P0235 – Manifold absolute pressure (MAP) sensor A, TC system circuit malfunction

By Reinier (Contact Me)
Last Updated 2018-02-07
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0235 Mean?
2. Where is the P0235 sensor located?
3. What are the common causes of code P0235 ?
4. Get Help with P0235

What Does Code P0235 Mean?

OBD II fault code P0235 is a generic code that is (most commonly defined) as “Manifold absolute pressure (MAP) sensor A, TC system circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a general failure or malfunction in the forced induction boost control circuit that is labelled “A”. Note that “forced induction” can refer to either a turbo charger or a supercharger, both of which are devices that force highly pressurized, additional air into the engine intake system.

NOTE #1: Note that since car manufacturers do not label circuits, sensors, or parts in the same way, it is important to refer to the manual for the affected application to determine exactly which part of the boost control circuit is labelled “A” on that particular application. Failure to do this could result in confusion and a misdiagnosis, since some applications use multiple pressure sensors in the boost control circuit that all have to supply closely correlating readings for the boost control system to work properly.

NOTE #2: While code P0235 is a generic code, it may have definitions that differ in their specific wording from the one given above, depending on the source consulted. For instance, some sources may list this code as “Turbocharger/Supercharger Boost Sensor “A” Circuit”, while others may list it as “Turbocharger (TC) boost pressure sensor A/supercharger (SC) boost pressure sensor A – circuit malfunction”.

NOTE #3: Note also that on many, if not most VAG group applications, code P0235 may be displayed as 16619/P0235/000565 – “Boost Pressure: Regulation Range not Reached”. Regardless of the specific wording of the definition though, all definitions mean the same thing, i.e., the PCM is unable to control the boost pressure due to a failure of, or malfunction in, the boost control circuit.

Regardless of the specifics of the forced induction device (turbo charger/supercharger) used on any given application, the purpose of the additional, pressurized air is to increase the engine’s power output without using significantly more fuel. Although forced induction is widely used, the technology is particularly useful on small capacity engines (particularly the small diesel engines in compact cars), since forced induction produces huge power gains at relatively low cost, and without significant penalties in terms of weight and fuel consumption.

However, and again regardless of the specific device involved, the boost pressure from the forced induction device has to be controlled precisely both to derive the maximum benefit from it, and to protect the engine from the extremely harmful (and sometimes fatal) effects of over boost conditions. While both turbo-, and superchargers use built-in mechanical devices to bleed off excess boost pressure, these devices are electronically controlled by the PCM in such a way that the boost pressure that enters the engine never exceeds the maximum allowable limit.

Typically, boost pressure sensors are pressure sensitive, and their resistance changes in direct proportion to changes in pressure. In practice, these sensors are supplied with a 5-volt reference voltage and a ground that can in some cases, be supplied by the PCM also. In terms of operation, the pressure sensor will allow close to 5 volts to pass back to the PCM via a dedicated signal circuit when no boost pressure present, or when the engine is at idle. As the boost pressure increases with increasing engine speed however, the sensor’s also resistance increases thereby decreasing the current being passed back to the PCM, which interprets the changing signal voltage as changes in pressure. Note however that on some applications, the boost pressure (and other pressure) sensors are electrically opposite, in the sense that their electrical resistance decreases as the boost pressure increases.

To accomplish effective control of the boost pressure, the PCM uses primary input data from the MAP (Manifold Absolute Pressure) sensor, or a dedicated boost pressure sensor to monitor the actual boost pressure in the inlet system. This value is then compared to input data from (among others) the engine speed sensor, IAT (Intake Air Temperature) sensor, and TPS (Throttle Position) sensor(s) to calculate the required boost pressure, as well as appropriate fuel delivery and ignition timing strategies to suit current operating conditions.

If the boost control system is fully functional, the PCM will then use the cumulative input data from all implicated sensors to either fully/partially open/close the devices that are used to bleed off excess boost pressure to maintain the boost pressure within the allowable range, or to increase/decrease the boost pressure to meet the desired boost value.

However, if the boost control circuit fails, the PCM is no longer able to control the boost pressure effectively, and it will set code P0235 and illuminate a warning light as a result. Note that depending on the application and the nature of the problem, the PCM may also set additional codes, some of which may cause the PCM to initiate a failsafe or limp mode both as a safety precaution, and to protect the engine.

Where is the P0235 sensor located?

The image above shows a typical boost pressure sensor, such as might be found on almost any application that uses forced induction. Note that since boost pressure sensors often resemble MAP (Manifold Absolute Pressure) sensors, great care must be taken not to confuse the two sensors with each other, because doing so will lead to a misdiagnosis.

Nonetheless, boost pressure sensors are typically located on the inlet manifold, but note that this is not always the case, and that the sensor could be located elsewhere in the inlet tract. Moreover, since there may be more than one sensor on the inlet manifold (or in the inlet tract) on some applications, it is important to refer to the manual for the affected application to locate and identify the boost pressure sensor correctly.

What are the common causes of code P0235 ?

NOTE: Be aware that while code P0235 rarely indicates a defective turbo-, or supercharger, defects in, or malfunctions of these devices should not automatically be ruled out as possible causes of this code.

Nonetheless, some possible causes of code P0235 could include the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Defective boost pressure sensor
• Damaged, split, or perforated inlet ducting between the turbocharger and the inlet manifold
• Damaged or worn turbo-, or supercharger
• Damaged or defective boost pressure relief device, such as a waste gate, or waste gate actuator
• Failed or failing PCM. Note that this is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced

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