P0138 means the Bank 1 downstream oxygen-sensor circuit remained above its expected voltage/response range. It does not automatically identify a bad downstream sensor or catalyst. Check actual rich operation, purge and fuel faults, exhaust integrity, connector routing and circuit shorts. Toyota documents two RAV4-specific paths: listed 2006–2010 applications where incorrect upstream A/F response may be involved, and listed 2009–2012 2AR-FE applications with emissions/calibration limits.
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 sensor or signal-circuit short | Signal stays high or circuit fails controlled test | Inspect heat-damaged routing and load-test circuits |
| Actual rich exhaust from fuel or purge fault | Negative trims, fuel odor/deposits or rich companion codes | Test pressure, injectors and purge sealing |
| Incorrect upstream A/F response on covered older application | Matched VDS/year with P2195/P2197 context | Apply T-SB-0001-10 diagnostic sequence |
| Covered 2AR-FE calibration condition | Matched 2009–2012 VDS/emissions and older calibration | Follow T-SB-0151-18 applicability and repair |
| Crossed connector, contamination or exhaust fault | Recent service or abnormal routing/leak | Verify Bank 1 Sensor 2 identity and exhaust integrity |
RAV4 P0138 can be a downstream report of an upstream error
Determine whether exhaust is genuinely rich using trims, fuel/purge evidence and companion codes. If not, inspect the downstream circuit and response. On a fully matched older application, Toyota guidance shows that upstream A/F-sensor behavior and calibration can influence the downstream high-voltage judgment.
Confirm Bank 1 Sensor 2 physically before disconnecting anything. Four-cylinder and V6 layouts differ, and heat-damaged or crossed connectors after exhaust work can imitate sensor failure.
How to read RAV4 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 RAV4 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, the catalyst or ECM from one P0138. Toyota's RAV4 bulletins require exact VDS, engine, emissions and calibration matching and can point upstream rather than to the named downstream sensor.
Why the model year changes the test
RAV4 spans several unrelated diagnostic platforms: early 1AZ/2AZ four-cylinders, the 2006–2012 2GR-FE V6 option, 2AR-FE, 2019+ A25A-FKS Dynamic Force engines, hybrids and RAV4 Prime. Airflow, EVAP, VVT, fuel and start-control strategies differ. Confirm model year, VIN, engine code, hybrid/Prime status and emissions certification before selecting service information.
- T-SB-0001-10 Rev1 includes listed 2006–2010 RAV4 VDS applications and possible P2195/P2197/P0606 context.
- T-SB-0151-18 includes 2009–2012 RAV4 2AR-FE applications; match VDS, emissions and calibration and do not replace the ECM as part of its procedure.
- Later RAV4 sensor strategies require current engine-specific data.
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 — RAV4 P0138 diagnostic tipsListed 2006–2010 RAV4 VDS applications; incorrect upstream A/F-sensor response may accompany P0138/P0158 and P2195/P2197↗02Toyota T-SB-0151-18 — RAV4 2AR-FE P01382009–2012 RAV4 2AR-FE applications with detailed VDS/calibration limits; follow emissions and calibration table↗03Toyota RAV4 manuals and warranty guidesOfficial Toyota model-year document lookup; use VIN-, engine- and powertrain-correct repair information↗