EZBIMMER
Fault code

BMW 278B — VANOS Pressure Accumulator Valve

P104B · P104A · P104C · P104D

Description

BMW fault code 278B relates to the electrical control circuit of the VANOS pressure accumulator valve. This solenoid-operated valve manages hydraulic oil flow to the VANOS pressure accumulator, which stores pressurized oil to enable rapid camshaft timing adjustments. The accumulator ensures the VANOS system has sufficient hydraulic pressure available for fast actuation, particularly during sudden throttle changes when the oil pump alone may not respond quickly enough.

The DME uses an ATIC35 driver chip to control this valve. The driver chip monitors the electrical circuit continuously and can detect short circuits, open circuits, and thermal overload conditions. Fault code 278B covers all four electrical failure modes of this valve, each identified by a separate P-code.

This code has 4 variants:

278B / P104B
Short to B+
Valve circuit shorted to battery voltage — voltage detected with valve off
278B / P104A
Short to ground
Valve circuit shorted to ground — excessive current with valve energized
278B / P104C
Open circuit
Valve circuit open — insufficient current with valve energized
278B / P104D
Overtemperature
DME driver chip thermal protection triggered — overtemperature condition

278B / P104B — Short to B+

The DME's ATIC35 driver chip detected battery voltage on the VANOS pressure accumulator valve output circuit when the valve was commanded off. Under normal operation, the circuit should see no voltage when the driver is not energizing the solenoid. The presence of voltage indicates an unwanted connection to a power source somewhere in the wiring or valve assembly.

Symptoms (4)
  • MIL (check engine light) may illuminate after two engine cold-start cycles
  • VANOS pressure accumulator valve remains energized when it should be off, potentially causing incorrect oil pressure management in the VANOS system
  • Possible audible VANOS rattle or ticking at cold start due to improper pressure accumulator behavior
  • In some cases, subtle loss of low-end torque or throttle response if VANOS timing is affected

Common Causes (3)
  1. Chafed wiring harness — the most common cause. The valve wiring runs through the engine bay near heat sources and moving components. A wire with damaged insulation that contacts a power feed (such as an adjacent actuator circuit or fused power line) creates an intermittent or permanent short to B+.
  2. Corroded or contaminated connector — moisture or oil contamination at the valve connector can create a conductive bridge between the signal pin and a power pin, presenting as battery voltage on the output.
  3. Internal solenoid short — less common, but an internal fault in the valve solenoid can create a path that feeds voltage back to the DME output when the valve is de-energized.

Diagnosis Steps (6)
  1. Locate the VANOS pressure accumulator valve on the engine. It is mounted on the VANOS unit at the front of the cylinder head. Refer to BMW TIS for the exact location and connector identification for your engine family.
  2. Disconnect the valve connector. With the ignition off, unplug the electrical connector from the valve.
  3. Measure voltage at the valve connector (harness side). Turn the ignition to terminal 15 (key on, engine off). Using a multimeter set to DC voltage, measure between the signal wire and ground. With the valve disconnected and not commanded, you should read 0 V. If battery voltage (~12 V) is present, the short to B+ is in the wiring harness — trace the harness from the DME to the valve connector, looking for damaged insulation or contact with power-carrying wires.
  4. If no voltage at the harness side, the fault may be intermittent or internal to the valve. Reconnect the valve. Measure voltage at the DME connector pin (consult the wiring diagram for your engine in BMW TIS for the specific pin assignment). If voltage is present at the DME pin with the valve connected but the wire is clean, the valve has an internal fault.
  5. Inspect the full wiring run from DME E-box to the valve, paying particular attention to areas where the harness passes near exhaust components, sharp edges, or other connectors. Look for heat damage, pinched wires, or chafed insulation.
  6. Check the connector pins at both ends for corrosion, bent pins, or oil contamination. Clean with electrical contact cleaner if needed.
  • Damaged wiring: Repair the harness — splice in new wire if the damage is localized, or replace the harness section if the damage is extensive. Use heat-shrink tubing and proper automotive-grade wire. Re-route away from the chafe point.
  • Corroded connector: Clean the connector pins and housing with electrical contact cleaner. If the pins are damaged or the connector housing is cracked, replace the connector.
  • Faulty valve: Replace the VANOS pressure accumulator valve. Obtain the correct part number from BMW ETK for your engine. No coding or programming is required after valve replacement — the DME recognizes the new valve automatically.

After repair, clear fault codes and run the engine through at least two cold-start cycles to confirm the fault does not return.

278B / P104A — Short to ground

The DME's ATIC35 driver chip detected excessive current flow when energizing the VANOS pressure accumulator valve. This indicates a low-resistance path to ground in the circuit, which draws more current than the solenoid coil's normal operating range. The driver chip flags this as a short-to-ground condition to protect itself from thermal damage.

