Why Does My Car Overheat in Slow-Moving Traffic?

If your temperature gauge creeps up when you’re crawling through traffic but settles back down once you’re moving again, the problem is almost always the cooling fan. At low speed there’s little airflow coming through the grille naturally, so the engine relies on the electric fan to pull air through the radiator. When that fan isn’t doing its job properly, the engine starts to overheat while you’re sitting still or moving slowly.

This is a genuinely common fault, especially on older cars or those with high mileage, and it’s worth taking seriously rather than just cranking the heater up and hoping for the best.

Likely causes

A handful of faults tend to cause this exact pattern of overheating at low speed but not at higher speed.

• A failed or failing cooling fan motor that no longer spins, or spins too slowly
• A faulty fan relay or blown fuse that stops the fan getting power when it should switch on
• A broken or stuck coolant temperature sensor giving the fan control module wrong information
• Low coolant level, which reduces the system’s ability to shed heat even with a working fan
• A clogged or damaged radiator restricting airflow even when the fan is running properly

Why this matters

An engine that overheats in traffic isn’t a minor inconvenience. If the temperature keeps climbing and you don’t act, you risk warping the cylinder head, damaging the head gasket, or causing more serious internal engine damage that costs far more to fix than a fan. It’s also a safety issue if the gauge climbs into the red while you’re stuck in a queue with nowhere to pull over safely.

Watch for the temperature needle rising specifically at idle or low speed, a warning light on the dash, or the smell of hot coolant. If you notice any of these, get the car looked at before it becomes a breakdown.

What a mechanic checks

A proper diagnosis starts with confirming the fan actually runs when it should, both on its own and when triggered by the temperature sensor. A mechanic will check the fan motor, the wiring and relay, the coolant temperature sensor, and the coolant level and condition. They’ll also inspect the radiator for debris or damage blocking airflow, since a fan working perfectly can still leave you overheating if the radiator itself is choked.

What it typically costs to fix

A failing cooling fan or fan relay is one of the most common causes of overheating specifically in slow traffic, since airflow through the radiator relies on the fan at low speed. Replacing a fan motor or relay typically costs £100-250. Low coolant from a small leak is often cheaper to fix, £80-200 depending on where the leak is. A radiator that’s partially blocked internally is a bigger job at £200-400.

What happens if you leave it

Traffic is exactly the situation where a car that overheats has nowhere to go for relief, since there’s no airflow and no easy way to pull over safely on some roads. Repeated overheating cycles cause more damage each time, gradually stressing the head gasket and cylinder head even if the car cools back down afterwards, so this is one to get diagnosed properly rather than just topping up coolant and hoping.

What you can check yourself first

Listen for the cooling fan kicking in when the car has been idling for a few minutes, since you should be able to hear it running, especially with the bonnet up. Check the coolant level when the engine is cold, and if the temperature gauge starts climbing in traffic, turning the heater on full can help draw some heat away from the engine as a short-term measure while you find somewhere safe to stop.

It’s worth noting whether the temperature comes back down once you’re moving again at a normal speed, since that pattern strongly points toward a fan or airflow issue rather than a more serious internal fault. If the temperature keeps climbing even once you’re moving, that’s a different and more urgent pattern worth mentioning when you book the car in.

How electric cooling fans are controlled

Most modern cars use an electric cooling fan controlled either directly by a coolant temperature sensor or, more commonly on newer vehicles, by the engine control unit reading temperature data and switching the fan on through a relay once a set threshold is reached. This is different to older mechanical fans, which were physically driven by a belt and spun constantly whenever the engine ran, providing airflow regardless of actual need. The electric system is more efficient but introduces more potential failure points: the sensor itself, the wiring, the relay, and the fan motor all need to work correctly in sequence for the fan to do its job when it’s genuinely needed.

Why a stuck-on fan is a different problem to a fan that won’t switch on

It’s worth understanding that a fan can fail in two opposite ways, and only one of them causes overheating in traffic. A fan that’s stuck permanently on, running even when the engine is cold, is usually a relay or sensor fault, and while it’s inefficient and can drain the battery if the car sits for a long time with the fan running, it doesn’t cause overheating. A fan that fails to switch on when it should is the fault that actually causes the specific overheating-in-traffic pattern covered here, since the engine loses its main source of airflow at exactly the low-speed conditions where it needs that airflow most.

Why radiator condition matters even with a working fan

A fan pulling air through a radiator that’s internally clogged with scale, corrosion, or old degraded coolant still won’t achieve proper cooling, since the airflow has nothing effective to cool if coolant can’t flow freely through the radiator’s core in the first place. This is why a mechanic checking a fan-related overheating complaint will also assess radiator condition and coolant quality, rather than assuming a working fan automatically rules out the radiator as a contributing factor. Old coolant that’s overdue for a change loses some of its heat transfer properties over time, quite aside from any corrosion protection it’s meant to provide.

