Corrosion and MOT Failures: A Guide

The annual MOT test is a crucial part of ensuring roadworthiness and safety for vehicles across the UK. While many drivers are familiar with common failure points like worn tyres or faulty lights, the complexities surrounding structural corrosion can often leave owners confused. This article delves into the specifics of how corrosion, particularly on cross members and integral body structures, can lead to MOT failures, drawing on practical examples and official guidance.

Understanding Corrosion-Related MOT Failures

The MOT test has stringent requirements for vehicle structure, aiming to prevent vehicles with compromised integrity from being on the road. Corrosion, if severe enough, can significantly weaken a vehicle's chassis and bodywork, making it unsafe. The key is to understand what level of corrosion constitutes a failure.

Cross Member Corrosion: A Common Concern

A frequent question arises regarding the condition of cross members, especially when they support critical components like engine mountings. The Driving and Vehicle Standards Agency (DVSA) provides guidance on this matter. For instance, on vehicles like the Nissan Primera and Micra, the engine mounting is attached to a cross member. If this member is heavily corroded, does it automatically fail under section 6.1 of the MOT testing manual, as it might be considered insecure?

The official stance is that engine mountings themselves are not a 'prescribed area' for rejection based on corrosion. Therefore, excessive corrosion in the vicinity of an engine mounting bracket isn't an automatic fail. However, the situation changes if the corrosion is so severe that the security of the engine mounting bracket is compromised, making it likely to detach. Similarly, if the cross member itself is so corroded that it's likely to detach from its fixings at either end, this would indeed be a Reason for Rejection.

Vehicle Structure and Rigidity

Beyond specific components like cross members, the overall structural integrity of the vehicle is paramount. Section 6.1.1 (c) (i) of the MOT manual addresses this directly. If the vehicle structure is corroded to the extent that the rigidity of the assembly is significantly reduced, it will fail. This is a broad category, and testers will assess whether the corrosion has compromised the fundamental strength and safety of the car's structure.

Similarly, section 6.2.2 (d) (i) deals with integral body structure or chassis corrosion. If there is excessive corrosion, seriously affecting its strength within 30cm of a body mounting, seatbelt mounting, or cross member, this will also result in a failure. This highlights the importance of inspecting areas around mounting points, as these can be susceptible to rust.

Case Study: Land Rover Defender MOT Issues

A real-world example illustrates these points effectively. A 2002 Land Rover Defender 110 TD5 Station Wagon failed its MOT due to several issues, including steering problems and significant corrosion.

Dangerous Defects

The most critical failure was:

• Steering rack has relative movement to chassis/bodywork persistently moving when jacked up (2.1.2 (a) (ii)): This is a dangerous defect, meaning the vehicle must not be driven until repaired. This indicates a serious issue with the steering system's connection to the chassis, potentially caused by worn mounting points or a weakened chassis structure itself.

Major Defects (Repair Immediately)

Several major defects were identified, requiring immediate attention:

• Offside Front Axle swivel pins and bushes excessively worn (5.1.2 (b) (i)): Worn swivel pins and bushes can affect steering accuracy and stability.
• Nearside Rear Wheel bearing has excessive play offside rear (5.1.3 (a) (i)): Excessive play in wheel bearings is a safety hazard, potentially leading to wheel detachment.
• Nearside Rear Shock absorbers has an excessively worn bush (5.3.2 (c)): Worn shock absorber bushes can affect ride quality and handling.
• Offside Front Vehicle structure corroded to the extent that the rigidity of the assembly is significantly reduced (6.1.1 (c) (i)): This aligns with the general structural integrity requirements mentioned earlier.
• Offside Rear Integral body structure or chassis has excessive corrosion, seriously affecting its strength within 30cm of a body mounting crossmember at body mounting corroded (6.2.2 (d) (i)): This is a clear indication of structural weakness due to rust, specifically near a mounting point.
• Offside Front Integral body structure or chassis has excessive corrosion, seriously affecting its strength within 30cm of a body mounting nearside front crossmember corroded (6.2.2 (d) (i)): Further evidence of significant chassis corrosion.
• Front Intermediate drop arm insecure (2.1.3 (a) (i)): An insecure drop arm poses a direct threat to steering control.

Advisories (Monitor and Repair if Necessary)

Additionally, some items were flagged as advisories:

• Nearside Rear Lower Shock absorbers has a slightly worn bush offside rear lower (5.3.2 (c)): Less severe wear on shock absorber bushes.
• Nearside Rear Integral body structure or chassis corroded within 30cm of a mounting, but not seriously affecting overall security seatbelt mounting at crossmember corroded (6.2.2 (d) (i)): Corrosion present but not yet deemed a structural failure. This is a prime example of an area that requires monitoring.
• Nearside Front offside front hub seals slight leak: A minor fluid leak.
• clutch slave cylinder leaking: Another minor fluid leak.

