Cause
Perished seal or O-ring
A rubber seal hardened, cracked or flattened with age and heat, so a joint that was sealed no longer is. It is deliberately a general cause, because the same failure appears across hydraulic, fuel, vacuum and coolant circuits and the mechanism does not change with the fluid.
Seals fail by losing compliance rather than by breaking. A seal works because it is squeezed and pushes back; heat cycling and time make the rubber hard, and it takes a permanent set in the shape of its groove. So the leak appears when cold and closes as things expand, which is the pattern to recognise and the reason an intermittent leak is so often a seal rather than a crack.
Two practical points. A seal is single-use — one that has taken a set will not reseal on reassembly, so any joint opened wants a new one, and reusing an O-ring is a common reason a repair leaks immediately. And the material must match the fluid: a seal correct for oil may swell or dissolve in brake fluid or a gaseous fuel, so a generic O-ring of the right dimensions is not necessarily the right part.
Typically specialist work. Needs equipment or training most home workshops do not have.
Some of these codes mean you should stop driving
1 of the 4 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
- P2712 Hydraulic Power Unit Leakage Likely for this code Stop driving High
- P277A Electric/Auxiliary Transmission Fluid Pump B Hydraulic Leakage Likely for this code Moderate
- P27E1 Electric/Auxiliary Transmission Fluid Pump A Pressure Too Low Moderate
- P27E2 Electric/Auxiliary Transmission Fluid Pump B Pressure Too Low Moderate
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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