Ford S-Max Catalytic Converter

The catalytic converter is a key part of the Ford S-Max's emissions system. It filters harmful pollutants like carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) from exhaust gases, converting them into less harmful substances before they exit the exhaust system. It is positioned in the exhaust line so exhaust gases pass through it before being released into the atmosphere. Gasoline engines use a three-way catalytic converter to treat CO, HC, and NOx, while diesel engines use two-way catalytic converters combined with additional NOx control technologies like EGR and SCR with AdBlue in Euro 6 models.

According to Ford and technical literature, the catalytic converter contains:

• A ceramic honeycomb monolith core with many small channels.
• A high surface area alumina washcoat layer.
• A coating of precious metals such as platinum combined with rhodium or palladium with rhodium, typically at a 5:1 ratio.
• A stainless steel (INOX) heat shield housing, often in two parts with an elastic layer to handle thermal expansion.

Its weight varies from about 1 to 10 kg depending on technology and size.

Exhaust gases from the engine flow through the honeycomb monolith, where toxic molecules contact precious metals and undergo oxidation-reduction reactions:

• Reduction of NOx into nitrogen (N₂) and oxygen, mainly via rhodium and platinum.
• Oxidation of unburned hydrocarbons (HC) into carbon dioxide (CO₂) and water (H₂O).
• Oxidation of carbon monoxide (CO) into carbon dioxide (CO₂).

The catalytic converter works best at approximately 400–450 °C. Below this temperature, reactions slow and clogging or fouling may occur.

Ford S-Max models, like other recent Ford vehicles, may have multiple catalytic converters positioned along the exhaust line (near the engine and further downstream). Diesel models often combine the catalytic converter with a Diesel Particulate Filter (DPF) inside the same housing (especially Euro 5/6), and Euro 6 versions also add SCR technology with AdBlue injection for NOx reduction. Ford notes the catalytic converter heats very rapidly to high temperatures after start-up and stays hot well after shutdown.

Lambda sensors are placed before and after the catalytic converter to:

• Measure oxygen levels in exhaust gases for accurate air-fuel mixture control.
• Monitor catalytic converter efficiency by comparing gas composition upstream and downstream.

If the catalytic converter malfunctions or the mixture becomes incorrect, the engine warning light activates.

Typical causes include:

• Excess fuel in the exhaust (due to ignition misfire, leaking injector, or prolonged starting), which can melt the ceramic monolith.
• Soot blockage, especially in diesel models, that restricts gas flow and reduces conversion efficiency.
• Using inappropriate sealants upstream that harden and damage the monolith.
• Mechanical shocks, corrosion, and repeated overheating.

Ford highlights risky behaviors that accelerate damage, such as repeated running with an empty tank, long starter cranking, running the engine with spark plug wires disconnected, push-starting or towing start, and turning off the engine under heavy load.

Common signs include:

• Opaque, often dark smoke from the exhaust.
• Metallic rattling or vibrations felt under the vehicle (caused by broken monolith fragments).
• Loss of power, trouble accelerating, and frequent engine stalling.
• Illumination of the engine warning light with fault codes related to catalytic efficiency or lambda sensors.

These symptoms require prompt inspection and repair to avoid further engine damage.

Ford's user manuals include important warnings:

• Do not park or run the vehicle on dry vegetation or flammable surfaces due to risk of fire from exhaust heat.
• Never work on the exhaust system until it is fully cooled to avoid burns.
• If you smell exhaust fumes inside the cabin, stop driving and have the system checked immediately due to risk of intoxication.
• The exhaust system and catalytic converter emit intense heat during and after operation; handle with care.

Following these precautions ensures your safety and prolongs system life.