The Pierburg controller will latch a dry-run/overspeed or comms fault and refuse to run until power is removed. Pulling battery clears that latch. It’s not that air magically disappears; the controller just reboots. If there’s a marginal condition (tiny air pocket, weak cap, or control mismatch) it may prime and run after the reset, until the next event.
A system that holds 25 inHg for hours can still ingest micro-air at the pump inlet quick-connect when hot and under suction. Those two O-rings can seal fine static and still let a bubble past dynamically. (No coolant drip, no residue.) If that hose/coupler is original, I’d still replace the inlet hose assembly, Volvo doesn’t sell those O-rings separately for a reason.
Do these in order (no VIDA required):
- Part number sanity check (most common root cause).
Match the pump to your VIN via dealer/VIDA. Pierburg sells multiple CWA variants with different logic (PWM vs LIN/LIN profile). An “OEM” Pierburg can be electrically identical but the firmware/handshake is wrong for Volvo → intermittent no-start that a battery pull “fixes.” If there’s any doubt, return it and install the Volvo-boxed pump that supersedes your original PN. - Replace the pump inlet hose assembly.
New coupler + fresh O-rings eliminates the micro-ingestion wildcard. Lightly lube O-rings with coolant and make sure it fully snaps/locks. - Vacuum-fill again, but add these two tricks:
- Raise the front of the car ~6–8″ and clock the pump the same as OE (outlet slightly “uphill”).
- Run the cabin heat on HI, bring it through two full fan cycles, cool completely, recheck level.
A weak expansion-tank cap can also encourage localized boiling/cavitation, cheap to replace; I’d do it.
- Electrical supply check while it’s “not running.”
Back-probe the pump connector during a failed start:- B+ to ground ≈ battery voltage (KOEO/KOER).
- Ground drop <50 mV (measure between pump ground pin and battery negative).
- Gently tug each terminal in the connector, make sure no backed-out pins, no green crust.
If B+/ground are solid and it still won’t spin until you power-cycle: that screams pump variant/firmware mismatch.
- Quick A/B to end the guesswork:
If your old pump still worked, reinstall it briefly.- Old pump = 100% reliable → the new pump(s) are the problem (compatibility/firmware).
- Old pump shows the same intermittent → chase inlet hose/cap/wiring.
If/when you can get VIDA (or a Volvo-capable scan tool):
- Run the bleed/activation routine and watch Commanded pump speed vs Actual.
- Commanded >0 and Actual = 0 → pump/harness/compatibility.
- Commanded = 0 when hot → ECM logic/sensors/software.
- Pull manufacturer DTCs (the generic OBD app won’t show the good stuff).
Software updates are worth doing once the hardware is correct, but software won’t make a wrong-profile pump talk on the network. The “works after battery pull” pattern is much more consistent with pump controller behavior than with a missing update.
Bottom line to be done with it:
- Install Volvo-boxed supersession for your VIN,
- replace the inlet hose assembly (fresh O-rings),
- new expansion-tank cap,
- vacuum-fill and heat-cycle.
That combo has cured this exact intermittent on VEA XC60s more than once.
Hang in there, you’re close.