P00C6 – Fuel Rail Pressure Low During Cranking

By Reinier (Contact Me)
Last Updated 2022-06-30
Automobile Repair Shop Owner

What Does Code P00C6 Mean?

OBD II fault code P00C6 is a generic trouble code that is defined as, “Fuel Rail Pressure (FRP) – Engine Cranking – Pressure Too Low” or sometimes, as “Fuel Rail Pressure Low During Cranking”, and is set when the PCM (Powertrain Control Module) detects a condition in which the fuel pressure in a fuel rail is too low to-

• allow the fuel injectors to open
• to cause the fuel to be properly atomized during injection

In both conditions, the PCM will not complete the starting circuits during cranking, effectively preventing the engine from starting.

While modern fuel injection systems have made cars more fuel-efficient than they have ever been, the biggest drawback all fuel injection systems have is that the fuel pressure must be maintained within a very narrow band on either side of an ideal pressure for the fuel injectors to work as intended.

In older fuel injection systems, the fuel pressure was supplied by a fuel pump in the fuel tank that delivered fuel to the fuel rail and from there to the injectors, at a near-constant rate and pressure. In practice, both the volume and pressure of the supplied fuel were somewhat higher than the engine needed even under wide-open throttle conditions, so to manage the fuel pressure when the demand for fuel suddenly decreased, fuel rails were typically fitted with pressure relief valves that vented excess pressure back to the fuel tank through dedicated return lines.

We need not delve into the details of the various mechanisms that controlled these pressure relief valves but suffice it to say that in a fully functional fuel system, the pressure relief valves could control the fuel pressure to within one or two percent of the ideal fuel pressure under all operating conditions.

In modern sequential fuel injection systems, however, the fuel pressure is primarily regulated by the fuel pump. To achieve this, the PCM or dedicated fuel control module varies the speed of the fuel pump to adapt the fuel pressure to suit the current fuel demand. In practice, the implicated control module increases the speed of the fuel pump under high-demand conditions to increase the volume of fuel, as well as its pressure. When the demand for fuel drops as the engine speed decreases, the implicated control module reduces the speed of the fuel pump, thus reducing both the volume and pressure of the fuel delivered to the injectors to suit the changing operating conditions.

Since the fuel pump regulates the fuel pressure on sequential injection systems, these systems have neither fuel regulators nor return lines, since the fuel pressure is always maintained within a very band on either side of the ideal pressure by the speed of the fuel pump.

Therefore, the actual fuel pressure plays a critical role in efficient engine operation, but the actual fuel pressure available to the injectors plays an equally critical role in the highly complex series of events that must all occur in a fixed order before a modern engine will start.

The process of starting a modern engine is a highly complex and technical one, and as such, the complete process falls outside the scope of this article. However, we can break a generic process down into bullet points to make at least a small part of the process clearer-

• Assuming that all security passcodes are recognized and verified by several control modules, the PCM activates a part of the starting circuits
• One function of this part of the starting circuit is to activate the fuel pump for a second or two to verify that the pump works and that there is fuel in the tank
• If the fuel pump works and delivers pressurized fuel to the fuel system, the PCM and other control modules will verify that the static pressure falls within a specified, and extremely narrow range of pressures
• If the fuel pressure is within the specified range, the PCM will allow the engine to crank, during which time it records the cranking speed and dozens of other parameters, During this time the PCM also determines the position of piston #1 to calculate a suitable fueling and ignition strategy, depending on the engine temperature
• Only if all enabling criteria are met, the most important being the actual fuel pressure in the rails, the position of piston #1 is suitable for starting, and the cranking speed is within an allowable range, will the PCM and several other control modules complete the stating circuits to allow the engine to start

The above scenario might sound like a long-drawn-out affair, but all of it (and more) actually occurs within about two seconds or less, so none of it is noticed by the driver when they start a modern vehicle.

As a practical matter, however, the PCM and other control modules will not allow the starting procedure to progress beyond the engine cranking stage in cases where the static fuel pressure is too low for the fuel injectors to work efficiently. In some cases though, the PCM and one or more other control modules will not even allow the engine to crank when the fuel pressure is too low.

Insufficient fuel pressure during engine cranking can have dozens of possible causes, but in practice, code P00C6 only indicates a low fuel pressure condition. It takes no account of any possible or probable causes, most of which set dedicated trouble codes, but note that some do not. Regardless of the cause, however, when the PCM or other control module detects an insufficient fuel pressure condition during cranking, it/they will recognize that they cannot control the engine management and/or fuel delivery system effectively, and the PCM and/or dedicated fuel control module will set code P00C6, and illuminate a warning light.

Where is the P00C6 sensor located?

This image shows a fuel rail from a Hyundai Elantra application, with the fuel injectors attached. In this example, the large yellow arrow indicates the fuel rail, which is just a hollow tube. The red arrow indicates the fuel intake point, the green arrow indicates the electrical connector of the fuel pressure sensor, and the blue arrow indicates the vacuum-operated fuel pressure regulator.

While most fuel rails follow this general pattern, the location of fuel intake points and pressure sensors vary between vehicle makes and models. Moreover, on vehicles with returnless fuel systems, there may not be a fuel pressure regulator fitted to the fuel rail.

Be aware that while fuel rails are always attached to cylinder heads, it may be difficult to access the rails on some applications. In some cases, it may necessary to remove several cosmetic engine covers, as well as some major unrelated engine components, which often include intake manifolds and intake air ducting. Therefore, we strongly recommend that you consult reliable service information for the affected vehicle before attempting any repairs or servicing of fuel rail components to avoid making potentially expensive mistakes, or inadvertently creating fuel leaks that could cause engine fires.

What are the common causes of code P00C6?

Some common causes of code P00C6 are many and varied, and could include one or more of the following-

• Damaged, burnt, shorted, corroded, or disconnected wiring and/or connectors that affect the operation of the fuel pump
• Defective or failing fuel pump
• Clogged, dirty, or restricted fuel filter
• Low fuel levels
• Restricted or blocked fuel pick-up in the fuel tank
• Pinched, restricted, or otherwise damaged fuel lines
• Damaged, failed, or malfunctioning fuel pressure regulator
• Damaged or malfunctioning fuel pressure sensor
• Defective or corrupted fuel pump driver circuits in the PCM or dedicated fuel control module on returnless fuel systems
• Fuel system leaks

WARNING: Leaking injector seals or fuel line junctions can, and very often does, create destructive fires, meaning that when a strong smell of fuel is present along with code P00C6 (or any other fuel-related trouble code, for that matter), the origin of the leak must be found and corrected immediately. Moreover, the vehicle must NOT be operated until the leak is found and repaired, since fuel fires can start without warning, and at any moment.   

What are the symptoms of code P00C6?

Common symptoms of code P00C6 are similar across all applications and could include one or more of the following-

• Stored trouble code and an illuminated warning light
• In some cases, additional codes may be present along with P00C6
• The engine may crank normally, but not start
• In some cases, the engine may not crank at all

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