Addressing Specific Concerns from the Defender Case

The owner of the Defender had several questions:

Steering Rack Movement

The "Steering rack has relative movement to chassis/bodywork persistently moving when jacked up" defect is concerning. This often points to worn mounting bushes for the steering rack or issues with the rack itself. In some cases, it could also be a symptom of a chassis that has corroded around the mounting points, leading to looseness.

Steering Damper vs. Shock Absorber

The observation that the Defender has a Bilstein gas shock instead of an oil-filled steering damper is interesting. While a steering damper's primary role is to absorb steering vibrations (shimmy), a shock absorber's function is different. If the steering damper has been replaced with an inappropriate component, it could potentially affect steering feel and stability, especially on a vehicle like a Defender that is prone to steering play.

Chassis Replacement and Suppliers

The prospect of needing a new chassis is daunting. For Land Rover Defenders, chassis wear and corrosion are well-documented issues. Reputable suppliers like Richmond are indeed known for providing quality replacement chassis. When considering a chassis replacement, it's crucial to research the reputation and quality of the manufacturer and ensure the work is carried out by experienced professionals.

Swivel Pins and Clutch Slave Cylinders

While the owner correctly identifies that replacing swivel pins and clutch slave cylinders might not seem overly complex, it's essential to approach these tasks with care. Swivel pin wear can affect steering geometry, and clutch slave cylinders are critical hydraulic components. Proper diagnosis and execution are key to a successful repair.

What to Look Out For

Given the MOT report, other areas to scrutinize on the Defender would include:

• Suspension mounts: Check all mounting points for corrosion or stress fractures.
• Brake lines and fuel lines: These are often vulnerable to corrosion and can lead to serious failures.
• Bulkhead: Another common area for rust on older Land Rovers.
• Body mounts: Ensure the body is securely attached to the chassis without excessive corrosion at the mounting points.

Understanding MOT Terminology

The question about what a "Nearside Rear Headlamp" means highlights common confusion with MOT terminology. In the UK, 'nearside' refers to the side of the vehicle nearest to the kerb (typically the left side), and 'offside' refers to the side furthest from the kerb (typically the right side). So, 'nearside rear' would be the rear light on the left side of the vehicle.

Preventative Measures and Regular Checks

To avoid costly MOT failures related to corrosion, regular vehicle maintenance is key. This includes:

• Cleaning the underbody: Especially after driving in wet or salty conditions, cleaning the underside of the vehicle can remove corrosive elements.
• Waxoyl or underseal treatment: Applying protective coatings to the chassis and vulnerable areas can significantly slow down the progression of rust.
• Regular inspections: Even if not a professional, periodically looking under the car for signs of rust, especially around suspension mounts, sills, and chassis rails, can help identify issues early.
• Addressing minor rust spots: Small patches of surface rust can be treated and painted over before they develop into more serious corrosion.

In conclusion, while corrosion can be a disheartening MOT failure, understanding the specific criteria used by testers and performing regular maintenance can help keep your vehicle safe and on the road. For significant structural corrosion, seeking professional advice and using quality replacement parts is essential.

Frequently Asked Questions

Q1: If my car has minor rust on a cross member, will it fail its MOT?

A: Not necessarily. Minor surface rust or corrosion that does not affect the structural integrity or the security of attached components is usually advisories. However, if the corrosion is deep, compromises the strength, or makes a component insecure, it will likely fail.

Q2: What is considered 'significant' corrosion in the context of an MOT?

A: 'Significant' corrosion is generally defined as rust that has penetrated the metal, significantly weakened the structure, or is likely to lead to the detachment of a component. Testers look for excessive corrosion that reduces the strength or rigidity of the vehicle's structure.

Q3: Can I repair corroded areas myself?

A: For minor surface rust, DIY repairs might be feasible. However, for structural corrosion, especially on chassis members or load-bearing components, professional welding and repair are often required to ensure safety and compliance with MOT standards.

Q4: What's the difference between a 'major defect' and a 'dangerous defect'?

A: A 'dangerous defect' means the vehicle must not be driven until repaired due to immediate safety risks. A 'major defect' requires immediate repair but might not pose the same level of imminent danger as a dangerous one, though it still prevents the vehicle from passing the MOT.

Q5: Are all areas of a vehicle inspected for corrosion during an MOT?

A: Yes, the MOT test includes a thorough inspection of the vehicle's structure, including the chassis, bodywork, suspension mounts, and other critical areas for signs of significant corrosion.