P2203 OBD-II Trouble Code: NOx Sensor Circuit High Input Bank 1

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P2203 is a diagnostic trouble code (DTC) for "NOx Sensor Circuit High Input Bank 1". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P2203 code definition

NOx Sensor Circuit High Input Bank 1

What the P2203 code means

This code indicates the NOx module is maintaining a high voltage to the NOx (Oxides of Nitrogen) sensor.

What causes the P2203 code?

This code is set either because there is too much NOx in the exhaust system or the NOx sensor itself has failed. Too much NOx in the exhaust stream can be a result of many factors:

  • Failed catalytic converter
  • Diesel Exhaust Fluid (DEF) has run out
  • Rich fuel mixture
  • Catalytic converter is not within the correct operating temperature

What are the symptoms of the P2203 code?

The Check Engine Light will be on. Some manufacturers choose to have the Engine Control Module (ECM) place the vehicle into a pre-programmed low power output mode to give the owner the incentive to repair the Diesel Oxidation Catalyst (DOC) system. Aside from this, there will typically not be any other symptoms.

How does a mechanic diagnose the P2203 code?

The first step is to connect a scan tool and verify the code. If the code is present, the freeze frame data should be recorded, the code should be cleared and the vehicle should be taken on a test drive to see if the code returns. If the code doesn’t return immediately, the conditions in the freeze frame data should be duplicated, if possible. Often times this code can be set under certain extreme driving conditions. One example is towing a large load up an extended grade.

If the code returns during the test drive or before, it will be necessary to perform the following tests:

  • Check the other sensors in the exhaust for correct operation with a scan tool (e.g., Exhaust Gas Temperature (EGT) sensors)

  • Verify there are no intake leaks

  • Check for exhaust leaks before the NOx sensor

  • Inspect the NOx and EGT sensor wiring harnesses

  • Remove the NOx sensor and look for a soot plugged sensor

  • Check wiring harness from PCM to the DOC module.

Common mistakes when diagnosing the P2203 code

If this is the only code set, it is easy to dismiss any other data that seems incorrect. If the scan tool data shows multiple sensors out of spec, time should be spent considering what data is the source of the problem and what data is a result of the actual problem. Many DOC systems utilize as many as seven sensors of various types as well as the ECM and DOC modules.

The DOC system has a warm up state, a maximum efficiency state and an extreme conditions state. It’s important to refer to the freeze frame data as a guide to what conditions occurred when the code was set. Failing to consider the state of the DOC system at the time of the code being set will almost definitely result in a misdiagnosis.

To add to the confusion, the DOC system consists of the Diesel Oxidation Catalyst (DOC) stage, the Selective Catalyst Reduction (SCR) stage and a Diesel Particulate Filter (DPF) stage. Knowing what each stage does and how it will affect the scan tool data is paramount to the correct diagnosis of this system. Each stage has a variety of processes that need to be completed during specific conditions to sustain operation at peak efficiency. If the on board diagnostics performed by the computer fail, a corresponding code will result with a Check Engine Light illuminated. This code is set because it failed one of those ECM tests.

How serious is the P2203 code?

The only real problem with this code is the emissions that affect the environment, if this code is a hard code and the NOx sensor is in total failure. Some manufacturers choose to program a low power output mode into the ECM as an incentive for the owner to have this system repaired. It is common for this code and others in the DOC system to be set because of extreme conditions. Once those conditions are no longer present, this code will no longer apply.

What repairs can fix the P2203 code?

  • Clearing the code and test driving to see if the code returns (it may have been set under extreme conditions that are not longer present)

  • Replacing the NOx sensor due to soot build up

  • Replacing the NOx sensor due to internal electrical failure

  • Repairing any damaged wiring in the NOx sensor harness

  • Repairing any problems with the catalyst temperature control system

  • Repairing any intake manifold leaks

  • Repairing any exhaust leaks before the sensor

  • Replacing DOC module

  • Replacing the ECM

  • Replacing EGT sensor or sensors

  • Repairing fuel system problems related misfires or a rich condition.

Additional comments for consideration regarding the P2203 code

The DOC systems are exclusively used on diesel powered vehicles. The EPA (Environmental Protection Agency) required manufacturers of diesel powered vehicles to add NOx reduction systems by the year 2010. The manufacturers experimented with three different designs but eventually the consensus was the DEF system was the best. Other systems reduced power output or were more expensive to maintain. The only drawback of the DEF system is it requires the owner to add DEF fluid to a tank, not unlike the washer fluid reservoir. If DEF should run out, this emissions system will not operate and a check engine light will turn on.

Need help with a P2203 code?

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