P0155 on Toyota: How to Diagnose & Fix
O2 Sensor Heater Circuit (Bank 2, Sensor 1)
1 Toyota model · 1 documented issue · $220-$2,200 repair
P0155 on Toyota by model: symptoms, diagnosis and fix
High-mileage XL20 Siennas commonly set P0420 (Bank 1) and sometimes P0430 (Bank 2) for catalyst efficiency below threshold. On this V6 layout Bank 1 is the rear (firewall) bank and Bank 2 the front, and the van carries four sensors: two upstream air/fuel ratio sensors and two downstream heated oxygen sensors. The most frequent real cause is a lazy downstream sensor whose switching has slowed with age, not a dead converter - but shops routinely quote catalytic converter replacement first, which on this van is a four-figure job. Exhaust leaks upstream of the rear sensor and unresolved misfires also set the code.
Common Symptoms
- Check engine light with catalyst efficiency code
- Slightly reduced fuel economy
- Failed state emissions inspection
- No drivability symptom in many cases
How to Fix
Before replacing a converter, graph the downstream sensor on a scan tool - a healthy rear sensor should sit relatively flat around 0.6-0.7V, while a sensor that mirrors the upstream switching is worn out. Also smoke-test or listen for exhaust leaks at the manifold and flange gaskets and clear any misfire codes first. A Denso or OEM downstream sensor is a few hundred dollars installed versus $1,200-$2,200 for the rear manifold converter, which sits against the firewall and is the labor-intensive one. No recall exists, though the converter may be covered under the federal 8-year/80,000-mile emissions warranty on vehicles still in that window.
Sources for this issue (3)
P0155 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 P0155 mean on a Toyota?
P0155 stands for "O2 Sensor Heater Circuit (Bank 2, Sensor 1)." The PCM has tested the heater circuit of the Bank 2, Sensor 1 upstream O2 sensor and detected a malfunction. The heater brings the sensor to operating temperature for closed-loop control. Indicates a heater element or circuit fault. On Toyota it is documented across 1 model: Sienna.
What are the most common causes of P0155?
The most common causes of P0155 are: Failed O2 sensor heater element, Open or shorted heater wiring, Blown heater fuse, Corroded connector, Poor heater power/ground. The specific cause varies by vehicle.
How much does it cost to fix P0155 on a Toyota?
Published repair estimates on Toyota range from $220 to $2,200 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0155 on a different make?
View P0155 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.