|Trouble Code||Fault Location||Probable Cause|
|P1452||Unable To Bleed Up Vacuum in Tank (Ford, Mazda)
Secondary Air Injection System Circuit Open (Audi, Volkswagen)
Fuel Tank Pressure Sensor Low (Toyota, Lexus)
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What Does Code P1452 Mean?
OBD II fault code P1452 is a manufacturer specific code that is defined by carmakers Ford and Mazda as “Unable To Bleed Up Vacuum in Tank.” On these applications, code P1452 is set when the PCM detects an excessive vacuum in the fuel tank and/or EVAP (Evaporative Emissions Control) system. “Bleeding” in these definitions refers to the PCM’s inability to maintain a specified vacuum in the EVAP system only when the engine is running at high-speed cruising RPM’s.
NOTE: Ford also uses related code P1450 – “Unable To Bleed Up Fuel Tank Vacuum”, which is so similar to P1452 – “Unable to bleed up vacuum in tank”, that the diagnostic/repair information for P1452 apply to both codes. The only significant difference in the code setting parameters is that while P1452 sets during high-speed cruising conditions, P1450 sets at engine speeds above idle.
The purpose of the EVAP system is to capture and contain fuel vapors before they can escape into the atmosphere. In all EVAP systems, fuel vapors are temporarily stored in a charcoal canister until certain very specific conditions are met, which is when the PCM commands a purge valve open to allow engine vacuum to “suck” the stored vapors out of the charcoal canister into the inlet tract through a dedicated vacuum line to be combusted as part of the air/fuel mixture.
To assist this purging process, an EVAP system is fitted with a vent valve that is normally open, so that when the purging process starts, fresh air enters the system through the vent valve to help displace fuel vapors from the charcoal canister. During self-tests, the PCM closes the vent valve until the engine vacuum has created a specified vacuum by sucking fuel vapors into the engine through the purge valve. When the specified vacuum is attained, the PCM closes the purge valve, which then leaves the EVAP system in a sealed state. If the vacuum holds for a specified period of time, the PCM determines that no leaks exist and the vent valve is opened again, until the next self-test is run.
In a fully functional EVAP system, the vent valve remains open for the purpose of effectively purging the EVAP system and only the purge valve (which is normally closed), is commanded open and closed for the purpose of purging fuel vapors from the EVAP system. Therefore, if the vent valve remains closed for whatever reason, the inlet of the vent valve is blocked, or the charcoal canister and/or its pre-filter (where fitted) is/are sufficiently clogged to prevent air flowing through it/them, a vacuum that exceeds the specified value is created when the purge valve opens.
Thus, code P1452 sets, and a warning light is illuminated when the excessive vacuum cannot be relieved, or be allowed to decay to within the specified value. Note however, that specific conditions must to be met before this particular monitor can run. Refer to the manual for the application being worked on for detailed information on the specific conditions that must be in force on that application before this monitor can run.
The image below shows a much-simplified schematic of a typical EVAP system, such as might be found on Ford applications. Note the position of the vent valve relative to other major system components. This valve needs to be open in order for the EVAP system to be purged, as well as to vent the fuel system during filling. As the tank is filled, fuel vapors are absorbed by the charcoal canister, from where they are fed into the engine when the next purging cycle starts. One other major consequence of a blocked or closed vent valve is that filing the tank can become very difficult, if not impossible, since the air in the tank cannot be displaced by the incoming fuel.
What are the common causes of code P1452?
The typical causes of code P1452 are much the same across all applications, and could include the following-
- Blockages in the vacuum lines at one or more points between the vent valve inlet and the fuel tank.
- Defective vent valve
- Damage to the charcoal canister caused by over filling of the fuel tank
- Damaged, burnt, shorted, or corroded wiring and/or connectors
- Defective fuel pressure sensor. Note that this condition will almost always be indicated by a code other than P1452.
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.
What are the symptoms of code P1452?
Apart from a stored trouble code and an illuminated warning light, one other typical symptom of code P1452 could be-
- In some cases, the engine may be difficult to start directly after filling the tank
- Apart from setting code P1452, over filling the fuel tank could damage the charcoal canister on some applications
How do you troubleshoot code P1452?
SPECIAL NOTES: Ford Explorer, Taurus, Flex, Edge, MKT, MKS, MKX, and Mustang vehicles of the model years 2012-2013 fitted with 3.5L Twin-independent Variable Cam Timing (TiVCT) or 3.7L engines made from 1/3/2012 through 7/31/2012, may exhibit code P1452 repeatedly, even though the vent valve and charcoal canister may be working perfectly. Note that on most Ford applications, but particularly Ford Escape models, the failure of the purge valve causes fuel to be sucked into the charcoal canister, which causes the charcoal charge to disintegrate. The remedy on affected applications is to replace the EVAP system purge valve. END OF SPECIAL NOTES.
NOTE: Non-professional mechanics should take note that on some late model Ford Escape models, gaining access to the charcoal canister and vent valve for the purposes of testing and/or removal or replacement may require partial or complete removal of the rear axle. With this in mind, it is always a good idea to check if the EVAP system is still covered by the manufacturer’s warranty whenever code P1452 appears on a late model Ford Escape, since the equipment required to remove the axle from the vehicle is seldom available to the average non-professional mechanic.
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: In some cases, there may be other EVAP system related codes present as well, some of which may be the result of code P1452, while others may have contributed to the setting of code P1452. Refer to the manual for detailed information on any additional codes that may be present, but be sure to follow the directions in the manual exactly if the manual calls for other codes to be resolved first, before a diagnosis of P1452 is attempted. Failure to do this could result in a misdiagnosis, and the almost-certain unnecessary replacement of parts and components.