Symptoms (4)
  • MIL (check engine light) may illuminate after two engine cold-start cycles
  • VANOS pressure accumulator may not function correctly if the DME disables the output to protect the driver chip
  • Possible VANOS rattle at startup or under load due to insufficient accumulator pressure
  • Subtle loss of power or torque response, especially during rapid throttle transitions

Common Causes (4)
  1. Wiring harness short to chassis ground — the valve signal wire contacting a grounded metal surface (engine block, bracket, bolt) through damaged insulation. This is the most common cause, particularly where the harness is routed near engine mounting hardware or along the valve cover.
  2. Internal solenoid coil short — a winding-to-winding or winding-to-case short in the valve solenoid reduces the coil resistance, drawing excessive current. This occurs as the valve ages and the coil insulation degrades from heat cycling.
  3. Connector pin bent or pushed back — a misaligned pin in the valve connector can contact the connector housing (which is grounded through the engine), creating a direct short to ground.
  4. Water or oil ingress into the connector — conductive contamination between the signal pin and the ground/housing creates a low-resistance leakage path.

Diagnosis Steps (6)
  1. Disconnect the valve connector at the VANOS pressure accumulator valve.
  2. Measure solenoid coil resistance. Using a multimeter set to ohms, measure across the two valve pins. Compare the reading to the specification for your engine (refer to BMW TIS wiring diagrams for the expected resistance value). A reading significantly below spec indicates an internal coil short. An extremely low reading (near 0 ohms) confirms a dead short in the solenoid.
  3. Measure resistance from each valve pin to the valve body (ground). The reading should be very high (megohms or open line). A low reading from either pin to the valve body indicates a winding-to-case short — the valve must be replaced.
  4. If the valve resistance is within spec, the short is in the wiring. Measure resistance from each harness-side connector pin to chassis ground. A low reading (near 0 ohms) on the signal wire confirms a wiring short to ground.
  5. Trace the wiring harness from the valve connector back toward the DME. Look for points where the harness contacts the engine block, brackets, bolts, or sharp edges. Check for heat-damaged insulation near exhaust components.
  6. Inspect the connector for pushed-back pins, bent terminals, or contamination that could bridge to the housing.
  • Wiring short to ground: Locate the damaged section and repair. Ensure repaired wiring is properly insulated and routed away from grounding surfaces.
  • Internal solenoid short: Replace the VANOS pressure accumulator valve. Source the correct part number from BMW ETK.
  • Connector damage: Repair or replace the connector. Ensure pins seat fully and make solid contact.

Clear fault codes after repair and verify through two cold-start cycles.

278B / P104C — Open circuit

The DME's ATIC35 driver chip commanded the VANOS pressure accumulator valve on, but measured insufficient current flow through the circuit. This indicates a break in the electrical path between the DME driver and the valve solenoid — either the wire is broken, a connector is disconnected, or the solenoid coil itself is open.

Symptoms (5)
  • MIL (check engine light) may illuminate after two engine cold-start cycles
  • VANOS pressure accumulator valve does not energize, so the accumulator cannot store or release pressurized oil on command
  • VANOS rattle or clatter, especially at cold start and during rapid throttle transitions
  • Reduced low-end torque and throttle response due to slower VANOS actuation without accumulator support
  • Possible rough idle if VANOS timing is significantly affected

Common Causes (4)
  1. Disconnected or loose connector — the most common and simplest cause. Vibration, previous service work, or a failed connector latch can leave the valve unplugged or making intermittent contact.
  2. Broken wire in the harness — the wiring between the DME and the valve passes through areas subject to vibration and heat cycling. A fractured conductor inside intact-looking insulation is a common failure, especially at flex points near connectors or where the harness crosses from the body to the engine.
  3. Corroded connector pins — oxidation on the connector pins creates a high-resistance junction that reduces current below the DME's detection threshold. Common in vehicles exposed to moisture or road salt.
  4. Open solenoid coil — the valve coil winding has broken internally, creating an open circuit. This is a wear-out failure and typically occurs at high mileage or after extended heat exposure.

