A 2007 Volvo S80 rolls into the bay with the check engine light on and two specific codes: P019000 and P151B68. The technician noted exactly what the customer saw, which was simply that the check engine light was illuminated. When our diagnostic platform processes a fuel pressure code alongside an idle speed code on this specific vehicle, it builds a focused diagnostic path based on known failure patterns. Here is how that path breaks down.
The Fuel Rail Pressure Sensor
Our analysis ranked a defective fuel rail pressure (FRP) sensor as the most likely cause at 75 percent probability. On the Volvo 3.2-liter six-cylinder B6324S engine, this sensor is mounted directly on the fuel rail. It measures instantaneous fuel pressure so the engine control module (ECM) can regulate the fuel pump speed via pulse-width modulation.
An internal electrical defect in the sensor can cause the ECM to register a signal voltage outside the accepted 0.2 to 4.8 V window, immediately generating P019000. Without reliable pressure feedback, the ECM defaults to a predetermined pump cycle. This leads to incorrect fuel pressure at low load, causing the engine to run lean at idle and fail to maintain target RPM, which triggers P151B68. The initial tests here are straightforward. Check the 5V reference and signal voltage at the sensor connector with a multimeter, compare the live data pressure value with a manual gauge connected to the rail, and inspect the vacuum port for internal fuel leaks.
Mechanical Wear in the Fuel Pump
Ranked next at 60 percent probability is a worn or defective in-tank fuel pump. The S80 3.2 uses an electronically controlled, returnless fuel system. Over time, mechanical wear in the impeller or the carbon brushes can reduce flow, making the pump unable to maintain the required 3.8 to 4.2 bar at low engine speeds. When rail pressure drops below the ECM target, P019000 is logged, and the resulting fuel starvation at idle triggers P151B68. To test this, you measure system pressure with a gauge at idle and under dynamic load, perform a flow test via the diagnostic tool, and check the pump current draw with an oscilloscope and amp clamp.
Wiring and Connector Integrity
At 45 percent probability, we look at the wiring harness to the FRP sensor. Sitting on top of the cylinder head, this harness absorbs constant engine vibrations and severe temperature changes. Damaged wires, loose connections, stretched terminals, or oxidation in the 3-pin connector can create transient voltage drops. If the signal drops below 0.2 V or spikes above 4.8 V, P019000 sets. The resulting intermittent pressure disturbances cause the ECM to miscalculate fuel delivery, dropping the RPM and setting P151B68. The best approach is a wiggle test on the harness while monitoring live data, alongside a physical inspection of the pins for corrosion and tension.
The Electronic Throttle Module Wildcard
Finally, at 40 percent probability, carbon deposits in the Electronic Throttle Module (ETM) can restrict the minimal air gap needed to hold a stable 650 to 700 RPM idle. When the engine is under load from the alternator or AC, or if the fuel mixture is marginal from a pressure issue, the restricted airflow is not enough. The ECM increases the throttle angle to compensate, but once it hits its adaptation limit, the RPM falls and P151B68 is set. You verify this by visually inspecting the throttle body and checking the Absolute Throttle Position in live data at a warm idle.
Have you seen this combination of fuel pressure and idle control codes on the Volvo 3.2-liter platform in your workshop?
Reference pages for the codes in this article
- P0190, what we measure for this code across real diagnostics