P204A: Reductant Pressure Sensor Circuit

What Does The P204A Code Mean?

A stored code P204A means that the powertrain control module (PCM) has detected a degree of voltage in the reductant pressure sensor circuit that’s not within the normal parameters. This code is exhibited in vehicles with a clean burning diesel engine, exclusively.

The catalyst system is responsible for reduction of (basically) all exhaust emissions although certain applications are also equipped with a NOx trap.

Exhaust gas recirculation (EGR) systems go another step further in the reduction of NOx. Nevertheless, today’s larger and more powerful diesel engines cannot meet strict federal (U.S.) emission standards with the EGR, diesel particulate filter/catalytic converter, and NOx trap alone. It was for this reason that selective catalytic reduction (SCR) systems were invented.

SCR systems inject a reductant compound or a diesel exhaust fluid (DEF) into the exhaust in front of the diesel particulate filter, NOx trap, and/or the catalytic converter via the reductant injection valve (solenoid). The precisely timed DEF injection elevates the temperature of the filtration element and allows it to perform more efficiently. It enhances filtration element longevity and allows fewer harmful exhaust emissions to be released into the atmosphere.

The entire SCR system is monitored and controlled by either the PCM or a stand-alone controller (which interacts with the PCM). Either way, the controller monitors the O2, NOx, and exhaust temperature sensors (as well as other inputs) to determine the appropriate time for DEF (reductant) injection. Precise DEF injection is necessary to maintain exhaust temperature within acceptable parameters and optimize pollutant filtration.

The reductant/regeneration supply pump is used to pressurize the DEF in the liquid reductant system for use, when required. Supply pump voltage is monitored by the PCM for consistent fluctuation and load percentage. The PCM also monitors one or more pressure sensors in the reductant supply system to determine if there is a leak in the system.

If the PCM detects a degree of voltage in the reductant temperature sensor circuit that is not within acceptable parameters, a code P204A will be stored and a malfunction indicator lamp (MIL) may be illuminated. Multiple ignition cycles (with a failure) may be required for MIL illumination.

What Are The Symptoms Of The P204A Code?

Symptoms of a P204A trouble code may include:

• Reduction in fuel efficiency
• Excessive black smoke from vehicle exhaust
• Diminished engine performance
• Other SCR related codes

What Are The Potential Causes Of The P204A Code?

Causes for this code may include:

• Defective reductant pressure sensor
• Open or shorted circuits in the reductant pressure sensor system
• Bad SCR controller/PCM or programming error

How Serious Is This P204A Code?

A stored code P204A should be considered severe and addressed as quickly as possible. The SCR system could be disabled because of it. Catalyst damage could result if the conditions which contributed to the code being stored are not rectified in a timely fashion.

How Can You Fix The P204A Code?

Tools required

A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle specific diagnostic information will be required to diagnose a code P204A.

You may use your source of vehicle information to locate a technical service bulletin (TSB) that matches the vehicle year, make, and model; as well as the engine size, code/s stored, and symptoms exhibited. If you find one, it could yield helpful diagnostic information.

Step-by-step guide

Use the scanner (connected to the vehicle diagnostic connector) to retrieve all stored codes and pertinent freeze frame data. It is a good idea to write this information down before clearing the codes then test-drive the vehicle until the PCM either enters readiness mode or the code is reset.

If the PCM enters readiness mode at this time, the code is intermittent and may be much more difficult to diagnose. If this is the case, the conditions which contributed to the code being stored may need to worsen before an accurate diagnosis can be made.

If the code is immediately reset, the next step of your diagnosis will require that you search your vehicle information source for diagnostic flow-charts, connector pin-out charts, connector face views, and component testing procedures/specifications.

Use the DVOM to test the reductant pressure sensor according to manufacturer specifications. Components which do not test within maximum allowable parameters should be considered defective.

If actual reductant system pressure is within specifications, the code P204A persists, and the sensor in question is in working order, use the DVOM to test input and output signal circuits between the pressure sensor and the PCM/SCR controller. Disconnect all controllers prior to using the DVOM for testing.

Note: Reductant pressure sensor codes are usually attributed to a reductant system pump that is leaking internally but complete your diagnosis

Recommended Parts

Below are some recommended auto parts to help you address the trouble code affecting your vehicle and get it running smoothly again:

>>> Twippo 372Pcs Waterproof Wire Connectors Kit
>>> SCR System
>>> ECU
>>> Autel Scanner MaxiCOM MK808S
>>> KAIWEETS Digital Multimeter

Note: During the purchasing process, please check carefully whether the part you want to buy fits your car!

Reference Sources

P204A Reductant Pressure Sensor Circuit, OBD-Codes.