P25AA: Piston Cooling Oil Control Circuit Low

P25AA flags a voltage at the piston cooling oil control valve output that sits too low for the commanded state, typically a short to ground between the module and the valve. This valve decides whether the oil jets under the pistons spray. GM's 2018 diesel engine table 18OBDG04 tests the low-side driver while it is switched off: if the output behaves like a connection of 0.5 ohm or less to ground in 40 of 50 checks taken every 100 ms, with at least 11 V from the powertrain relay, the code is stored as Type B and the lamp follows on the second trip. Deutz EMR L1 industrial engines report the same fault on their high-side switched supply valve as P25AA-11. An open circuit is P25A9, a short to power P25AB.

Medium severity. Can I keep driving?Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

Symptoms

Check engine lamp after two drives
Possible power limitation to protect the pistons

Most likely causes

Valve control wire chafed through and touching ground
Internal short in the piston cooling valve
Water or oil in the valve connector bridging the pins
Faulty output stage in the engine control module

What to check, in order

Start with the simplest. You can tick off each step.

With the engine cold, check the coolant level; never do this when the system is hot
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
Measure the sensor and replace it if its readings do not make sense

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P25AA means

P: Powertrain: engine and transmission
2: Generic code: same on every make
5: Speed and idle
AA: Specific fault in that system

Frequently asked questions

Can I keep driving with code P25AA?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P25AA?

The most common causes are: valve control wire chafed through and touching ground; internal short in the piston cooling valve; water or oil in the valve connector bridging the pins.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.