Toyota RAV4 P1135 and P1155 Codes After O2 Sensor Replacement: Causes, Diagnosis, and Repair Direction

On a Toyota RAV4, P1135 and P1155 usually point to a problem with the air-fuel ratio sensor heater circuits, not necessarily a bad oxygen sensor element itself. If the sensors have already been replaced and the wiring has been checked, the next most likely causes are a heater power supply issue, a blown fuse, poor ECM control, connector pin damage, or an exhaust leak or engine condition that is making the sensors appear faulty when they are not.

These codes do not automatically mean both new sensors are defective. On many RAV4 applications, especially those using Toyota air-fuel ratio sensors rather than traditional narrowband O2 sensors, the heater circuit is monitored closely by the engine control module. If the heater does not draw current correctly, warms too slowly, or loses power or ground control, the codes can return even with new parts installed. The exact diagnosis depends on the RAV4 year, engine, and whether the codes are for Bank 1 Sensor 1 and Bank 2 Sensor 1 on a V6 model or the single upstream sensor on an inline-four configuration.

Direct Answer and Vehicle Context

P1135 and P1155 on a Toyota RAV4 most commonly mean the upstream air-fuel ratio sensor heater circuits are not operating as expected. On four-cylinder RAV4 models, the fault may involve the primary upstream sensor circuit depending on the engine family and calibration. On V6-equipped RAV4s, the codes usually separate by bank, which means the issue may affect both banks or may be caused by a shared power supply rather than two failed sensors at the same time.

If the sensors have already been replaced, the problem is often not the sensing element itself. The heater inside the sensor needs battery voltage and ECM control to bring the sensor up to operating temperature quickly. A sensor can be new and still set the same code if the heater feed is missing, the ground control is interrupted, the fuse is weak or open, the connector terminals are loose, or the ECM is not driving the circuit properly. The engine type matters because Toyota used different sensor layouts across RAV4 generations, and the bank/sensor assignment must be verified before assuming the same repair path on every model year.

How This System Actually Works

The upstream sensor on a Toyota RAV4 is not just measuring exhaust content; it also needs to heat up quickly after startup. That heater is built into the sensor and is separate from the sensing element. When the engine starts cold, the ECM expects the heater to bring the sensor online fast enough for accurate fuel control.

The heater circuit usually has a power feed and a control side. Depending on the exact RAV4 engine and year, the ECM may switch the ground side of the heater or monitor current flow through the circuit. If the heater circuit does not draw the correct current, the ECM interprets that as a fault and stores a code. This is why a sensor replacement alone does not always solve the problem. The sensor is only one part of the circuit; the fuse, relay, harness, connector, and ECM control strategy all matter.

On some RAV4 engines, especially those with two banks, the heater feed may be shared while the ECM controls each sensor separately. That creates an important diagnostic clue: if both P1135 and P1155 appear together, a shared power supply problem becomes more likely than two simultaneous sensor failures.

What Usually Causes This

The most realistic causes are usually electrical rather than exhaust-related. A blown heater fuse, a weak fuse connection, or a relay that is not supplying stable voltage can trigger these codes. A heater circuit can also fail because of terminal spread inside the connector, corrosion in the plug, or a wire that looks intact externally but has broken strands inside the insulation.

Incorrect sensor installation is another common cause. Some replacement sensors have the correct thread and connector but the wrong heater resistance or calibration for the specific RAV4 application. That is especially important on Toyota air-fuel ratio sensors, where the part must match the engine and model year closely. A sensor that physically fits is not always electrically correct.

Heat damage near the exhaust is also common. The upstream sensor harness runs close to hot exhaust components, and repeated heat cycling can harden the insulation or damage the connector pins. Even when the wiring checks out visually, a load test may reveal a voltage drop that does not show up in a simple continuity check.

Engine operating conditions can matter too. Excessively rich or lean operation, exhaust leaks ahead of the sensor, or ignition misfire can confuse sensor readings and contribute to heater-related fault logic on some Toyota systems. That said, a heater code should still be treated first as a circuit problem unless live data or testing shows otherwise.