West Yorkshire traffic patterns and this specific fault

Bradford’s city centre, along with busier routes through Leeds Road and around the ring road during peak hours, regularly puts cars into exactly the low-speed, stop-start conditions where a fan fault shows itself most clearly. Drivers whose daily commute involves a lot of this kind of traffic are more likely to notice a developing fan issue sooner than someone who mostly drives on faster, more open roads, simply because they’re spending more time in the conditions that expose the fault. If your commute regularly involves crawling traffic and you’ve noticed the temperature gauge behaving differently than it used to, that’s worth mentioning specifically when booking a check.

What happens during fan replacement

Replacing a cooling fan assembly typically involves accessing it from the front of the engine bay, disconnecting the electrical connector, and unbolting the fan shroud assembly before fitting the replacement and reconnecting everything. On most cars this is a reasonably accessible job that doesn’t require removing major surrounding components, though the exact process varies by model. Once fitted, a mechanic will typically test the fan’s operation both manually and by letting the engine reach its normal operating temperature to confirm it switches on correctly at the right threshold.

What you can check yourself before booking

With the engine idling and up to temperature, listen carefully for the fan actually running, you should be able to hear a distinct whirring sound, particularly noticeable with the bonnet up. Check your coolant level when the engine is cold, and note whether the temperature specifically climbs in traffic and comes back down once you’re moving, which is the classic pattern pointing toward a fan issue rather than a more general cooling fault.

Prevention and long-term cooling system care

Having your cooling fan tested as part of routine servicing, rather than only discovering a fault once it’s causing overheating, is a simple way to catch this before it becomes a genuine problem. Keeping the radiator’s exterior clear of debris, leaves, insects and road grime that can build up over a season, also helps maintain proper airflow even when the fan is working correctly, since a fan pulling air through a partially blocked radiator surface still won’t cool as effectively as one with a clear path.

Why fan faults are sometimes intermittent at first

A cooling fan motor or relay does not always fail completely and suddenly, it can begin by working inconsistently, running fine some days and failing to engage on others, particularly in relation to temperature or humidity affecting a marginal electrical connection. This intermittent pattern can make the fault frustrating to diagnose if it happens not to occur during a single test, which is why describing the pattern you have noticed, including whether it seems worse in certain weather or after the car has been running a while, genuinely helps a mechanic recreate the fault reliably during their check.

Battery drain from a stuck-on fan

While this article focuses on a fan that fails to switch on, it is worth being aware that the opposite fault, a fan stuck permanently running, can drain your battery flat if the car sits unused for a day or two, since the fan continues drawing power even with the engine off in some fault conditions. If you have noticed your battery unexpectedly flat after the car has been parked for a short period, alongside any cooling concerns, that combination is worth mentioning together rather than assuming they are unrelated issues.

Keeping a simple record of when the temperature gauge behaves unusually, including the approximate speed, traffic conditions and weather at the time, helps build a pattern that makes intermittent fan faults considerably easier to diagnose accurately.

Why fan faults are often cheaper to fix than owners expect

Despite the seriousness of what an ignored fan fault can lead to, the fan assembly itself, the relay, and the temperature sensor are all relatively affordable components compared with the engine damage that results from prolonged overheating. Owners sometimes delay booking a check out of concern the repair will be expensive, when in reality this is one of the more cost effective faults to address early, especially compared with the alternative of a warped cylinder head from repeated overheating cycles.

A well maintained cooling system, checked periodically rather than only when a symptom appears, is one of the more reliable ways to avoid an unexpected overheating incident catching you out at an inconvenient time or place.

It is also worth noting that a fan fault occurring right as warmer weather arrives is a common enough pattern that it should not be dismissed as coincidence, since summer heat genuinely does expose marginal fan components sooner than milder months would.

Getting this checked before summer arrives, rather than after the first hot spell exposes it, is a sensible piece of proactive maintenance.

SSK Mechanics comes to you across Bradford and West Yorkshire to run this check on your driveway or at work, so you’re not left waiting at a garage while they work through the possibilities.

SSK Mechanics comes to you across Bradford and West Yorkshire to run this check on your driveway or at work, so you’re not left waiting at a garage while they work through the possibilities.

When to book vs when it can wait

If overheating only happens occasionally and briefly at very low speed, it’s still worth booking a check soon rather than ignoring it, because fan faults rarely fix themselves. If the gauge is regularly climbing towards the red, or a warning light has appeared, treat it as urgent and avoid long periods stuck in traffic until it’s sorted.

The RAC’s drive advice on overheating recommends pulling over and switching off the engine if the temperature gauge goes into the red, rather than pushing on and hoping it settles. Once a fan fault is confirmed, our cooling system service covers a full Cooling Fan Assembly replacement, fitted at your home or workplace.

Source: RAC drive advice on overheating