P0138 means the ECM saw Bank 1 Sensor 2—the downstream oxygen-sensor circuit—remain too high for the monitored condition. Possibilities include a genuinely rich exhaust, contamination, an exhaust/sensor issue or a signal short. Do not immediately replace the downstream sensor. Toyota T-SB-0001-10 Rev1 gives a particularly important diagnostic branch for 2008–2010 Highlander 2GR-FE: after checking companion codes, technicians compare exact freeze-frame A/F Sensor Bank 1 Sensor 1 and O2 Sensor Bank 1 Sensor 2 values. When the bulletin's precise value window is met, Toyota directs replacement of the upstream A/F sensor only, not the downstream O2 sensor. Outside that window—or on other years/engines—follow normal circuit diagnosis.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Save freeze frame and compare upstream/downstream values before clearing.
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 |
|---|---|---|
| Bank 1 Sensor 2 signal short/high-resistance circuit issue | Voltage remains high with controlled circuit tests | Inspect and electrically test signal/ground wiring |
| Rich exhaust from fuel, purge or misfire fault | Negative trims, fuel odor, deposits or companion codes | Fuel-control and injector/purge diagnosis |
| Upstream A/F sensor fault in exact bulletin window | 2008–2010 2GR-FE plus specified FFD values | Follow T-SB-0001-10 decision tree |
| Downstream sensor contaminated or failed | Circuit passes but controlled response remains wrong | Engine-specific sensor response test |
| Connector swap, water or exhaust damage | Recent repair, melted harness or contamination | Trace Bank 1 Sensor 2 circuit physically |
The official bulletin reverses the obvious parts guess
T-SB-0001-10 tells technicians to check all DTCs and compare A/F B1S1 with O2 B1S2 in freeze frame. In its defined window, the appropriate repair is the upstream A/F sensor only.
This is exactly why code-title parts replacement fails. The branch depends on model, year, engine, companion codes and exact values—not just P0138.
Separate a rich exhaust from a high circuit
Negative fuel trims, fuel odor, deposits and both-bank behavior support a real rich condition. A signal pinned high despite controlled mixture changes points back toward sensor/circuit testing.
Correct rich operation before prolonged road testing. Raw fuel can overheat and damage the catalyst.
Connector identity and routing matter
Multiple similar connectors near a V6 exhaust can be misrouted or swapped after repair. Trace the circuit rather than trusting reach or color alone.
Heat shields and clips protect the harness. A repaired wire left against the exhaust may pass briefly and fail again as insulation softens.
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.
- A/F Sensor B1S1 voltage/current as supported
- O2 Sensor B1S2 voltage
- Both-bank fuel trims
- RPM, load, coolant temperature and speed
- P2195/P2197, P0606, misfire and mixture codes
- Recent exhaust, engine or wiring work
A practical diagnostic workflow
- 01Confirm Bank 1 Sensor 2
Use the exact engine diagram; Bank 1 contains cylinder 1 and Sensor 2 is downstream.
- 02Save all freeze-frame values
The 2008–2010 bulletin depends on exact paired sensor readings and companion codes.
- 03Decide whether exhaust is truly rich
Review trims, plugs, fuel pressure, purge and injector behavior before blaming the circuit.
- 04Inspect the hot-zone harness
Look for melted insulation, connector damage and incorrect routing near the exhaust.
- 05Apply T-SB-0001-10 only when exact
Its value window can point to the upstream A/F sensor even though P0138 names the downstream circuit.
- 06Run a controlled response test
Verify the correct sensor changes and the code remains absent after monitor completion.
Do not replace Bank 1 Sensor 2 solely from the code title. On the exact 2008–2010 2GR-FE bulletin branch, Toyota may direct replacement of the upstream A/F sensor instead. On every application, confirm rich operation and circuit integrity before parts.
Why the model year changes the test
Highlander diagnostics span the first-generation 2.4L 2AZ-FE and 3.0L/3.3L MZ engines, later 2.7L 1AR-FE and 3.5L 2GR-family V6 engines, several hybrid systems, the 2020+ 2.5L A25A four-cylinder hybrid, and newer 2.4L turbo applications. Bank layout, fuel delivery, EVAP hardware, VVT control and safe hybrid procedures differ sharply. Confirm model year, VIN, engine code, hybrid status, drivetrain and emissions label before choosing tests or parts.
- 2008–2010 Highlander 2GR-FE: T-SB-0001-10 Rev1 supplies exact freeze-frame and companion-code diagnostic logic.
- The bulletin does not cover every Highlander year, four-cylinder, hybrid or turbo powertrain.
- Bank 1 location must be identified by cylinder 1 on the exact engine, not a universal front/rear claim.
- Sensor terminology matters: Toyota A/F sensors upstream do not behave like conventional narrowband downstream O2 sensors.
P0138 FAQ
Is P0138 the downstream sensor?+
The code monitors Bank 1 Sensor 2, but the root cause can be wiring, rich exhaust or—under Toyota's exact bulletin logic—the upstream A/F sensor.
Which side is Bank 1?+
The bank containing cylinder 1; confirm with the VIN-correct engine diagram.
Can I drive?+
Usually briefly if operation is normal, but stop for flashing misfire, smoke or raw-fuel odor.
Can a leaking injector cause it?+
Yes. A truly rich exhaust can keep downstream oxygen-sensor voltage high.
Can a purge valve cause it?+
A purge valve admitting excessive vapor can enrich the mixture, particularly around refueling events.
Does the bulletin cover a 2012 Highlander?+
No. The cited application includes 2008–2010 Highlander 2GR-FE.
Why would Toyota replace the upstream sensor?+
The bulletin's exact paired freeze-frame pattern identifies an upstream A/F measurement fault that can drive the downstream reading/code logic.
What confirms repair?+
Correct fuel control, sound wiring, normal controlled sensor response 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 — P0138/P0158 diagnostic tipsIncludes 2008–2010 Highlander 2GR-FE; uses companion codes and exact freeze-frame sensor values to distinguish the A/F sensor from the downstream O2 circuit↗02Toyota Highlander manuals and warranty guidesOfficial Toyota model-year document lookup; confirm the VIN, engine, hybrid status and emissions certification↗03Toyota Technical Information SystemToyota's current North American repair manuals, bulletins, wiring diagrams and calibration information↗