Most Likely Culprits:
* Front Heater Core: This is the most probable cause. The heater core is a small radiator located within the dashboard that warms the air blown by the blower motor. If it's clogged, leaking, or bypassed, no heat will reach the front vents.
* Front Heater Core Control Valve (if equipped): Some vehicles have a valve that controls the flow of coolant to the heater core. If this valve is malfunctioning or stuck closed, no coolant will reach the heater core.
* Low Coolant Level: Check your coolant level in the overflow reservoir. Low coolant means the system isn't circulating enough fluid to heat properly. This will affect both front and rear, unless you have separate coolant circuits (less likely in this vehicle). A leak somewhere in the cooling system (including a leaking heater core) would also explain this.
* Blending Door Issue: The blend door controls the mix of hot and cold air. If it's stuck on cold or malfunctioning, you'll get cold air regardless of the coolant temperature. This would affect only the front if the blend door is specific to the front system.
* Blower Motor Issues (Less Likely): A weak blower motor *could* result in weak warm air, but it wouldn't stop warm air entirely if the heater core is working.
Troubleshooting Steps:
1. Check Coolant Level: Open the hood and locate the coolant overflow reservoir. If it's low, add coolant (use the correct 50/50 mix of coolant and distilled water). Do not open the radiator cap when the engine is hot! If the level is consistently low, you likely have a leak somewhere in the system.
2. Feel the Heater Core Hoses: With the engine running and the heater on full blast, feel the two hoses that connect to the heater core. They're usually located behind the dashboard, accessible near the firewall on the passenger side. One should be hot (the supply hose) and the other slightly cooler (the return hose). If *both* are cold, the problem is likely with the heater core, the control valve, or the flow of coolant.
3. Check for Leaks: Inspect the heater hoses for cracks or leaks, particularly where they connect.
4. Inspect the Heater Core (Difficult): Accessing the heater core in a 1998 Navigator is a difficult task usually involving partial dashboard disassembly. This should be a last resort. You might be able to detect leaks by looking for stains or dampness near the heater core area, but it is challenging.
5. Test the Blend Door (If possible): This is difficult to test without diagnostic tools or service manuals.
If you're not comfortable working on your vehicle yourself:
* Take your Navigator to a qualified mechanic. This is especially advisable if you suspect a heater core leak or if you're not familiar with auto repair. Replacing a heater core is a major job.
The rear heat working suggests the overall heating system is functional. The problem is likely isolated to the front heater core and its associated components. Prioritize checking the coolant level and the hoses first to rule out simple fixes before tackling the more complex issues.
