* Synchronizer wear (most likely): The synchronizers in your gearbox help match the speed of the gears before they mesh. Wear and tear, especially on a higher-mileage car, can cause difficulty shifting, particularly between gears that are further apart in speed like fourth and third. This is often manifested as a crunching or grinding noise when attempting the shift.
* Clutch issues: A slipping clutch or one that isn't fully disengaging can make it hard to select gears smoothly. This would likely manifest as difficulty shifting into *any* gear, not just fourth to third, and might also involve slipping under acceleration.
* Gearbox linkage problems: Worn linkage components (cables or rods) can lead to imprecise gear selection. This could cause a slight misalignment preventing smooth engagement.
* Low gearbox oil: Insufficient or contaminated gearbox oil can cause increased friction and make shifting difficult. Check your oil level and condition. If it's low or dirty, it needs attention.
* Bent shift fork: A bent or damaged shift fork inside the gearbox can prevent a gear from engaging properly. This is a more serious internal problem.
* Worn gear teeth: Extreme wear on the gear teeth themselves can make shifting difficult. This is usually accompanied by noticeable noise and is a more advanced stage of gearbox wear.
What to do:
1. Check the gearbox oil: This is the easiest and cheapest thing to check first. Make sure the level is correct and the oil is clean. A proper gearbox oil change might resolve the issue if it's low or contaminated.
2. Clutch test: Observe if the clutch is slipping (RPMs rise without a corresponding increase in speed) or if you have difficulty getting the car into gear when starting off.
3. Inspect linkage: Check for any obvious damage or looseness in the gear shift linkage.
4. Professional inspection: If the problem persists after checking these points, take your car to a mechanic specializing in transmissions. They can diagnose the problem accurately and recommend the appropriate repair, which could range from a relatively inexpensive repair (e.g., oil change, linkage adjustment) to a more costly gearbox overhaul or replacement.
Known gearbox issues for the 2007 Ford Focus 1.6 diesel: While there aren't widespread, universally acknowledged gearbox issues specific to this model and engine combination that cause the *specific* fourth-to-third problem, general gearbox wear and tear, particularly the synchronizer issue mentioned above, are common with age and mileage. The mechanic will be able to determine if your issue is a common wear-related problem or something more unusual.
