P0036 Code: HO2S Heater Control Circuit (Bank 1 Sensor 2)

What the P0036 code means

The sensor behind the catalytic converter gets gases that have already cooled, and on many vehicles it is mounted under the floor, where moving air cools it even more. Without its heater element it would take too long to go into service, or on short trips it would never do so.

The P0036 code flags a general fault in the control circuit of that heater, without saying whether the voltage is high or low, which is what P0038 and P0037 spell out.

The electronics are identical to the upstream sensor, but the location changes the order of suspects. This sensor lives underneath, exposed to water, mud, salt and stones, so the wiring and the connector explain a much larger share of cases than they do up front.

Its practical effect is the typical one for any downstream sensor fault: the engine runs normally, but the catalyst monitor does not complete and the vehicle cannot pass an emissions test.

Another thing that sets this sensor apart from the upstream one is that its heater works much less. The one up front has to keep the sensor hot almost continuously because its signal corrects the mixture in real time. The one in the back only has to reach temperature so the system can evaluate the catalytic converter, a one-time check that happens once per drive cycle. That is why its element usually lasts more miles, and why when the P0036 code appears it pays to suspect the wiring before the element.

Keep in mind that on most vehicles this circuit shares a fuse with other heaters. That is why a single blown fuse produces three or four simultaneous codes that look like independent failures of different sensors.

The check takes a minute and organizes the whole job: if several heater codes show up at once, start with the fuse and the shared power supply. If there is only one, the diagnosis goes to that specific sensor and its wiring.

P0036 code symptoms

• Check engine light on.
• No noticeable change in how the engine runs.
• Slight increase in fuel use on some vehicles.
• The catalyst monitor does not reach ready status.
• Failed emissions test.
• The downstream sensor signal takes a long time to settle.
• A P0037 or P0038 may be stored too.
• Several heater codes at once if the shared fuse blew.

Likely causes of the P0036 code

1. Wiring damaged under the vehicle by rubbing, stones or salt.
2. Connector with water or corrosion, very common because of its position.
3. Heater element open from age.
4. Blown fuse in the heater circuit.
5. Thermal shock from puddle water hitting a hot sensor.
6. Rodents chewing the harness on vehicles that sit unused for long periods.
7. Control wire shorted to ground.
8. Poor ground in the circuit.
9. Sensor contaminated by oil consumption.
10. Blown shared fuse in the heater circuit, which leaves several sensors without power at once.

How to diagnose the P0036 code step by step

1. Lift the vehicle and inspect the wiring along its whole route. This is the check that solves the most cases for this particular code.
2. Check the connector for water, green corrosion and backed-out pins.
3. Measure the heater resistance with the connector unplugged.
4. Check whether other heater codes are stored, which would point to the fuse or a shared power supply.
5. Verify the 12 volt supply with the key on.
6. Wait a minute or two before you measure current, because this sensor heater is switched on after a deliberate delay.
7. Rule out a short to ground on the control wire.
8. Flex the wire with the engine running and watch whether the code comes and goes.
9. Count how many heater codes there are. Several at once point to the shared fuse. A single one points to that sensor and its wiring.

How to fix the P0036 code

• Repair damaged wiring with solder and heat-resistant sleeving, and secure it along the original route so it does not rub again.
• Clean or replace the connector and protect it with dielectric grease.
• Replace the sensor when the heater resistance is open.
• Replace the fuse and find out why it blew.
• Restore the circuit ground.
• Deal with oil consumption if it contaminated the sensor.
• Loosen the sensor with the exhaust warm and penetrating oil applied ahead of time.
• Check the shared fuse before you remove any sensor when several heater codes appear.

Technical reference data

• Typical cold heater resistance: 2 to 14 ohms.
• This sensor heater is switched on one or two minutes after startup.
• Expected sensor signal with a healthy catalytic converter: steady between 0.6 and 0.8 volts.
• P0037 specifies low voltage and P0038 high voltage on the same circuit.
• Usual torque: 40 to 50 N·m (about 30 to 37 lb-ft).
• On most vehicles the heater circuit shares a fuse, so one blown fuse sets several codes.

Brand-specific notes

On this sensor it is worth spending ten minutes lifting the vehicle and following the wire with a flashlight before you order the part. Rubbing against the chassis, chewed wires and connectors full of water explain a high share of cases, and the repair costs a fraction of what the sensor costs.

The threads seize easily because of exposure to water and salt. Apply penetrating oil ahead of time, because if the sensor snaps off inside the pipe, extraction can force you to replace the whole section.

Frequently asked questions about P0036