P201A – Reductant injector – circuit range/performance Bank 2 Unit 1

By Reinier (Contact Me)
Last Updated 2017-12-31
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P201A Mean?
2. Where is the P201A sensor located?
3. What are the common causes of code P201A?
4. Get Help with P201A

What Does Code P201A Mean?

OBD II fault code P201A is a generic code that is defined as “Reductant injector – circuit range/performance Bank 2 Unit 1, and is set when the PCM (Powertrain Control Module) detects an electrical voltage anywhere in the exhaust reductant injection system that falls outside of an expected limit or range. “ Bank 2” refers to the reductant injection system that is fitted to the exhaust system of the bank of cylinders that does not contain cylinder #1.

The purpose of the reductant injection system on modern vehicles is to introduce metered amounts of a gaseous or liquid reductant to reduce harmful exhaust emissions beyond the reductions that are possible with catalytic converters and diesel particulate filters alone. Note that on gasoline engines, the reductant is introduced into the catalytic converter, while on diesel engines the reductant is most commonly introduced into the diesel particulate filter.

NOTE: While some applications use somewhat pressurized atmospheric air to assist in the oxidization of harmful exhaust emissions, the word “reductant” specifically refers to substances such as liquid ammonia or urea that reacts with the exhaust gasses to convert oxides of nitrogen into water vapor and pure nitrogen.

Since their invention in the early 2000’s, many different selective catalytic reduction (SCR) systems have been developed, and many SCR systems in use today depend on proprietary technologies to monitor and control the injection of reductant fluids. However, all systems consist of the same basic components, such a reductant tank, a heater element to heat the reductant fluid to a set temperature, liquid feed lines, an injector, dedicated pressure/temperature sensors, electrical wiring/connectors, and one or more control modules that work in conjunction with the PCM to control and/or monitor the operation of the reductant injection system.

In terms of operation, the PCM depends mainly on input data from the exhaust gas temperature and pressure sensors to determine when to introduce a metered amount of reductant fluid to the exhaust stream. Early SCR systems (and therefore the most basic systems) relied on a basic pressure differential between the exhaust stream and the reductant fluid supply system to determine the amount and timing of fluid to be introduced. Since the exhaust gas temperature and pressure sensors’ resistance changes in direct response to changing temperatures and pressures, the PCM uses the altered voltages to calculate the actual pressures and temperature of the exhaust gas as the basis on which to calculate an appropriate reductant injection strategy.

However, since the caustic environment inside the exhaust system causes sensors to decay and/or degrade, thereby causing incorrect data to be sent to the PCM, newer systems have the reductant pump and pressure sensor upstream of the reductant injector in the exhaust system. The practical advantage of this arrangement is that when the engine is started and the reductant pump is disabled, the actual exhaust pressure should agree with the output (signal) voltage of the exhaust pressure sensor. If there is any disagreement between the actual exhaust pressure and the output of the pressure sensor signal voltage, the disagreement almost always indicates a defective sensor, as opposed to a general malfunction in the electrical control circuit(s) of the reductant injection system.

Moreover, modern SCR systems have the ability to adjust the timing and duration of reductant injection in order to compensate for small deviations between the actual exhaust pressure and the signal voltage generated by the exhaust pressure sensor(s), thus improving reductant injection control. In practical terms, this means that the PCM and other control modules can correct for limited amounts of sensor degradation, and should a level of pressure sensor degradation occur that exceeds a predefined level or threshold, the PCM and other control modules in the reductant injection system can still control the system completely independently of the exhaust pressure sensor(s).

Note that while a range or performance problem in the control circuit(s) of the reductant injection system will cause the PCM to set code P201A on the first failure on most applications, the fault needs to occur several times on some other applications before the PCM will illuminate warning light at the same time it sets code P201A. In the absence of a warning light, code P201A will be set and stored as a “pending” code.

Where is the P201A sensor located?

The image above shows a simplified schematic diagram of a typical modern SCR system fitted to the exhaust system of a passenger vehicle. Note though that since catalytic converters, diesel particulate filters, and mufflers often resemble each other, it is important always to refer to the manual for the affected application to locate and identify parts, components, and particularly sensors correctly. Failure to do this will almost certainly result in confusion, wasted time, misdiagnoses, and the distinct possibility that expensive parts and components may be replaced unnecessarily.

What are the common causes of code P201A?

From the image above it should be obvious that the possible causes of code P210A are many and varied, especially since sensors such as the TPS (Throttle Position) sensor(s), MAF (Mass Airflow) sensor, MAP (Manifold Absolute Pressure) sensor, oxygen sensors, and systems such as both the engine and diagnostic CAN (Controller Area Network) bus systems are also involved in controlling and/or monitoring the reductant injection system.

For this reason, it is not possible to list all possible or even probable causes of code P210A beyond the causes that occur most commonly, which could include the following-

• Damaged, shorted, disconnected, shorted, or corroded wiring and/or connectors almost anywhere in the reductant injection system, or in associated systems/sub-systems/sensors/circuits listed immediately above
• Defective reductant injection pump
• Defective reductant injector
• Defective reductant pressure sensor
• Defective reductant heater element and/or control system
• Defective reductant temperature sensor
• Defective reductant injector solenoid
• Exhaust leaks upstream of the reductant injector
• Failure of one or more control modules. Note that while this is not altogether impossible, it is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced

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