Diagnosis Steps (5)
  1. Check the valve connector first. Physically verify that the connector is fully seated on the VANOS pressure accumulator valve. Disconnect and reconnect it, checking for a positive click from the latch. Inspect the connector for cracks or a broken latch that would prevent it from staying seated under vibration.
  2. Measure solenoid coil resistance. With the connector unplugged, measure resistance across the valve's two pins. A reading within the specification (refer to BMW TIS) confirms the coil is intact. An infinite (OL) reading means the coil is open — the valve must be replaced.
  3. If the coil is good, test wiring continuity. Measure continuity from the DME connector pin through the harness to the valve connector pin (harness side). Test both the signal wire and the ground return path. Any break in continuity identifies the failed wire.
  4. Wiggle test. With the multimeter connected for continuity, flex and wiggle the harness along its run — at the connector entry points, at any tie-down points, and where the harness crosses from body to engine. An intermittent break will show as a flickering continuity reading during the wiggle test.
  5. Inspect connector pins at both the valve end and the DME end for corrosion, green oxidation, or receded pins that aren't making contact. Clean with electrical contact cleaner and verify pin tension with a terminal pick tool.
  • Disconnected connector: Re-seat and verify the latch. If the latch is broken, replace the connector or use a zip-tie retention method as a temporary measure.
  • Broken wire: Locate the break using the continuity and wiggle tests, then splice in new wire with proper solder or crimp joints and heat-shrink insulation. If the break is in a high-flex area, add a service loop to prevent recurrence.
  • Corroded pins: Clean with electrical contact cleaner and apply dielectric grease to prevent future corrosion. If pins are too degraded, replace the connector terminal.
  • Open solenoid coil: Replace the VANOS pressure accumulator valve. Source from BMW ETK.

Clear fault codes and run through two cold-start cycles to confirm the repair.

278B / P104D — Overtemperature

The ATIC35 driver chip inside the DME that controls the VANOS pressure accumulator valve has triggered its thermal protection shutdown. This means the chip's internal temperature exceeded its safe operating threshold. The driver chip reduces or disables its output to prevent permanent damage to itself and the DME circuit board. This fault is stored to record the event.

Symptoms (4)
  • MIL (check engine light) may illuminate after two engine cold-start cycles
  • VANOS pressure accumulator valve may be intermittently disabled, especially under sustained high-load driving or in hot ambient conditions
  • Symptoms may be intermittent — appearing only during extended highway driving, track use, or in very hot weather, and resolving after the DME cools down
  • Possible VANOS-related symptoms (rattle, reduced torque response) only during the overtemperature events

Common Causes (4)
  1. Marginal wiring fault increasing current draw — a partial short to ground or slightly reduced wiring resistance that is not severe enough to trigger P104A (short to ground) but draws enough extra current to overheat the driver chip over sustained operation. This is the most likely electrical cause.
  2. Restricted E-box airflow — the DME is typically located in the E-box (electronics box) in the engine bay. If the E-box cover is missing, improperly sealed, or if the ducting that provides fresh air to the E-box is blocked or disconnected, the DME runs hotter than designed.
  3. High ambient or under-hood temperature — sustained high-performance driving, track sessions, or operating in extreme heat can push the E-box temperatures beyond the DME's thermal design. This is more of a contributing factor than a root cause — if the wiring and airflow are correct, the driver chip should handle normal operating conditions.
  4. Failing DME driver chip — in rare cases, the ATIC35 driver chip itself is degraded and triggers overtemperature at lower-than-normal thresholds. This is a DME hardware fault.

Diagnosis Steps (6)
  1. Check for co-stored codes. If P104A (short to ground) or P104B (short to B+) are also stored, address those first — a marginal wiring fault is likely causing elevated current draw that triggers the overtemperature protection.
  2. Measure solenoid coil resistance. If coil resistance is at the low end of spec or below spec, the solenoid is drawing more current than normal, contributing to driver chip heating. Consider replacing the valve.
  3. Measure wiring resistance from DME pin to valve connector. Compare against the expected round-trip wire resistance (should be very low — under 1 ohm for a short harness run). Elevated wire resistance causes the driver chip to work harder to maintain current through the solenoid.
  4. Inspect the E-box. Confirm the E-box cover is properly installed and sealed. Check that the fresh air duct to the E-box is connected and not blocked. Look for heat sources near the E-box — aftermarket exhaust headers, missing heat shields, or relocated components that radiate heat toward the electronics.
  5. Check ambient conditions when the fault occurs. If 278B / P104D only appears after sustained high-load driving in hot weather, and the wiring and E-box check out, the driver chip may be marginal. Monitor for recurrence under similar conditions.
  6. If no wiring or environmental cause is found, the DME driver chip itself may be degrading. This is a DME-level repair — professional assessment required.
  • Marginal wiring fault: Repair or replace the affected section of the harness. Even a partial short that doesn't trigger P104A/P104B can cause chronic overtemperature.
  • E-box airflow: Restore the cover, seals, and ducting to factory specification. Ensure no aftermarket modifications are directing heat toward the E-box.
  • Solenoid at low end of resistance spec: Replace the VANOS pressure accumulator valve as a preventive measure to reduce current draw.
  • DME driver chip failure: Requires DME repair or replacement. DME replacement requires programming and coding to the vehicle via ISTA. Some specialist electronics repair shops can replace individual driver chips on the DME circuit board — this is significantly less expensive than a full DME replacement.

Clear fault codes after repair. If the issue was environmental or load-related, attempt to reproduce the original driving conditions and confirm the fault does not return.

Last updated Mar 27, 2026 · Suggest an edit
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