P0629 indicates the Powertrain Control Module (PCM) has detected an abnormally high voltage in the fuel pump control circuit. The fuel pump is normally switched on through a relay or an electronic driver module, and this code sets when the monitored signal falls outside the expected range. The condition can prevent the pump from delivering proper fuel pressure, leading to drivability or no-start symptoms.
An internal short, high resistance, or mechanical binding in the fuel pump motor can alter the control circuit's voltage and is the most common hardware failure associated with P0629.
Chafed, broken, or corroded wiring between the PCM, relay or driver module, and the fuel pump—particularly a short to voltage on the control side—commonly produces a high-voltage reading.
Many vehicles use a relay or a dedicated Fuel Pump Driver Module (FPDM) to switch the pump. A stuck or shorted switching device can leave the circuit in a high state.
Moisture intrusion, pin damage, or a poor terminal fit at the fuel pump harness, relay socket, or PCM connector can cause the circuit to read high and set the code.
A broken, loose, or corroded ground path at the fuel pump can force the PCM's monitoring circuit to register an abnormally high voltage on the control line.
Although the PCM monitors this circuit, an internal driver fault is rare compared with the field components above and should only be considered after they are ruled out by direct testing.
Often, yes. A failing or internally shorted fuel pump is one of the more common causes of P0629 because abnormal pump current directly affects the control-circuit voltage the PCM is monitoring.
Almost never. The PCM monitors the circuit but is rarely the failed component. Drivers are sometimes steered toward a PCM replacement, but the actual fault is almost always in the pump, relay, driver module, wiring, or connectors. A PCM should be replaced only after every other possibility has been ruled out by direct circuit testing.
It is not advisable. The fuel pump may run intermittently, at reduced output, or not at all, which can leave you stranded or cause engine damage from a lean operating condition.
With a digital multimeter or lab scope, voltage and ground are measured at the pump connector while the pump is commanded on, and the control wire is back-probed at the relay or driver module. Resistance and insulation tests on the harness identify shorts, opens, or high-resistance connections.