P202A – Reductant tank heater control – open circuit

Reinier

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

Trouble CodeFault LocationProbable Cause
P202A Reductant tank heater control - open circuit Wiring, reductant tank heater, ECM

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What Does Code P202A Mean?

OBD II fault code P202A is a generic code that is defined as “Reductant tank heater control – open circuit”, and is set when the PCM (Powertrain Control Module detects an open circuit in the reductant tank’s heater control system. Note that the word “reductant” refers specifically to the liquid reductant fluid (commonly known as DEF, or Diesel Exhaust Fluid) that is injected into the exhaust stream to reduce exhaust emissions.

SPECIAL NOTES: All diesel exhaust fluids that meet current SAE/ISO standards consist of urea and de-ionized water that is mixed in the ratio of 32.5% urea to 67.5% de-ionized water. While this mixture freezes at 12°F (-11°C), the very precise ratio if 32.5% : 67.5% ensures that the liquid urea and water both freeze and thaw at exactly the same rate, which means that the concentration of urea in the mixture never changes as the fluid freezes and thaws. However, since winter temperatures in some parts of the USA regularly dip below 12°F (-11°C), the reductant tank is equipped with a high-efficiency heater element to prevent the reductant from freezing during vehicle operation.

Note though that if the reductant freezes when the vehicle is parked, for instance overnight, engine operation and starting times will generally not be affected in the time it takes for the heating system to thaw the frozen reductant fluid. For this reason, additives to prevent freezing of the reductant fluid should NOT be added to the reductant tank, since the addition of any additive(s) will upset the urea/water balance of the reductant fluid on the one hand, and may even damage some SCR (Selective Catalytic Reduction) system components, on the other. END OF SPECIAL NOTES.

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.

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 an open circuit or loss of continuity in the control circuit(s) of the reductant injection heating system will cause the PCM to set code P202A 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 P202A. In the absence of a warning light, code P202A will be set and stored as a “pending” code.

Where is the P202A 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, with the reductant tank heater element, and its associated temperature sensor indicated with red arrows.

Note though that since the reductant temperature and level sensors often resemble each other, it is important 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 P202A?

Note that since code P202A specifically refers to an open circuit or loss of continuity in the heater control circuit of the reductant tank, the probable causes of the code are far more likely to involve failures and malfunctions only in this particular circuit, rather than in any other parts, components, circuits, or subsystems elsewhere in the reductant injection system. Therefore, the most likely causes of this code could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or electrical connectors in the heater control circuit
  • Defective reductant tank heater element
  • Defective reductant temperature sensor
  • Defective reductant temperature thermostat (where fitted)
  • Failed or failing PCM or other control unit. Note that this is an exceedingly rare event, and the fault must therefore be sought elsewhere before any control unit is replaced

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