Our editors pick the products & services we write about. When you buy through our links, we may get a commission. Learn more.

P0030 Code: Meaning, Symptoms, Causes, Diagnostics, And Fixes

In vehicles equipped with the OBD-2 codes, P0030 is set when there is a problem in the heater control circuit. This circuit involves heating oxygen and maintaining the fuel-to-oxygen ratio. The code could be triggered by trouble with the wiring of the sensor and the consistently wrong fuel-to-oxygen ratio.

P0030 is triggered when there is a problem with the H02S on the O2 sensor.
P0030 is triggered when there is a problem with the H02S on the O2 sensor.

P0030 Code Definition

  • P0030 Generic: HO2S Heater Control Circuit (Bank 1 Sensor 1)
  • P0030 Subaru: HO2S 11 Heater Control Circuit

What Does P0030 Mean?

The OBD-2 code P0030 refers to a problem with the heated oxygen sensors, called the HO2S. These sensors are responsible for detecting oxygen in the exhaust. The problem arises when the oxygen content moves beyond the preferred air: fuel ratio of 14.7:1. When the oxygen content is too high, the sensors send a message to the ECM. Then, the ECM adjusts the fuel to match the oxygen ratio as it goes to the engine.

To keep the ratio correct, the heated oxygen sensor circuit warms the oxygen to decrease emissions. There are several parts of the engine that monitor and power the O2 sensor. They include a 12-volt fused supply and grounded computer control. P0030 is set if there is a problem in bank-1 near the sensor-1 heater circuit.

In a Subaru, the P0030 code is triggered when the oxygen sensors detect a problem with the voltage signal. This signal controls the heater circuit that brings the oxygen to 750°F. The oxygen needs to be heated quickly. As the oxygen heats, the signal sends temperature readings to the ECM. If there is a problem with the heater element, the ECM will receive a message from the sensors.

What Are The Symptoms Of The P0030 Code?

Since the P0030 code is affected by the fuel-to-oxygen ratio, the symptoms usually involve fuel efficiency. The vehicle will use more fuel than it usually does.  When there is a problem, the check engine light will illuminate. Other symptoms include:

  • Decreased response time or no response from the O2 sensor
  • ECM cannot control fuel-to-air ratio
  • ECM sets a fixed fuel mix

What Are The Causes Of P0030?

The reason the P0030 code shows up is a problem with the H02S on the O2 sensor. These are the possible causes of the code:

  • A faulty heater circuit element
  • Irreparable heater circuit
  • Problem with the heater’s ground circuit
  • Battery short on the O2 sensor
  • ECM wiring issues
  • Sensor failure

How Serious Is P0030 Code?

You can still drive your vehicle with the P0030 code triggered. But, the problem can become exacerbated if not fixed. Eventually, the sensor loop will fail. The vehicle will use too much fuel and it might not run well. Other engine and exhaust components can be damaged, too.

How To Diagnose And Fix The Code P0030


 We use an OBD2 scanner to diagnose the P0021 Code.
We use an OBD2 scanner to diagnose the P0030 Code.


  • The first step is to use the OBD-2 scanner to verify the presence of the P0030 code. Then, reset the codes and take the car for a test drive. Upon return, recheck the code to see if it sets again.
  • Before making any repairs, do a visual inspection of the wiring. It could be damaged by exhaust heat or catalytic converter. If the wiring is ok, the next step is to check the voltage and ground of the sensor.
  • Using a voltmeter, check that at least 12 volts are going from the battery to the heater. The key should be in the ignition when checking the volts. If there aren’t any volts, the 12-volt feed needs to be repaired. Replace the fuse if necessary. But, before replacing the sensor or fuse, be sure there is a ground to the sensor.
  • If you check the voltmeter and the battery is ok, then the problem might be with the ECM sensor. To check this, take the ground from the ECM. Then, look for resistance. Infinite resistance means the ECM needs repairing.
  • If the ECM is working properly, then the O2 sensor probably needs to be repaired. The best way to take care of this is to replace it. Then, retest with the OBD-2 scanner.

Common Mistakes While Diagnosing The Code P0030

After the check engine light turns on, the first step should be double-checking the code. Skipping this step is the first mistake that mechanics make when diagnosing P0030. There are other common mistakes.

  • Do not replace the HO2S prior to checking the wiring around it.
  • Be sure that the HO2S has 12 volts and that the groundworks.
  • Check connectors prior to replacing or repairing them. The connector might not have any problems.

If the vehicle has driven over 100,000 miles, there could be temporary sensor issues. These often happen when starting a car or the drive train is stressed, which may turn on the check engine light. Before making any expensive repairs, check the OBD-2 code and never skip the verify-and-reset steps to avoid diagnostic mistakes. 

Tips To Avoid P0030 In The Future

There isn’t much that drivers can do to avoid the P0030 code. Driving on bumpy or dirt roads could cause problems with wiring sensors. It is wise to have the manufacturers’ recommended maintenance performed. Even with regular maintenance, the heating elements cannot be controlled by the driver.

If the code is present, the best thing to do is have it checked. Use a certified mechanic because amateur mechanics can make mistakes and create more issues. Since P0030 is difficult to avoid, the best thing to do is to make necessary repairs before more problems occur.

Leave a Comment

Your email address will not be published.