Exhaust smell inside the cabin: treat it as urgent

Most articles on this site can reasonably suggest monitoring a symptom. This one cannot.

Exhaust gas contains carbon monoxide, which has no smell of its own. What you smell is the other combustion products alongside it — which means the CO is arriving too.

Carbon monoxide causes headache, drowsiness and confusion before it causes collapse, and the confusion prevents you recognising the problem. In a car, that is a driving hazard as well as a health one.

Related: burning smell car diagnosis.

Immediately

1. Open the windows.
2. Switch the ventilation to fresh air, not recirculation.
3. Turn the fan up.
4. Do not drive further than necessary.

Then find the cause before using the car normally again.

Where it gets in

An exhaust leak

The most common cause. A leak anywhere along the system releases gas under the vehicle, and it finds its way in through any opening in the floor or the boot.

Where they occur:

• The manifold or its gasket. Often audible as a tick that is loudest cold: ticking noise at idle.
• Joints and clamps along the system.
• The flexible section, which cracks with age.
• The silencer, which rots from the inside.

Exhaust rattle and heat shields.

Body seals and grommets

Exhaust gas under the car enters through:

• Perished boot or tailgate seals.
• Missing or damaged grommets where cables pass through the floor.
• Rust holes in the floor pan or the boot.
• A poorly sealed repair.

Rust holes are the serious version, because they indicate structural corrosion as well: underbody rust inspection.

The tailgate aerodynamic effect

Specific to hatchbacks and estates, and worth knowing.

Driving with the tailgate open — or with a poorly sealing one — creates low pressure at the rear that actively draws exhaust gas into the cabin. This is a documented and genuinely dangerous effect.

Never drive with the tailgate propped open for a long load. If you must, open windows fully and set ventilation to fresh air at high fan speed.

Loading the car safely.

The ventilation intake

The intake sits at the base of the windscreen. In traffic it draws in exhaust from the vehicle ahead.

That is not a fault with your car. Switching briefly to recirculation in heavy traffic is the intended answer.

Blocked cabin ventilation outlets

Cars have one-way vents, usually in the rear quarter panels or behind the bumper, that let air out. Blocked ones prevent proper flow-through and allow gas to accumulate.

Cabin filter and air quality.

The specific danger cases

Idling in an enclosed space. A garage, even with the door partly open, accumulates CO quickly. Never run an engine in a closed garage.

Sleeping in a car with the engine running. For heat, in cold weather. This kills people every winter, particularly when snow blocks the exhaust outlet.

Snow around the exhaust. Clear it before running the engine while stationary.

Winter starting tips and emergency kit for the car.

A CO alarm

If you use your vehicle for sleeping — camping, long-distance work, a van conversion — a battery carbon monoxide alarm is inexpensive and appropriate.

Finding the leak

Listen. A manifold leak ticks, loudest on a cold start.

Look underneath, safely supported, for black sooty deposits around joints — that marks where gas escapes.

Check the boot seals and the spare wheel well, which is also where you find water: water in the spare wheel well.

Check for rust holes in the floor pan.

An exhaust leak also affects the oxygen sensors and can set codes: P2195 and P2197 oxygen sensor biased lean.

Sequence

1. Ventilate immediately and stop using recirculation.
2. Listen for an exhaust leak on a cold start.
3. Inspect the exhaust for soot marks at joints.
4. Check boot and tailgate seals.
5. Check the floor for corrosion.
6. Do not drive with the tailgate open.
7. Have it repaired before normal use.

Try it on the free tier of the app — pair an ELM327 adapter and check fuel trims and oxygen sensor data, which often reveal an exhaust leak. But do not wait for data before ventilating the car.