P0138 means the Bank 1 downstream oxygen-sensor circuit remained above its expected voltage range/response. It does not prove that sensor failed or that the catalyst is bad. Check companion rich/lean and A/F-sensor codes, exhaust integrity, connector/wiring shorts and controlled downstream response. Toyota published two narrow older-Camry paths: 2007–2010 V6 guidance involving upstream A/F-sensor response, and 2010–2011 non-PZEV 2AR-FE calibration guidance.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Stop extended driving and diagnose actual rich operation/injector or pressure faults.
When the P0138 diagnosis is complete
The repair is complete when the original operating condition is reproduced safely, the monitored values behave normally and no pending P0138 or related fault returns. An extinguished lamp immediately after clearing is not proof.
Recheck for fuel vapor, intake, exhaust, coolant and oil leaks plus anything disturbed during diagnosis. If weather, temperature or fuel level prevents the original monitor from running, document that limitation rather than claiming certainty.
If the code returns, compare the new freeze frame with the original. A matching pattern suggests incomplete root-cause correction; a different load, temperature or companion-code pattern may reveal a second fault.
What can set P0138—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Downstream oxygen sensor or circuit short | Signal/circuit fails controlled electrical test | Inspect routing and load-test signal/ground circuits |
| Actual rich exhaust from fueling fault | Fuel trims, smoke/deposits or companion rich codes | Diagnose fuel pressure, injectors and purge |
| Incorrect upstream A/F sensor response on covered older V6 | Matched VDS/year and companion P2195/P2197 pattern | Apply T-SB-0001-10 applicability and test sequence |
| Calibration condition on covered non-PZEV 2AR-FE | Matched 2010–2011 application/calibration | Follow T-SB-0151-18 after normal checks |
| Connector mix-up, contamination or exhaust issue | Recent work, damaged harness or abnormal leak/temperature | Verify sensor identity, connections and exhaust integrity |
Downstream high voltage may originate upstream
First determine whether the exhaust is actually rich. If fuel control is normal, inspect the downstream circuit for a short and verify response during a controlled test. If companion A/F codes and an exact older application match, Toyota's bulletin may redirect diagnosis to upstream sensor response/calibration.
Bank 1 Sensor 2 is downstream of the catalyst, but location and connector routing change with the four-cylinder/V6 layout. Confirm identity before unplugging or ordering parts.
How to read Camry scan data for P0138
Preserve the first freeze frame for P0138: rpm, calculated load, coolant and intake-air temperature, vehicle speed, fuel level, battery voltage and time since start. Those values reveal whether the monitor failed after an overnight soak, during warm-up, at idle, during cruise/deceleration or under high load. Testing in a different state can make an intermittent fault look normal.
Read companion codes as a system. Circuit and reference-voltage faults move electrical diagnosis ahead of component replacement. Repair active misfire and mixture faults before judging catalyst or oxygen-sensor performance. On a hybrid, distinguish engine run time from vehicle READY time because the gasoline engine may cycle off.
Graph related signals on one time scale and label units. Compare desired with actual VVT angle, both fuel trims, temperature sensors after a cold soak, or commanded valve state with tank-pressure response. A single screenshot at warm idle rarely proves a performance code.
Useful owner checks and professional boundaries
With the engine cold, an owner can inspect oil and coolant levels, the air-filter housing and duct, battery terminals, accessible grounds, the fuel cap/seal where equipped, and connectors or hoses disturbed during recent service. Photograph the original condition before moving anything.
Use trained service equipment for regulated EVAP smoke, bidirectional purge/vent commands, fuel-pressure testing, energized electronic throttle work, oscilloscope cam/crank correlation, cooling-system pressure testing, compression/leak-down and hybrid high-voltage areas. Fuel vapor, hot coolant, moving engine parts and high voltage create separate hazards.
Ask which test failed, under what operating condition, and which engine-specific Toyota procedure was followed. A diagnosis written only as ‘code present—replace sensor’ does not separate the sensor from wiring, plumbing, calibration or the upstream condition it reported.
How Toyota bulletins fit the diagnosis
Match the complete applicability table: model year, engine, transmission, VDS/WMI, emissions certification, production range and calibration where listed. Similar code wording on another generation does not extend the document.
Follow the sequence and exclusions. Some Toyota bulletins apply only when no additional DTCs are present or only after a specific symptom is reproduced. If the conditions fail, return to the normal repair-manual diagnostic tree instead of forcing the bulletin repair.