How the Correct Diagnosis Is Separated From Similar Problems

The key is separating a heater circuit fault from a sensor signal fault. P1135 and P1155 are not the same as mixture codes, response codes, or catalyst efficiency codes. A sensor can produce a reasonable signal once warm and still fail because the heater circuit never reaches the expected operating state.

A good diagnosis starts by confirming whether the code is for the upstream sensor heater and whether the RAV4 is a four-cylinder or V6 model. That determines whether the problem is on one bank, both banks, or a shared circuit. If both codes return together, the most useful question is whether both heaters are losing the same power feed. If one code returns by itself, the focus should shift to that sensor’s connector, wiring, and ECM control side.

Live data and heater circuit testing separate an electrical fault from a mechanical exhaust or engine issue. If the sensor stays cold too long, heater current is low, or voltage is missing at the connector during key-on or startup, the fault is in the circuit. If the heater circuit tests correctly but the code returns, then ECM control, pin fitment, or a wrong sensor part number becomes more likely.

An exhaust leak ahead of the sensor can mimic a mixture problem, but it does not usually create a true heater circuit code by itself. Likewise, a vacuum leak or fuel trim issue can cause drivability complaints, but those are different from a heater fault unless the scan data shows the sensor is not reaching operating temperature or the ECM is reacting to an implausible signal.

What People Commonly Get Wrong

A common mistake is replacing the sensor twice without verifying heater voltage and current draw under load. A continuity check only proves the wire is not open at rest; it does not prove the circuit can carry current when the heater is energized. Another frequent error is assuming that a code on both banks means both sensors failed at the same time. In real service, that pattern often points to a shared fuse, relay, or power feed problem.

Another misunderstanding is treating all O2 sensors as interchangeable. Toyota upstream air-fuel ratio sensors are not generic oxygen sensors, and the heater characteristics matter. A sensor that clears the code briefly but returns after a cold start may simply be the wrong specification for that RAV4.

Connector condition is also underestimated. A terminal can look clean and still fail to grip the sensor pin tightly enough to carry heater current. That kind of fault often shows up only when the circuit is loaded and heat-soaked. Loose pin fit, corrosion inside the connector body, or an aftermarket repair splice with poor crimp quality can all create an intermittent heater code.

It is also easy to overlook the ECM side of the circuit. If power and ground are present at the connector but the code persists, the control side may not be switching correctly, or the ECM may be seeing an abnormal current pattern. That is less common than a wiring or fuse issue, but it becomes more relevant after sensors and basic harness checks have already been ruled out.

Tools, Parts, or Product Categories Involved

The most useful diagnostic tools are a scan tool with live data, a digital multimeter, and ideally a test light or current-capable circuit tester for load checking the heater circuit. For repairs, the common parts categories involved are air-fuel ratio sensors, heater fuses, relays, connectors, pigtails, and in some cases ECM-related electrical components.

Depending on the exact RAV4 engine layout, additional inspection may involve exhaust gaskets, sensor mounting threads, and harness clips or heat shields that protect the wiring from exhaust heat. If the vehicle has a bank-specific setup, it is also important to verify the correct sensor location before replacing any component.

Practical Conclusion

On a Toyota RAV4, P1135 and P1155 usually mean the upstream sensor heater circuit is not functioning correctly, and the problem is often somewhere outside the new sensor itself. If both codes are present after replacement, the most likely direction is a shared power supply issue, connector problem, or wiring fault under load rather than two failed sensors. The engine type and model year must be confirmed because the bank and sensor assignments are not identical across all RAV4 versions.

The next logical step is to test heater power, ground control, and current draw at the sensor connector during startup, then verify the related fuse and relay under load. If those checks pass and the correct sensors are installed, the remaining focus should shift to terminal fitment, harness damage near the exhaust, and ECM control of the heater circuit.