P0056 is a somewhat common O2 sensor-related OBD2 trouble code. It is a generic code, which means it has the same definition for the Mazda Miata as it would any other vehicle.
This code is almost always caused by a bad heated oxygen sensor or wiring issue and almost never creates drivability issues.
P0056 should NOT be considered a breakdown risk if it appears by itself. While your Miata’s engine relies heavily on the upstream sensor for engine tuning, the downstream O2 sensor is there to verify catalytic converter efficiency.
P0056 Definition: Heater Control – Circuit – B1S2
P0056 indicates there is no signal coming from your Mazda Miata’s Bank2 Sensor 2 oxygen sensor’s heater circuit.
The heated element within the B2S2 oxygen sensor heats the sensor faster than the exhaust can. This reduces emissions by letting the fuel trim enter closed loop operation more quickly.
The A/F sensor is responsible for measuring the oxygen content of the exhaust. The PCM uses this data to adjust the air-fuel mixture to the optimal 14.7 parts air to 1 part fuel.
Circuit
In the case of P0056, the PCM has detected a fault in the circuit that activates the oxygen sensors heater.
Bank Two
Bank one is the part of your Miata’s engine with the second cylinder. You only need to concern yourself with “banks” if there is more than one cylinder head, and in this case, you do need to determine which bank is bank 2.
Bank one is usually the cylinder head facing a little more forward than the other (toward the engine accessories), while bank 2 will face a little further back. This is a general rule, and manufacturers can determine the cylinder location without any sort of standardized system.
Sensor 2
The second sensor is easier to identify. Start from bank two and follow the exhaust. It’ll be right after the catalytic converter.
Symptoms of P0056 in the Mazda Miata
The heated element in the oxygen sensor enables closed-loop operation faster, decreasing engine emissions.
You probably won’t notice any drivability issues with your Miata since B2S2 is there to verify the catalytic converter is doing its job.
If your Miata had P0050 (the Bank 2 Sensor 1 version of this code), there would be more noticeable symptoms.
Mazda Miata: P0056 Causes + Diagnosis
Here are the most common causes of P0056 in the Mazda Miata, presented to you in a somewhat proper order of diagnosis:
1. Check the Wiring on Bank 2 Sensor 2
Since the PCM is not getting a response from the heated oxygen sensor circuit, check the wiring harness for damage.
Pay extra close attention to where the harness may have come into contact with something. Look for burnt or damaged wiring. Wiring issues are a VERY COMMON cause for P0056.
The hot exhaust can make the wiring harness brittle, and road debris can damage them.
Take a look at where your Miata’s oxygen sensor plugs into the wiring harness and see if there has been any damage to the pins or if the wiring harness is breaking at the pigtail.
If the pins appear corroded, get some electrical contact cleaner and clean them up. If they are damaged, you will need to repin or replace your wiring harness.
2. Check the Fuses and Voltage
Check the heated oxygen sensor fuse to ensure that it has not blown for some reason. Then, verify with the key on that there’s voltage at the B2S2 oxygen sensor. If there is no voltage, you’ll need to find where the short/open is.
If there is voltage at the heater circuit, you’ll need to test the ground for resistance. If you find infinite resistance, you’ll need to repair the ground circuit.
3. Optional – Swap Test
With P0036, you can do the “swap test.” This involves clearing the codes and seeing if the code “jumps” to bank 2. It’s a great test because it’s foolproof and easy.
Here’s how the swap test is done:
- Clear the DTC codes with your scanner.
- Swap the Bank 1 Sensor 2 O2 sensor with Bank 2 Sensor 2’s sensor.
- Run the your Miata’s engine until the check engine light comes back on.
- If the code changes to P0036 (The Bank 1 version of this code), that’s proof enough you need to replace the oxygen sensor you swapped.
- If the code remains P0056, you’ll need to continue pursuing your diagnosis, knowing that both O2 sensors are working fine.
4. Bad Oxygen Sensor
If the oxygen sensor wiring checked out or failed the swap test, it’s time to replace the heated oxygen sensor. Oxygen sensors go bad all the time.
Conclusion
P0056 in the Mazda Miata is almost always caused by a bad wiring harness or oxygen sensor. Good luck!