1997 Ford Expedition 5.4L 4WD OBD2 Code P1299: No Heat and Heater Core Failure Diagnosis

What does OBD code P1299 specifically indicate? I'm trying to understand the exact nature of this fault before proceeding with repairs.

P1299 indicates a cylinder head overtemperature condition. Have you checked your coolant level and ensured it's at proper capacity? Low levels can contribute to overheating and poor heater performance.

A thermostat stuck in the closed position is a common cause of this issue, along with potential water pump failure. Given the vehicle’s age, consider flushing the cooling system to remove rust or scale buildup that may be restricting flow. A full flush and refill can often resolve these problems. Good luck—please update on your progress.

I’ve experienced repeated electrolysis issues in my Ford Expedition, which led me to replace the radiator five times—last one over a year ago. I’ve already flushed the system and replaced the thermostat. After driving for about 15 minutes, the temperature gauge spikes to full then drops suddenly. Is this normal behavior when a thermostat is faulty? And could a bad thermostat be directly responsible for no heat from the heater core?

It sounds like a defective thermostat is likely at fault—this can cause both overheating and loss of heater heat. However, I’m puzzled: why wasn’t this issue addressed earlier when multiple radiators were replaced? You should check for voltage drops in ground connections and wiring before investing in another radiator. Stray current can accelerate corrosion and damage components.

If the thermostat fails to open at the correct temperature, coolant won’t flow through the engine or heater core—resulting in no heat. The temperature gauge rising sharply and then dropping may indicate a stuck thermostat that eventually opens. This is consistent with a faulty unit. Additionally, stray current from electrolysis can cause radiator corrosion (especially aluminum or copper), damaging ground straps and wiring. Are all engine-to-body ground connections intact? Please verify this to rule out electrical faults.

I took it to a mechanic who diagnosed a faulty heater core plug—replaced that, then the thermostat. Still getting P1299. I now have a P0446 code, which may point to an EVAP canister valve issue, but I don’t believe these are directly related. After driving with the thermostat installed, the temperature gauge spiked rapidly, the oil light came on, and the engine ran roughly. Shutting it off and restarting resolved the roughness—but P1299 remains. This is getting frustrating! Any suggestions? Thanks for all the help—knowing people support each other makes a big difference.

Unless you're using an aftermarket thermostat (which can sometimes trigger false P1299 codes), consider whether there's a restriction in your cooling system. I recently encountered a similar case with a Ford Transit—no heat, temperature gauge spiked, fan running constantly, and bubbles in coolant. The issue was a broken plastic impeller in the water pump. With no coolant flow, the heater core received no hot fluid—only cooled air passed over it. The coolant temperature sensor registered high temps because the surrounding coolant heated up quickly with no circulation. This explains both overheating and lack of heat. Water pump failure is a common but overlooked cause.

How can I test if my water pump is functioning? I tried running the engine on an angled driveway, raising the radiator above the engine with the overflow cap off to release trapped air. Then I drove it for a while with the cap back on. Afterward, I felt the hose after the thermostat leading into the radiator—it was hot and full. If the water pump were failing, would this still happen? I’ve confirmed good system pressure—how can I confirm whether flow is actually occurring?

If all else fails, simply replace the water pump. This is a reliable and often effective fix for persistent cooling issues in older Ford Expeditions.