P0174 on Volvo: How to Diagnose & Fix

How to Fix

Replace the entire PCV breather box assembly along with associated hoses ($150-300 in parts). This is a common maintenance item - include it with timing belt service. DIY takes 2-4 hours. Also replace flame trap if equipped. Use OEM or quality aftermarket parts.

Owner tips & cautions

• TipDIY repairs can save significantly - dealer charges $150-300 but DIY costs are typically 50-70% less
• TipThis is a common maintenance item - include it with timing belt service.
• TipConsider OEM parts for critical components like sensors and electrical parts - aftermarket can be unreliable

How to Fix

Check oil level every 1,000 miles and top up as needed. Use Volvo-approved 0W-20 synthetic oil only. If consumption exceeds 1 quart per 1,000 miles, document with dealer for possible warranty claim. Piston ring replacement if out of warranty: $3,000-6,000. Later production engines (late 2017+) have updated rings.

Owner tips & cautions

• TipDIY repairs can save significantly - dealer charges $3,000-6,000 but DIY costs are typically 50-70% less
• TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes

How to Fix

Replace the PCV oil trap assembly, which is integrated into the valve cover on these engines (Volvo part 31338685 for oil trap, or 31375560 for complete updated valve cover assembly). Clean intake manifold of oil deposits. Replace spark plugs if fouled. Use Volvo-approved 0W-20 oil and shorten oil change intervals to 5,000 miles. Updated oil trap design from mid-2022+ should be used as replacement part.

How to Fix

Monitor oil level weekly and top up as needed. Visit dealer for engine software updates addressing boost pressure and valve timing. Replace PCV oil trap (Volvo part 31338685) if clogged. Shorten oil change intervals to 5,000 miles with 0W-20 synthetic. If consumption exceeds 1 qt per 1,000 miles, dealer should investigate piston ring condition under powertrain warranty. Document all oil additions for warranty claims.

How to Fix

Diagnose LH-Jetronic idle problems by smoke-testing the intake for vacuum leaks, inspecting all breather/PCV hoses and the flame trap for blockage, and checking the idle air control valve for sticking or no response; also verify the air mass meter output and connector condition, as worn AMMs commonly cause unstable idle and rich running. Clean or replace the flame trap service parts and cracked vacuum hoses first, then clean the throttle body and IAC valve, and replace a faulty Bosch AMM or IAC valve if testing confirms failure. After repairs, base idle and throttle switch operation should be verified and the system rechecked for leaks and proper idle speed. Typical cost is about $50-$150 for hoses/flame trap service, $150-$350 for an IAC valve, and $250-$450 for a Bosch AMM plus 1-2 hours labor.

How to Fix

Diagnose crankcase ventilation restriction by checking for pressure at the oil filler cap or dipstick tube at warm idle (a glove test over the filler neck should show slight vacuum, not inflation), then inspect for sludge in the breather box/oil trap and clogged hoses or banjo fittings. The proper repair is a full PCV service: replace the oil trap/breather box, all PCV hoses, flame trap components where fitted, intake manifold gasket, and clean the block breather port and manifold vacuum passages; if seals are already leaking, camshaft, front crank, or other affected seals may also need replacement. On the Volvo 850 5-cylinder, this is typically a 3-6 hour job depending on access and sludge severity, with parts usually around $120-$250 for a complete kit and total shop cost commonly $400-$900, or more if oil leaks have progressed and seals must be repaired.

How to Fix

Diagnose the PCV/oil trap system by checking for excessive crankcase vacuum or pressure at the oil filler cap, listening for a whistling diaphragm, and inspecting for oil seepage at the cam, crank, and valve cover seals; scan for lean-mixture or idle-related faults as supporting evidence. Repair typically requires replacing the oil trap/PCV separator assembly and breather hoses, and on the 3.0L/3.2L SI6 this often involves removing the intake manifold for access; any failed seals fouled by crankcase pressure should be replaced at the same time. After repair, clear fault codes, verify normal crankcase pressure, and confirm idle quality and oil leaks are resolved. Typical independent-shop cost is about $700-$1,400 depending on hose/seal replacement, with dealer pricing often higher.

How to Fix

Diagnose the PCV system by checking for crankcase pressure at the oil filler cap or dipstick tube at idle; excessive pressure, whistling, or smoke from seals usually indicates a clogged oil trap/breather system. Repair requires replacing the complete PCV service kit: oil trap/breather box, hoses, banjo fittings, intake manifold gasket, and thoroughly cleaning the block breather port and related vacuum passages; on turbo models, also inspect the turbo inlet pipe and compressor housing for oil contamination and verify turbo seals were not damaged by prolonged crankcase pressure. After repair, confirm slight crankcase vacuum at idle and address any already-failed cam seals or turbo oil leaks separately if they persist. Typical cost is about $150-$300 in parts for a full kit, or roughly $600-$1,200 at an independent shop depending on labor and whether seal or turbo-related follow-up repairs are needed.

How to Fix

Inspect the crankcase ventilation system by removing the flame trap, breather hoses, and oil separator/breather box under the intake manifold; if the flame trap is restricted or the hoses are soft and sludged, the system is overdue for service. Clean or replace the flame trap, all PCV hoses, and ideally the breather box and intake manifold gasket, then verify crankcase vacuum at the oil filler cap or dipstick tube at idle to confirm proper flow. If the engine has been leaking for a while, also inspect and replace pushed-out cam, front crank, or intermediate shaft seals as needed after restoring ventilation. Parts typically run about $50-$150 for a basic flame trap/hoses service or $150-$300 with breather box and gaskets, while shop repair is commonly $300-$700 depending on access and how many seals are already leaking.