P018C – Fuel Pressure Sensor “B” Circuit Low

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By Reinier (Contact Me)
Last Updated 2016-10-22
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P018C Fuel Pressure Sensor "B" Circuit Low
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Fuel pressure sensor, Wiring, PCM failure

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Table of Contents

  1. What Does Code P018C Mean?
  2. What are the common causes of code P018C?
  3. What are the symptoms of code P018C?
  4. How do you troubleshoot code P018C?
  5. Codes Related to P018C
  6. Get Help with P018C

What Does Code P018C Mean?

OBD II fault code P018C is a generic code that is defined as “Fuel pressure sensor “B” – circuit low”, and is set when the PCM (Powertrain Control Module) detects an abnormal signal voltage from the fuel pressure sensor.  The letter “B” in this definition refers to the signal voltage circuit, as opposed to the input circuit voltage.  Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors. Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.

Since the pressure in the fuel rail varies constantly as the demand for fuel changes during normal driving, the PCM expects to see a signal voltage from the pressure sensor that accurately reflects these changes in pressure in order to be able to maintain an appropriate fuel delivery strategy at all times.

Thus, when the fuel pressure sensor registers a signal voltage that does not agree with the signal voltage the PCM expects to see under any given set of operating conditions/parameters such as engine speed, throttle position, and intake air temperature (among many others), the PCM will set code P018C and illuminate a warning light.

In terms of operation, fuel is delivered to the fuel rail at a constant rate and pressure, which pressure is converted into a signal voltage by the fuel pressure sensor. However, in practice, the actual pressure in the fuel rail changes constantly as the PCM makes fuel trim adjustments by altering the fuel injectors’ pulse width to increase, or decrease the amount of fuel being injected into the engine, and in a fully functional system, there is a direct correlation between the signal voltage the fuel pressure sensor generates, and the actual pressure in the fuel rail.

As a rule, the signal voltage rises as fuel pressure increases, and decreases as fuel pressure drops. For instance, if the engine is running at high RPM’s and the throttle is closed suddenly, the PCM immediately reduces the injectors’ pulse width to reduce the amount of fuel being injected. However, since fuel is still being delivered to the rail by the fuel pump at the same rate [as before], the pressure in the rail rises steeply, which causes an equally steep rise in the signal voltage, and vice-versa.

The sudden rise in pressure is however relieved by the fuel pressure regulator (not to be confused with the fuel pressure sensor) that diverts the excess pressure back to the tank. Provided the pressure regulator is fully functional, it will maintain the pressure in the rail within the range specified by the manufacturer to enable the PCM to satisfy the demand for fuel at all times.

From the above, it should be obvious why the fuel pressure sensor needs to reflect changes in the actual fuel pressure accurately within the time limits set by the manufacturer. Invalid (or delayed) signals from the fuel pressure sensor directly affects fuel delivery through the injectors, since the injector pulse width is directly related to the fuel pressure as indicated by the fuel pressure sensor, as opposed to the actual pressure in the fuel rail.

Note that while the actual fault setting parameters vary between manufacturers, all manufacturers base their code setting parameters on the time (measured in seconds) during which no changes, or changes that do not meet expected values in fuel pressure (measured in units of pressure) are indicated by the fuel pressure sensor while the fuel pump is running at either its maximum, or minimum rated voltage limits.

The image below shows a typical fuel pressure sensor (circled in red) that is located directly on the fuel rail. Note that while all fuel pressure sensors have the same general appearance, there may be slight differences between the example shown here, and fuel pressure sensors made for other applications.


What are the common causes of code P018C?

The most common causes of code P018C are much the same across all applications, and could include the following-

  • Defective fuel pressure sensor
  • Damaged, burnt, shorted, disconnected, and/or corroded wiring and connectors
  • PCM failure is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P018C?

Apart from a stored trouble code and an illuminated warning light, the typical symptoms of this code can vary greatly between applications. Common symptoms could include the following, but note that the severity of some symptoms may vary between applications-

  • Hard starting may be present under certain conditions
  • Depending on the application, a no-start condition may be present. In some cases, an invalid signal from the fuel pressure sensor could deactivate the fuel pump(s)
  • Engine may start, but shut off again almost immediately if an invalid signal from the fuel pressure sensor deactivates the fuel pump(s)
  • Rough idling, or hesitation and/or stumbling upon acceleration may be present on some applications
  • Engine might not idle at all in some cases
  • Frequent, or unpredictable stalling might be present on some applications
  • Fuel consumption might increase on some applications
  • On some applications, code P018C could cause sporadic oxygen sensor related codes to be set, especially when the PCM cannot execute fuel trim adjustments when the fuel pressure sensor feeds it invalid signals. Note that although this is a rare symptom, it does occur, and especially on applications where an aftermarket fuel pressure sensor is present.

How do you troubleshoot code P018C?

NOTE #1: Apart from a repair manual for the application being worked on and a good quality digital multimeter, you will need a dedicated fuel pressure gauge to diagnose this code.

