Bonnet Failures & Sticky Latches

The bonnet, or hood as it's known in some parts of the world, is a crucial component of any vehicle. It not only protects the engine and other vital components from the elements but also plays a significant role in vehicle safety. During the MOT test, the condition and functionality of the bonnet are thoroughly inspected. While it might seem like a minor detail, a faulty bonnet can lead to a failed MOT and, more importantly, pose a serious safety risk. This article delves into the reasons why a car might fail an MOT due to bonnet issues and explores common problems like sticky latches that prevent the bonnet from opening.

Bonnet Checks During an MOT Test

The Ministry of Transport (MOT) test is a mandatory annual inspection for vehicles in the UK to ensure they meet road safety and environmental standards. The bonnet is checked for several reasons:

• Security: The bonnet must be securely fastened and latched. It should not be possible to open it easily from the outside without operating the primary release mechanism.
• Secondary Latch: Most modern vehicles are equipped with a secondary or 'fail-safe' latch. This is designed to prevent the bonnet from accidentally flying open while the vehicle is in motion, even if the primary latch fails or is not properly secured. The presence and functionality of this secondary latch are critical.
• Condition: The bonnet itself must be in good condition, free from excessive corrosion or damage that could compromise its structural integrity or ability to stay closed.
• Visibility: While not a direct failure point for the bonnet itself, if the bonnet obstructs the driver's view, it could contribute to a dangerous driving situation.

When Should a Car Fail an MOT Due to the Bonnet?

A car will typically fail its MOT concerning the bonnet if:

• Primary Latch Malfunction: The primary release mechanism is broken, seized, or otherwise inoperable, preventing the bonnet from being opened by the tester.
• Missing or Faulty Secondary Latch: The secondary latch is missing, broken, corroded, or does not engage correctly. This is a significant safety defect. As one user noted, they "saw the aftermath of a failed one to a mate's Clio, I'm surprised more haven't." This highlights the critical safety aspect of the secondary latch.
• Bonnet Not Securing Properly: The bonnet cannot be closed and latched securely. This could be due to a faulty latch, damaged bonnet catches, or a misaligned bonnet.
• Excessive Corrosion or Damage: The bonnet has significant rust or damage that affects its structural integrity, making it unsafe.

It's important to note that testers will often 'pop' the bonnet for you, ready for them to inspect. As one comment suggests, the tester "leaves it for me to close properly when the MOT is finished." However, if the bonnet cannot be opened due to a fault, the test cannot be completed thoroughly, leading to a failure.

Why Isn't My Car's Bonnet Opening? Common Causes

If you've found yourself unable to open your car's bonnet, it can be a frustrating experience, especially if you need to perform routine checks or prepare for an MOT. The most common culprit behind a stubbornly shut bonnet is a sticking or seizing latch mechanism. Here's a breakdown of why this happens:

1. Latch Mechanism Seizing

The bonnet latch assembly consists of several moving parts, including cables, levers, and springs. Over time, these components can become contaminated with dirt, grit, road salt, and moisture. This ingress can lead to:

• Corrosion: Metal parts can rust, especially in areas exposed to road salt during winter. Rust can bind moving parts together.
• Dirt and Grime Buildup: Accumulations of dirt and grease can create a sticky, gummy residue that hinders smooth operation.
• Cable Issues: The cable that runs from the interior release lever to the bonnet latch can stretch, fray, or become corroded. If the cable becomes too slack, it might not have enough tension to operate the latch. If it seizes, it will prevent the latch from releasing.

When the bonnet is shut, the latch mechanism is under tension. If any part of this mechanism is stiff or seized, the force applied through the release cable might not be sufficient to overcome the resistance, making it difficult or impossible to open.

2. Misalignment

If the bonnet has been recently repaired or involved in a minor impact, it might be slightly misaligned. This misalignment can cause the bonnet to sit too tightly in the bonnet catch, putting excessive pressure on the latch mechanism and making it harder to release.

3. Broken Release Cable

While less common than seizing, the release cable itself can snap. If the cable has broken internally, you might find that pulling the interior release lever has no effect whatsoever.

4. Faulty Latch Component

Occasionally, a specific part within the latch assembly might fail due to wear and tear or a manufacturing defect. This could be a broken spring or a damaged lever within the mechanism.

Troubleshooting and Repairing a Stubborn Bonnet

Before assuming the worst, there are a few things you can try if your bonnet won't open:

1. Gentle Persuasion: Try applying gentle upward pressure on the bonnet near the latch release point while someone else operates the interior lever. Sometimes, this can help relieve pressure on the latch.
2. Lubrication: If you can access the latch mechanism (sometimes possible through the grille), spray a penetrating lubricant like WD-40 or a silicone-based spray directly onto the latch and cable connection points. Allow it to soak in for a while before trying the release lever again.
3. Check the Release Lever: Ensure the interior release lever is moving freely and feels connected. If it feels loose or disconnected, the cable might be the issue.
4. Inspect the Cable: If you can safely access the cable running from the lever to the latch, check for any obvious damage or kinks.

If these steps don't work, it's likely that a professional mechanic will need to diagnose and repair the issue. They have the tools and expertise to access and fix seized or broken bonnet latches and cables.

Preventative Maintenance for Bonnet Latches

To avoid the frustration and potential MOT failure associated with bonnet issues, regular preventative maintenance is key:

• Regular Lubrication: Periodically lubricate the bonnet latch mechanism and the secondary latch with a suitable lubricant. This helps prevent corrosion and seizing.
• Keep it Clean: Ensure the area around the bonnet catches is kept free from excessive dirt and debris.
• Check the Secondary Latch: Make it a habit to check that the secondary latch engages correctly each time you close the bonnet. You can usually do this by popping the bonnet again immediately after closing it.
• Inspect Cables: Occasionally check the bonnet release cable for any signs of wear or stretching.

Comparison: Bonnet Latch Issues vs. Secondary Latch Failure

It's important to distinguish between a failure of the primary latch (preventing opening) and a failure of the secondary latch (safety issue).

Frequently Asked Questions

Q1: Can a car fail its MOT if the bonnet won't open?

Yes, a car will fail its MOT if the bonnet cannot be opened for inspection by the tester. The tester needs to verify the condition of the latch mechanisms and the bonnet itself.

Q2: What is the secondary bonnet latch for?

The secondary bonnet latch is a safety feature designed to prevent the bonnet from accidentally opening while the vehicle is in motion, even if the primary latch is not fully secured.

Q3: My bonnet release cable snapped. What should I do?

You will need to have the release cable replaced. It's advisable to have this done by a qualified mechanic, as accessing and replacing the cable can be complex.

Q4: Is it expensive to fix a seized bonnet latch?

The cost can vary depending on the vehicle and the extent of the problem. Simple lubrication might resolve it, but a seized cable or latch mechanism may require replacement parts and labour, potentially costing anywhere from £100 to £300 or more.

Q5: How often should I lubricate my bonnet latch?

It's a good practice to lubricate the bonnet latch mechanism at least once or twice a year, particularly before the winter months when salt and moisture can cause corrosion.

In conclusion, the bonnet is more than just a cover for your engine; it's a vital safety component. Ensuring its proper function, including the critical secondary latch, is paramount for passing your MOT and, more importantly, for your safety on the road. Addressing issues like seized latches promptly will save you hassle and keep you legal.