This 2018 Renault Trafic with 45,000 miles came in with hard cold starting — a classic sign of failing glow plugs on a diesel. On bench testing, one plug was completely dead and another barely glowing. All four were replaced as a full set.
Glow plugs on this 1.6 dCi engine are accessible from the top with minimal stripdown — just the coolant bottle needs moving to reach plug four. The key tool is a quarter-drive 10mm socket; a full-size drive simply won't fit.
Symptoms of Failed Glow Plugs
- Hard starting on cold mornings — especially below 5°C
- Excessive white smoke on cold startup
- Engine management or glow plug warning light
- Rough running when cold, smooths out once warm
- Longer-than-normal cranking before firing
Also Applies To — Same 1.6 dCi Engine
- Vauxhall Vivaro (older shape)
- Nissan Primastar
- Fiat Talento
What Do Glow Plugs Do?
Glow plugs are a critical part of a diesel engine's cold-start system. Unlike petrol engines which use a spark plug to ignite fuel, diesel engines rely on compression heat — but when the engine is cold, especially in UK winter temperatures, the compressed air in the cylinder may not be hot enough to reliably ignite the diesel spray. Glow plugs are electric heating elements screwed into each cylinder that pre-heat the air inside to ensure reliable ignition from the first crank.
On a modern diesel van like the Renault Trafic, the glow plug system typically heats the cylinders for 1–5 seconds before the starter motor engages — shown by the coil/heater icon on the dash. Once the engine is warm, glow plugs are not used for combustion but some systems continue to run them briefly after start to reduce initial white smoke emissions and improve idle quality.
Symptoms of Failing Glow Plugs
Glow plugs don't always fail all at once — often one or two fail while the others are still functional. Even a single failed glow plug causes noticeable symptoms, particularly in cold weather. Common signs include: difficulty starting in cold weather, excessive white smoke on cold start (unburnt diesel due to incomplete combustion), rough running or misfiring in the first few minutes after a cold start, and an engine management warning light with a glow plug fault code. On a Trafic, a fault code P0670–P0677 range typically points to a glow plug or glow plug control circuit issue.
In mild weather, a single failed glow plug can go unnoticed for months — the remaining plugs compensate, especially if the engine is started warm. When temperatures drop below 5°C, however, the failure becomes obvious very quickly. If your Trafic has become harder to start in cold weather, the glow plugs should be one of the first things to check.
How Often Should Glow Plugs Be Replaced?
Glow plugs are typically not on a fixed scheduled replacement interval like spark plugs in a petrol engine. They are replaced on a condition basis — when they fail or show symptoms. However, on a high-mileage diesel van (over 100,000 miles), replacing all four glow plugs as a preventative measure during a major service is good practice. The cost of the plugs is modest, and replacing them as a set avoids the scenario where one is changed and another fails shortly after. Always replace all plugs at the same time to ensure equal cylinder performance.
Tools & Parts Needed
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Step-by-Step Guide
Disconnect All Four Glow Plug Connectors
Open the bonnet. The four glow plug connectors are small black tabs sitting on top of the engine — one per cylinder. Use long nose pliers to gently pull each one off. Plugs 1–3 are straightforward. Plug 4 is buried at the back — push the tab on the coolant bottle to release it and move it aside, then gently pull the small top coolant pipe free to create enough clearance.
Remove All Four Glow Plugs
Fit the 10mm quarter-drive socket on an extension and ratchet. Looking carefully down past each connector location you'll see the shiny glow plug hex head. Seat the socket fully, crack each plug loose, then spin out by hand. Use long nose pliers to lift each plug clear. Number 3 may be slightly tighter — persevere, it will come.
Test the Old Plugs
Connect a jump pack earth to the body of each plug and briefly touch positive to the tip. A healthy plug glows bright red within 1–2 seconds. On this Trafic: plug 2 was completely dead, plug 3 was barely glowing and nearly gone, plugs 1 and 4 were working. All four replaced as a service item regardless.
Fit the New Glow Plugs
Apply a tiny amount of copper slip to the threads of each new plug — this prevents future seizure. Drop each plug into its hole, let it find its seat, then use the 10mm socket to wind it in by hand first to guarantee no cross-threading. Tighten to a firm nip only — do not overtighten.
Refit Connectors & Test
Press each connector back onto its glow plug until you feel a click. Plugs 3 and 4 can be fiddly — use long nose pliers gripping where the cable meets the plastic and press firmly. Refit the coolant bottle and pipe, making sure the pipe clips in fully on both sides. Start the engine — it should fire readily with no warning lights showing.
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Job Summary
Frequently Asked Questions
Can I Drive with Failed Glow Plugs?
Technically yes — the engine will start and run if even one glow plug is still working, especially in milder weather. However, it's not advisable. You'll experience hard starting, poor fuel economy, and rough running in the first minutes after startup. The extra stress on the starter motor trying to crank a cold engine multiple times increases wear on the battery and starter. In winter conditions below 5°C, a failed glow plug makes the van unreliable. It's better to fix it promptly than struggle with failed starts at the roadside.
How Long Does the Job Take?
With reasonable DIY experience and the correct tools, glow plug replacement on the Trafic 1.6 dCi typically takes 1–2 hours. The fastest part is removing the old plugs (20 minutes for all four). Testing takes time if you want to bench-test each plug to confirm failure. Refitting is slower than removal because you must be careful not to cross-thread and not to overtighten. If you're less confident or working on a van with significantly corroded plugs, allow 2–3 hours. A skilled technician can do it in 45 minutes to an hour.
Do I Need a Torque Wrench?
No — a torque wrench is not essential. Glow plugs only need to be tightened firmly (a nip), not to a specific torque figure. This is actually safer than using a torque wrench, which could encourage over-tightening. Screw them in gently by hand using the quarter-drive socket and ratchet until they're snug and seated. You'll feel when they're in the right place. If you're worried about tightness, remember that glow plugs are soft brass and the head is mild steel — over-tightening risks snapping the tip off inside the head (a very costly failure). Gentle and firm is the rule.
What If a Glow Plug Snaps Off Inside the Head?
This is the nightmare scenario and one reason not to overtighten. If a plug snaps and the tip stays in the cylinder head, removal requires either drilling it out very carefully or, in some cases, a machine shop may be able to extract it with a screw extractor tool. If a piece breaks off inside the combustion chamber itself, the engine won't start and the debris will need to be flushed out. Prevention is much better than cure — never force a plug and only apply hand pressure with the quarter-drive ratchet. If a plug feels stuck, apply penetrating oil (like WD40) and let it sit for an hour before trying again.