NOTE #2:  Make sure that the vehicle has at least 2 gallons of fuel in the tank, to preclude the possibility of wasting time on diagnosing a fault when the vehicle has simply run out of fuel. Defective fuel gauges are far more common than anyone might think, so make sure that there is sufficient fuel in the tank before attempting to diagnose this code.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE:  This code is often accompanied by another code that indicates a request to illuminate a warning light has been made. These codes are sometimes manufacturer specific, so whenever P018C is accompanied by other codes, consult the manual for detailed information on the definitions and implications of codes other than P018C to avoid chasing codes that will clear automatically when the issue at hand is resolved.

Step 2

As a first step in the diagnostic/repair procedure, consult the manual to locate the fuel pressure sensor, as well as the location, function, color-coding, and routing of all associated wiring.

Perform a thorough visual inspection of all wiring and connectors, and look for damaged, burnt, shorted, disconnected, and/or corroded wiring and connectors. Make repairs as required, clear all codes, and rescan the system to see if the code returns.

Step 3

If no damage to the wiring is found but the code persists, consult the manual on the correct procedure to test the fuel pressure to either eliminate, or confirm the actual fuel pressure as the cause of the problem. However, both high and low fuel pressure conditions will almost always be confirmed by the presence of a pressure related trouble code, but there is no harm in testing the fuel pressure to be sure that it falls within specifications.

Step 4

If the actual fuel pressure checks out OK but the code persists, disconnect the sensor at the connector, and check its internal resistance across the relevant pins. Compare this value with the value stated in the manual, and replace the sensor if its resistance does not fall within specification.

WARNING: Do NOT remove the sensor from the fuel rail without first releasing the residual pressure in the fuel system. Failure to do this could result in fuel spraying all over the engine, which is an excellent way to start a fire, so consult the manual on the correct procedure to release the pressure in the fuel system before removing the pressure sensor. 

Step 5

If the fuel pressure sensors’ resistance checks out, check that the full reference voltage (usually 5 volts) reaches the sensor with the engine running at idle. If there is no voltage on this wire, check the ground circuit, to see that it is not shorted to the battery positive. Note that on most applications, the ground is supplied by the PCM.

WARNING: Consult the manual on the correct procedures to test these circuits to prevent damaging the PCM or other controllers.

Step 6

If the fuel pressure sensor appears to be functional (based on its internal resistance), do not assume that will actually work properly throughout its operating range. The only way to be sure that the sensor works as intended, is to connect the scanner to the vehicle, and set it so that it monitors the live data from the pressure sensor. Start the engine, but bear in mind that if the engine is running at a steady RPM, the signal voltage generated by the sensor should remain fairly constant.

Raising the engine speed sharply will cause a momentary drop in the actual pressure in the fuel rail, and this change in pressure should immediately be reflected in the display on the scanner. Similarly, suddenly decreasing the engine speed will momentarily raise the fuel pressure, which change should also be reflected on the scanner. In both cases, the change will be reflected in the form of a changing signal voltage.

If the change is slow, erratic, or if there is no change in the signal voltage, it is likely that the signal wire to the PCM is defective in some way. If this is suspected, prepare to perform resistance, continuity, and ground circuit checks on all associated wiring, but be sure to disconnect the wiring from the PCM to prevent damage to the controller.

Compare all obtained readings with the values stated in the manual, and make repairs as required to ensure that all electrical values conform to the manufacturer’s specifications. Clear all codes after repairs are complete, and retest the system to see if the code returns.

NOTE #1: The minimum and maximum signal voltages should both fall within the range specified by the manufacturer. If the signal voltage drops below the minimum specified value, or exceeds the maximum specified value and no other codes are present, suspect either a defective PCM (a rare event) or a defective/failing fuel pressure sensor. Note that many aftermarket fuel pressure sensors especially, do not always register changes in fuel pressure accurately. Even though a sensor’s resistance (at low fuel pressure conditions) might indicate that it is functional, internal defects might cause it to generate a signal voltage that does not accurately reflect the actual pressure in the fuel rail when the fuel pressure rises or fluctuates.

NOTE #2: If the above is suspected, disconnect the pressure sensor at the connector, and physically check the signal voltage it generates directly on the sensor when the fuel pressure fluctuates as the engine speed varies. Compare these readings with the manufacturer’s pressure-to-resistance chart. Replace the fuel pressure sensor if there are discrepancies between obtained readings and the specified values stated in the manual. Note that you may need to connect the fuel pressure gauge to the fuel rail to be able to monitor the actual fuel pressure during this step.     

Step 7

In some cases, it may be necessary to complete one or more drive cycles before the code can be cleared completely, so perform at least one drive cycle with the scanner connected to monitor the working of the fuel pressure sensor in real time.

If the code does not return, the repair can be considered as successful. Should the code return shortly after, it is likely that there is an intermittent fault present that could be extremely challenging to find and repair. In some cases, it may be necessary to allow the fault to worsen before an accurate diagnosis, and definitive repair can be made.

  • P018A – Relates to “Fuel pressure sensor “B” circuit – malfunction
  • P018B – Relates to “Fuel pressure sensor “B” circuit – range/performance”
  • P018C – Relates to “Fuel pressure sensor “B” circuit – low
  • P018D – Relates to “Fuel pressure sensor “B” circuit – high
  • P018E – Relates to “Fuel pressure sensor “B” circuit – intermittent/erratic

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