Glow Plug Circuit Diagnosis & Individual Plug Resistance Test (M57)

• Green — 0 to ~5,000 km after first fault code

  Single plug failure causes marginal cold-start roughness, white smoke for the first 30–60 seconds in temperatures below 5 °C. Engine runs normally once warm. Fault code P0671–P0676 (individual cylinder glow plug circuit) present. Driveable, no mechanical damage yet.

• Amber — ~5,000 to ~20,000 km, or second plug fails

  Two or more failed plugs produce persistent rough cold idle, misfire codes alongside glow plug codes, increased fuel consumption, and visible white/grey smoke on startup. Unburnt fuel begins to foul injector tips and EGR circuit. Risk of oil dilution begins if cold starts are repeated without full warm-up cycles.

• Red — Beyond ~20,000 km or three or more plugs failed

  Severe cold-start misfires risk hydrolocking a cylinder if injector continues delivering fuel into an unignited bore at low temperatures. Oil dilution (fuel contamination detected by a reduced oil viscosity on dipstick) can cause bearing damage. Catalytic converter and DPF face accelerated fouling from unburnt fuel. Repair cost escalates from a €50 plug set to potentially €800+ in downstream damage.

• Black — Ignored indefinitely, especially in northern-European winters (NL/DE below −5 °C)

  Complete failure to start, potential hydrolocking requiring cylinder head removal, injector replacement, and DPF/CAT replacement. What began as a €50–80 parts job becomes a €2,000–5,000 engine workshop bill.

• The glow plug electrical terminals on high-mileage M57s are almost always corroded — green or white oxidation on the brass nut and the ring connector. You will need to work each terminal nut free before you can get a clean multimeter probe contact. Budget 5 extra minutes per plug for cleaning with a small wire brush or fine sandpaper.
• On E53 and E70 X5 3.0d engines, the glow plugs on cylinders 4–6 (rear bank of the straight-six, nearest the firewall) are extremely difficult to reach with a standard multimeter probe. You will encounter the fuel rail, coolant hoses, and the intake manifold all competing for the same space. A right-angle probe tip or a short flexible extension lead soldered to your multimeter lead tip makes this feasible; without it, expect scraped knuckles and false contact readings.
• The most common mistake on this exact task is measuring resistance at the harness connector (the female side, after unplugging from the plug) rather than directly at the glow plug terminal. Measuring at the harness tests the wiring loom and GPCU output — not the plug itself. Many first-timers diagnose a 'failed' plug that turns out to be a wiring fault, or vice versa. Always measure plug-side resistance with the harness fully disconnected and your probe tip touching the plug's centre electrode terminal directly.
• At least one plug will almost always read slightly higher resistance than the others even when all are 'good' — this is normal manufacturing tolerance on ceramic glow plugs. Do not replace a plug purely because it reads 0.3–0.5 Ω higher than its neighbours. An open circuit (infinite/OL resistance) or a short to ground (0.0 Ω reading when measuring between the terminal and the engine block) are the failure signatures to act on.
• On vehicles over 8 years old or 120,000 km, the rubber boots/seals on each glow plug connector will have hardened and may crack or break when you pull them off. Have a roll of self-amalgamating tape or a set of replacement boots on hand — exposed plug terminals in the engine bay will corrode rapidly and cause the very faults you just diagnosed.

This is a diagnostic procedure — no replacement parts are required to perform it. However, if your diagnosis confirms one or more failed plugs and you are ordering replacements before beginning: \n\n1. CONFIRM YOUR M57 VARIANT FIRST. The M57 engine family spans three generations: M57 (1998–2003, 11V resistive plugs), M57TU/M57N (2003–2007, 4.4V fast-glow ceramic plugs), and M57N2 (2007–2013, 4.4V fast-glow ceramic plugs). Using a 11V plug in an M57TU application, or vice versa, will destroy the GPCU. Your engine variant is stamped on the engine block near the oil filter housing and listed on your vehicle's Fahrzeugschein (registration document) under engine code.\n\n2. COUNT THE PLUGS IN THE BOX. The M57 is a six-cylinder engine and requires six plugs. A box of four is for M47 four-cylinder applications — a common mis-ship error from online suppliers.\n\n3. COMPARE THE HEX SIZE. Place the new plug next to the old one (if you have an old one for reference). M57 glow plugs use a 10 mm hex on most variants. A plug with a 12 mm hex is the wrong application.\n\n4. CHECK THE THREAD PITCH AND LENGTH. M57 plugs use M10 x 1.25 mm thread. A visually similar plug with M10 x 1.0 thread will cross-thread and destroy the cylinder head on installation.\n\n5. VERIFY THE TERMINAL TYPE. M57 plugs have a threaded stud terminal (M4 or M5) accepting a ring connector and nut. If the plug has a push-on spade terminal, it is the wrong part.