A TSB is not automatically a recall or free repair. Warranty and campaign eligibility are VIN-, date- and mileage-specific. Verify current status with Toyota and keep the bulletin as technical evidence, not a promise of coverage.
Verification after the Camry repair
Clear P0138 only after preserving evidence and correcting a proven cause. Reinspect every disturbed hose, seal, connector, clamp and fluid level, then observe the relevant PIDs at idle before attempting a road test.
Repeat the safe portion of the original freeze-frame condition. An EVAP monitor may need a particular fuel level and soak; P0128 requires a true cold warm-up; a load misfire must not be reproduced aggressively if the lamp flashes; correlation faults require quiet, safe engine operation.
Check pending codes after the drive and allow readiness to progress without repeated resets. Permanent codes clear after enough passing monitor operation—not merely because battery power was removed. Retain before-and-after reports.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Confirmed, pending and permanent codes
- RPM, load, speed and coolant temperature
- Battery voltage and time since start
- Recent service, refueling and symptom history
- Bank 1 Sensor 2 voltage/current as supported
- Upstream A/F data, fuel trims and P2195/P2197/P0606 companions
A practical diagnostic workflow
- 01Map Bank 1 Sensor 2
Identify downstream sensor for the exact engine.
- 02Save companion-code context
Rich/lean, A/F and circuit codes change priority.
- 03Inspect wiring and exhaust
Check heat damage, crossed connectors and leaks.
- 04Test actual mixture
Use trims, plugs, fuel pressure and purge evidence.
- 05Apply narrow Toyota guidance
Match VDS, engine, emissions and calibration.
- 06Verify controlled response
Confirm normal downstream behavior and no pending code.
Do not replace Bank 1 Sensor 2 or the converter from P0138 alone. Toyota documents older cases where upstream A/F-sensor response or calibration influenced the downstream code, and wiring or real rich operation must also be excluded.
Why the model year changes the test
Camry is not one diagnostic application. Older 2AZ-FE and 1MZ/3MZ engines, 2AR-FE/2GR-FE, 2018+ A25A-FKS/2GR-FKS and hybrid powertrains use different sensors, fuel systems, EVAP hardware, timing controls and monitor logic. Confirm model year, VIN, engine code, hybrid status and emissions certification before selecting a repair procedure.
- T-SB-0001-10 Rev1 lists specific 2007–2010 Camry V6 VDS applications and possible P0138/P0158 with P2195/P2197; match the table.
- T-SB-0151-18 covers non-PZEV 2010–2011 Camry 2AR-FE only; identify emissions certification and calibration.
- Later wideband/downstream sensor strategies require their own Toyota repair-manual data.
- Confirm the engine family from the emissions label or VIN catalog before ordering parts; cylinder banks, sensor names, specifications and monitor conditions are not interchangeable.
- A Toyota bulletin is a diagnostic branch for its listed year, engine, VDS and calibration—not proof that every Camry with the same code needs that repair.
- Save confirmed, pending and permanent codes, readiness and the first freeze frame before clearing memory or disconnecting the battery.
P0138 FAQ
What does P0138 mean?+
The Bank 1 downstream oxygen-sensor circuit stayed higher than expected.
Is Sensor 2 downstream?+
Yes, but confirm the exact engine's layout and connector.
Does it mean a bad converter?+
No. The code is a sensor-circuit/response fault, not a catalyst verdict.
Can rich running cause it?+
Yes; diagnose fuel and purge causes when other evidence supports rich exhaust.
Can wiring cause it?+
Yes, heat damage or a signal short can hold voltage high.
Is there Toyota guidance?+
Yes for narrow older V6 and non-PZEV 2AR-FE applications.
Can I drive?+
Usually briefly if alone; stop for raw fuel, smoke or flashing misfire.
What proves repair?+
Normal controlled response, corrected mixture/circuit and no pending P0138.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Toyota T-SB-0001-10 Rev1 — Camry P0138 diagnostic tipsListed 2007–2010 Camry V6 applications; explains possible incorrect upstream A/F-sensor response with P0138/P0158↗02Toyota T-SB-0151-18 — 2010–2011 Camry 2AR-FE P0138Non-PZEV 2010–2011 Camry 2AR-FE only; requires application and calibration confirmation↗03Toyota Camry manuals and warranty guidesOfficial Toyota model-year owner-document lookup; repair procedures require VIN-correct Toyota service information↗