What do P0135 and P0155 together mean?

In Ford’s OBD System Operation Summary, P0135 identifies heater-circuit monitoring for Bank 1, Sensor 1, and P0155 identifies the corresponding circuit for Bank 2, Sensor 1. These refer to the upstream oxygen sensors in that document’s terminology. [S1]

Seeing both codes together points to two heater-circuit records, but it does not establish that both sensors are defective, that a shared electrical part has failed, or that the same explanation applies to every make and model. The vehicle’s circuit layout and monitoring strategy matter. Treat the pair as a diagnostic clue, not a parts-replacement instruction.

It makes a qualified technician’s comparison of the two circuits a reasonable question to raise. Whether the circuits share wiring, a power source, control, or another component depends on the specific vehicle. The cited Ford document describes Ford’s monitoring approach; it does not show that these codes always share a cause or that both sensors are good or bad. [S1]

Ask the technician to use service information for your exact vehicle rather than assuming a shared component or replacing both sensors from the code pair alone. Avoid probing circuits or attempting electrical tests yourself; this guide is for organizing information, not performing diagnosis.

Bring the year, make, model, and engine, plus the scan tool’s exact code display and status for each code, if shown. Include available freeze-frame context. Note when the warning light appeared and any relevant observations. The manufacturer reference specifically calls for confirming vehicle details, code status, and freeze-frame context, then using matching service information. [S1]

For example, your note could read: “Year: [year]; Make/model: [make/model]; Engine: [engine]; P0135 status: [status]; P0155 status: [status]; freeze-frame details: [copy from scan tool]; warning light first noticed: [when].” Keep the example as a template and fill it only with information from your vehicle.

You can conclude that the two codes identify heater-circuit monitoring records in the cited Ford reference. You cannot conclude from the pair alone which component, if any, needs repair. The source does not establish a universal cause, a diagnosis for your particular vehicle, or a safe do-it-yourself test procedure. [S1]

A useful request is: “Please confirm what P0135 and P0155 mean for my [year, make, model, engine], review their status and freeze-frame data, and use the matching service information to determine whether the two heater circuits have a related cause. Please explain the evidence before recommending parts.” Ask the technician to distinguish code definitions from test findings. Don’t bypass emissions controls or treat the code description as proof of a failed sensor.