P0138: How to Diagnose & Fix — O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
3 vehicles · 1 makes · $150-$1,400 repair
How to diagnose and fix P0138: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Toyota Camry · 2007–2010
Toyota T-SB-0001-10 Rev1 covers specified 2007-2009 Camry VDS BK46K and 2010 VDS BK3EK applications. P0138/P0158 may result from incorrect air-fuel-sensor response, and P0606 may also be present due to incorrect downstream O2-sensor operation. Toyota distinguishes these paths using the complete DTC set, freeze-frame air-fuel/O2 voltage and downstream O2 impedance, followed by circuit inspection. It does not describe every P0606 as a sensor fault or support the original 2AZ-FE, 2006-2012, VSC/TRAC or battery claims.
Common Symptoms
- Malfunction indicator lamp with a covered combination of P0138, P0158 and/or P0606
How to Fix
Confirm the covered VDS/2GR-FE application and capture all DTCs plus freeze-frame data. Follow T-SB-0001-10 Rev1: use the specified air-fuel/O2 freeze-frame gates for P0138/P0158, and for a qualifying P0606 path verify O2 Sensor 2 voltage above 1.10 V and/or impedance above 300 ohms at normal operating temperature, then inspect the circuit for open, short or high resistance. Replace only the bulletin-identified sensor when every gate is met; otherwise continue repair-manual P0606 diagnosis. Do not buy the prior sensor from this card.
Owner tips & cautions
- TipSave the complete DTC set and freeze-frame air-fuel/O2 values before clearing codes; the bulletin's path depends on the combination and measured data.
- WarningDo not treat a standalone P0606 or a non-covered VDS/engine as an oxygen-sensor diagnosis. Follow the applicable repair manual instead.
Sources for this issue (1)
Toyota Highlander · 2008–2013
Second-generation Highlanders with the 2GR-FE V6 commonly set a check engine light for an upstream air/fuel ratio sensor whose signal sticks on the lean side of its range. The characteristic Toyota pattern is a downstream oxygen sensor circuit-high code (P0138 bank 1 or P0158 bank 2) appearing together with, or instead of, the A/F sensor code (P2195 bank 1, P2197 bank 2) — the ECM flags the rear sensor because the front sensor's slow response drives the fuel trim, so shops that replace the downstream O2 sensor first fix nothing. Toyota published a service bulletin covering 2008-2010 Highlander for exactly this combination, attributing it to incorrect air/fuel ratio sensor response. Symptoms are mild — a light, slightly worse fuel economy, occasional rough idle — but the car will fail emissions testing.
Common Symptoms
- Check engine light on
- Slightly reduced fuel economy
- Occasional rough or hunting idle
- Fails state emissions test
- Downstream O2 code returns after replacing the rear sensor
How to Fix
Check the A/F sensor harness for opens, shorts and chafing first; if the wiring is sound, replace the affected bank's upstream air/fuel ratio sensor. Use a genuine Denso/Toyota sensor — aftermarket A/F sensors are a repeat-failure source on this engine. Anti-seize sparingly on the threads only, never on the sensor tip. Bank 1 is the front (radiator-side) bank on the transverse 2GR-FE; bank 2 is the firewall side and is the harder of the two to reach. This is not covered by any recall.
Sources for this issue (3)
Toyota Matrix · 2003–2013
P0420 is among the most frequently reported check-engine complaints on the Matrix and its Pontiac Vibe twin. On these cars it is rarely a converter that simply wore out in isolation. Two upstream causes dominate: the aged downstream oxygen sensor whose switching becomes too lazy for the ECM's efficiency comparison, and on higher-mileage 1ZZ-FE engines the oil consumption problem, where burned oil leaves phosphorus and zinc deposits that foul the sensors and coat the converter substrate. Vibe owners on the shared platform document long, expensive chases where a new converter and new sensors did not clear the code because the underlying oil burn was never addressed. Exhaust leaks upstream of the rear sensor produce the same code by admitting extra oxygen.
Common Symptoms
- Check engine light with P0420
- Failed emissions or OBD readiness test
- Rotten-egg sulfur smell from the exhaust
- Slight fuel economy drop
- Code returns weeks after clearing
How to Fix
Do not lead with a converter. Smoke-test or inspect for exhaust leaks at the flex pipe and manifold flange first; then look at the downstream oxygen sensor's live data - a lazy or slowly-switching rear sensor is a common and much cheaper fix than the converter. Confirm the engine is not consuming oil, because a converter installed on an oil-burning 1ZZ-FE will re-fail. If the converter is genuinely spent, note that these cars have a federally mandated emissions warranty on the converter that may still apply to lower-mileage examples, and that in CARB states a non-compliant aftermarket unit will not pass inspection.
Sources for this issue (3)
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P0138 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0138 mean?
P0138 stands for "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)." The PCM detects a persistently high voltage from the Bank 1, Sensor 2 downstream O2 sensor, indicating a short to voltage, contamination, or a rich condition at the post-catalyst sensor.
What are the most common causes of P0138?
The most common causes of P0138 are: Faulty downstream O2 sensor, Short to voltage in signal wire, Coolant/contaminant on sensor, Rich exhaust condition, Wiring/connector fault. The specific cause varies by vehicle.
How much does it cost to fix P0138?
Repair costs for P0138 range from $150 to $1,400, depending on the vehicle and root cause.
Which vehicles are affected by P0138?
Au7o has documented P0138 across 3 vehicle models from 1 manufacturers: Toyota.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.