Consult the manual for the application to locate all EVAP components, as well as the location, routing, function, and color-coding of all associated wiring, hoses, and vacuum/fuel lines. Once all parts are located, perform a thorough visual inspection of all associated wiring, connectors, hoses, and fuel lines.
Look for visibly damaged, shorted, disconnected, or corroded wiring/connectors, as well as for signs of damaged, kinked, pinched, or misrouted vacuum lines. Make repairs as required, clear all codes, and complete at least one drive cycle before rescanning the system to see if the code returns.
If no visible damage to wiring, connectors, or vacuum lines and hoses is found and the scanner has control functions, use the scanner command the vent valve closed. The scanner will indicate the opened/closed state of the valve, so if the vent valve is shown to be working on the scanner, it is likely that a blockage exists in the line leading up to the vent valve, in the valve itself, or in the charcoal canister and/or its pre-filter.
If no visible damage to wiring and/or connectors is found, and the vent valve does not react to control inputs via the scanner, consult the manual on the correct procedure to test the vent valves’ control circuit for ground connectivity, resistance, and continuity. Compare all obtained readings with the values stated in the manual, and repair/replace wiring as required to ensure that all electrical values fall within the manufacturer’s specifications.
NOTE: Vehicles that spend long periods in storage, or that are frequently driven in muddy conditions are particularly susceptible to getting the inlet of the vent valve blocked by mud, or by insects either living in the vent valve inlet, or by insects building nests in the valve inlet.
If a blockage is suspected, refer to the manual for the application on the correct procedure to remove the vent valve from the vehicle, and inspect it for signs of mud or insects having taken up residence. Remove the blockage if possible, but be sure to apply a direct current to the valve (refer to the manual for instructions) to verify that it closes properly. When the direct current is removed, the vent valve must open again- verify that this actually happens by blowing through the valve. If it is not possible to blow through the valve or if blowing through it is difficult, the valve is defective, and the likely cause of P1452. Replace the valve if it is suspect in any way.
NOTE: One reliable way to test the vent valve is to apply a vacuum to one opening with a hand-held vacuum pump when current is applied to the solenoid. If the test equipment is not defective in anyway, the vacuum should hold steady for at least 60 seconds; if it does not hold absolutely steady for this period, the valve is defective and it must be replaced.
If the vent valve and its inlet is clear and it hold a vacuum for 60 seconds or so, remove the charcoal canister (and its pre-filter where fitted), and check it for signs of damage caused by liquid fuel. If water, liquid fuel, or loose charcoal granules fall out when the canister is shaken, it is defective, and the likely cause of P1452. Replacement of the canister is the only reliable remedy.
Also, be sure to clean out or replace the pre-filter. It is very common for these filters to become clogged to the point where no air can flow through them, and since they are relatively inexpensive, replacement is almost always the better option.
NOTE: If there is no evidence of water, fuel, or loose charcoal granules falling out of the canister, attach a piece of new vacuum hose to the vent valve side of the canister, and try to blow through it using only lung power. There may be some initial resistance, but it should be possible to blow through the canister without any trouble at all. However, it is very important NOT to suck air through the canister- if you do, you WILL inhale residual fuel vapors, which is a very bad thing indeed.
If the vent valve itself, the line leading up to it, and the charcoal canister are clear and free of obstructions, remove all EVAP related vacuum hoses between the charcoal canister and the fuel tank from the vehicle, and use strong compressed air to blow them clear of whatever might be causing a blockage. If compressed air is not available, poke a length of soft, pliable wire through the hoses to be sure that all blockages are cleared out. Be careful however not to poke holes in any part of the vacuum line.
If there are any doubts about whether all blockages have been removed or not, the better option is simply to replace all vacuum hoses with OEM replacements.
If the vent valve works as intended and is not clogged up, the charcoal canister and its pre-filter are serviceable, and there are no blocked or damaged vacuum lines between the canister and the fuel tank, the diagnostic/repair steps up to this point will almost certainly have resolved the problem.
If the problem/code/symptoms persist, bear in mind that if everything else works as intended, a stuck open purge valve cannot cause this code. Stuck-open purge valves are almost always indicated by codes other than P1452 in any case, so if such codes are not present along with P1452, the purge valve and its operation are not likely to be implicated in the setting of P1452.
If the code reappears after a few drive cycles, it is likely that an intermittent wiring fault or an internal defect in the vent valve solenoid is causing the problem. Be aware though those intermittent faults are sometimes extremely difficult and time consuming to find and repair, and in some cases, it might be necessary to allow the fault to worsen considerably before an accurate diagnosis and definitive repair can be made.
Codes Related to P1452
- P1451 – Relates to “Evaporative emission control system vent control valve circuit fault”
- P1453 – Relates to “Fuel tank pressure relief valve malfunction”
- P1454 – Relates to “Evaporative system vacuum test malfunction”
Other Manufacturer Specific Definitions for P1452Unable To Bleed Up Fuel Tank Vacuum (Ford)
Fuel Tank Pressure Sensor Low Input (Toyota)
Secondary air injection (AIR) system - open circuit (Volkswagen)
Secondary air injection (AIR) system - open circuit (Audi)
Engine oil pressure – too low before start (BMW)
Diesel particulate filter (DPF) pressure differential sensor hose performance (Jeep)
Evaporative emission (EVAP) pressure pump – low current (Land Rover)
Fuel tank pressure sensor, low input- circuit malfunction (Lexus)
Evaporative emission (EVAP) system -unable to bleed fuel tank vacuum (Lincoln)
Evaporative emission (EVAP) system – fuel tank vacuum test failed (Mazda)
Diagnostic Module Tank Leakage Switching Solenoid Control Circuit Signal High (Mini)
Evaporative emission (EVAP) pressure sensor, in fuel tank – signal too low when depressurized (Saab)