• Digital multimeter
  Auto-ranging preferred; must have resistance (Ω) mode and DC voltage mode. A €15–25 unit from a German hardware store (Bauhaus, OBI) is adequate.
  Set to resistance (Ω) mode for plug testing. Zero your leads first: touch the two probe tips together and note the reading — it should be 0.0–0.2 Ω. Subtract this lead resistance from all subsequent plug readings. On auto-ranging meters, wait for the display to stabilise (2–3 seconds) before recording a value — the M57's cold glow plug resistance is very low (0.5–3 Ω) and the meter needs to settle.
• OBD2 diagnostic scanner
  Must read BMW-specific fault codes (not just generic OBD2). ISTA/D (BMW dealer software), INPA, or a third-party tool such as Carly, BIMMERCODE (read-only), or a professional-grade scanner with BMW coverage. A generic ELM327 dongle will only show generic P06xx codes and will miss BMW-specific glow plug relay codes.
  Connect before doing anything mechanical. Read and record ALL fault codes, not just the first one. Note the fault status (permanent vs. stored/intermittent) — an intermittent fault points to a wiring/connector issue rather than a dead plug element.
• 10 mm combination spanner or socket
  For loosening the glow plug terminal nuts if corroded. A quarter-drive ratchet with a 10 mm socket gives better control in the tight space above the valve cover.
  Use a 6-point socket, not 12-point, to avoid rounding the terminal nut on a corroded fitting. Apply penetrating oil if the nut does not break free within a quarter-turn.
• Small flat-blade screwdriver
  2–3 mm blade width, for gently prising the glow plug harness connector boots away from the plug terminals without cracking them.
• Wire brush / fine emery paper
  Small brass wire brush or 400-grit wet-and-dry paper for cleaning terminal contact surfaces before measuring.
• Penetrating oil
  WD-40, Caramba, or equivalent penetrating lubricant for corroded terminal nuts. Do NOT use WD-40 on rubber boots.
• Inspection light / head torch
  The glow plug terminals sit directly below the fuel rail and above the valve cover — ambient light is insufficient. A flexible neck inspection light is ideal.
• Pen and paper / phone camera
  For recording resistance readings per cylinder (1 through 6) systematically. You will not remember six numbers while working in an engine bay.

• Glow plugs (set of 6) — only required if diagnosis confirms failure
  • OEM / OE-equivalent (Beru, NGK, Bosch) — Beru and NGK supply the majority of M57 glow plugs to BMW at the factory. Beru GE101/GE153 (M57TU variant) and NGK Y-904J are widely used OE-equivalent references. Bosch 0 250 202 and 0 250 403 series cover M57 applications. Expect €50–80 for a set of 6 from a German parts supplier (TRW, Autodoc, Oscaro NL). These are the correct choice.
  • Budget (unbranded / marketplace) — Avoid. Ceramic glow plugs must meet precise resistance and voltage specifications matched to the GPCU. Off-spec resistance from a budget plug causes GPCU fault codes immediately or damages the unit over time. The cost saving of €15 is not worth the €200+ GPCU replacement risk.
• Glow plug terminal nuts (M4 or M5, set of 6)
  • Standard — These are standard metric nuts — available from any hardware store for cents each. Buy a set of 6 before starting. Corroded nuts frequently round off on removal and should be replaced rather than reused.
• Glow plug connector boots / insulators (optional but recommended for high-mileage vehicles)
  • OE-equivalent rubber insulator boots — Available from BMW dealers or as aftermarket rubber sleeves. Prevents terminal corrosion recurrence. Costs approximately €10–20 for a set.

• Check
  Fault code status after cold start and warm-up cycle
  Expected

  No stored or current P0671–P0676 codes or GPCU-related codes. Code memory is clean after a complete cold start and 15-minute warm-up cycle.
  If wrong

  If a code returns for the same cylinder you just tested and found a good resistance reading, re-examine the harness connector for that cylinder — particularly the ring connector crimp and the GPCU output terminal. If the code returns for a cylinder that read OL (open circuit) and you have not yet replaced the plug, the diagnosis is confirmed: replace that plug.
• Check
  Cold-start behaviour: cranking duration and exhaust smoke
  Expected

  Engine fires within 2 crankshaft revolutions (approximately 1 second of starter engagement) when cold at temperatures above 0 °C. No sustained white/grey exhaust smoke beyond 15 seconds of running. Idle settles smoothly within 10–15 seconds.
  If wrong

  If white smoke persists beyond 30 seconds, or the engine idles roughly for more than 30 seconds after a cold start, one or more plugs remain non-functional. Return to Phase 3 and retest the resistance of the cylinder(s) corresponding to any returning fault codes.
• Check
  Pre-glow indicator (dashboard coil symbol) behaviour
  Expected

  The pre-glow indicator illuminates for 1–4 seconds after turning the ignition to position 2 (before cranking), then extinguishes. The duration varies with ambient temperature — longer in cold weather, shorter when warm. This indicates the GPCU is completing a normal pre-glow cycle without cutting out early due to a detected fault.
  If wrong

  If the pre-glow indicator extinguishes immediately (under 0.5 seconds) even in cold conditions, the GPCU has detected a fault and aborted the cycle — recheck your fault codes and resistance readings. If the indicator never illuminates at all, the GPCU itself or its power supply fuse has failed.
• Check
  Terminal nut tightness visual and tactile check (post-reassembly)
  Expected

  Each ring connector boot sits flush against the plug body with no exposed metal visible. A gentle tug on each boot produces no movement — it is fully seated. No wires are routed under or against other engine components.
  If wrong

  If any boot is loose or lifted, remove it, check the terminal nut is snug (1–2 Nm), and reseat the boot firmly. A loose boot must be secured — exposed terminals will corrode and generate new fault codes within weeks.