke, a fuel smell, or fouled plugs. Trims outside roughly plus or minus 10 percent deserve investigation. A genuinely and severely rich exhaust will hold the downstream sensor high, but it almost always brings other codes with it.
ExpectedFuel trims within roughly plus or minus 10 percent and a normally cycling upstream sensor
Repair, clear, and run the drive cycle to completion
Tools Scan tool with readiness monitor status
After repair, clear the code and complete a drive cycle that lets the oxygen sensor and catalyst monitors run to completion. This matters especially before an emissions test — the code being gone is not the same as the monitors being ready, and a vehicle with incomplete monitors is rejected at inspection.
ExpectedNo return of P0138 and the oxygen sensor and catalyst monitors reporting complete
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Clean, dry, and reseal a water- or corrosion-damaged connector | Water or corrosion found inside the connector, with the reading returning to bias voltage after cleaning | $80–$280 | DIY |
| Repair a chafed or shorted signal wire in the sensor harness | Low resistance measured between the signal wire and a power or heater circuit, or visible chafe through to the conductor | $120–$420 | DIY |
| Repair a high-resistance or open signal return circuit | Excessive resistance measured on the signal return between the sensor connector and the module | $100–$350 | DIY |
| Correct a miswired aftermarket or universal oxygen sensor | Wiring traced against the factory diagram shows the signal wire connected to a heater or supply circuit | $80–$280 | DIY |
| Replace the bank 1 downstream oxygen sensor — the vehicle must be raised and supported safely, and the sensor removed while the exhaust is warm where possible to reduce the risk of shearing it off in the bung | Reading falls to bias voltage when the sensor is unplugged, with the harness and connector proven clean | $150–$420 | DIY |
| Diagnose and repair a genuine rich running condition | Circuit proven clean while fuel trims run strongly negative and the upstream sensor confirms a rich exhaust | $200–$1,300 | Hard |
| Repair or replace the powertrain control module | The input stays pinned high with the sensor disconnected and the harness verified good end to end from the connector to the module pin | $400–$1,500 | Shop |
This is one of the cheaper codes on the list when diagnosed in the right order. A downstream sensor is typically $150 to $400 fitted, and wiring repairs usually land under $400. The way people overspend here is by replacing the sensor, having the code return a week later, and then replacing it again — the disconnect test at the start would have pointed at the harness. Budget for a seized sensor: on a rusty vehicle the sensor can shear off in the bung, which turns a twenty-minute job into an extraction or a bung replacement and adds meaningful labour. Ranges are US independent shop, parts plus labour.
- Replacing the sensor first. It is the cheapest and most obvious part, so it gets replaced automatically — and on this particular code the wiring is the more likely culprit, because the voltage is higher than a working sensor can produce.
- Skipping the disconnect test. Reading the PID with the sensor unplugged takes thirty seconds and definitively separates a sensor fault from a harness fault.
- Missing water intrusion in the connector. It is a very common cause after driving through deep water or a high-pressure wash, and it will come back if the connector is not properly resealed rather than just dried out.
- Wiring a universal splice-in sensor incorrectly. Wire colours are not standardised across manufacturers, and crossing the signal wire with a heater feed produces exactly this symptom.
- Assuming a downstream sensor code means the catalytic converter is bad. This code describes the sensor circuit; the converter is a separate question entirely and requires the sensor to be working before it can even be evaluated.
- Clearing the code and going straight to an emissions test. Monitors need a complete drive cycle to set, and incomplete monitors fail inspection just as surely as a stored code.
- Ignoring harness chafe once the sensor is replaced and the code temporarily clears. If the wire is rubbing through, it will short again.
- Oxygen Sensor Downstream
- Oxygen Sensor Wiring
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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