u/FinalDevice

2018 Niro HEV. Yesterday I was driving in a storm. Driving in town, moderately heavy rain, wet (but not flooded) roads. Driving a steady 35mph. Suddenly the car dings and says "Check Hybrid System - Turn Engine Off". The engine did indeed turn off, but the car ran on electric power. I went a couple blocks, pulled into a parking lot, turned the car off and on again, and it ran normally and got me to work and back.

Here's where things get weird. Last night I pulled out a code scanner. There are no stored codes. There are no pending codes. All emissions checks are healthy. All diagnostic scans are healthy, including the engine, transmission, BCM, and inverter. The hybrid battery reads healthy.

So ... wtf? The car seems fine. I'd expect any condition severe enough to shut down the engine and put the car in limp mode should store a DTC. I wondered if my scanner is missing a proprietary DTC, but:

1. It recognizes the BCM.

2. It's able to read proprietary diagnostic codes, because it was able to tell me that the right front TPMS sensor is bad (which I already knew, and is unrelated).

Has anyone had the car go into limp mode without any stored faults?

Is your Niro blowing coolant out the exhaust? That heat exchanger in the catalytic converter was a bad design idea. Exhaust gases are acidic and highly corrosive. I believe this is the reason the heat exchanger leaks -- it develops pinhole leaks from corrosion.

Bypassing the heat exchanger is a well documented fix that I won't cover here. It's not a bad idea and it's my fallback plan.

But first, try adding a bottle of head gasket sealant to your coolant reservoir. I used K-Seal and it has been holding strong so far. These sealants are designed to stay a liquid until they come in contact with exhaust gasses. The exhaust gas hardens the sealant into a ceramic. It's a dubious fix for a head gasket leak. Honestly, it's a dubious fix in general - but our heat exchangers happen to be the perfect application of the stuff. The leak is sitting in a stream of hot exhaust gas.

I've heard stories of these "magic liquids" ruining engines. I think they're harmless here, for two reasons. First, I use the version that hardens when it meets exhaust gas. Don't use the sealants that start out thick and goopy, because they run a higher risk of blocking a coolant channel. Second, I'm assuming that you don't actually have a blown head gasket. This means your coolant system is still self-contained and isolated, so there is no risk of hardened sealant entering a combustion chamber and saying hi to your valves.

Will this last indefinitely? No, probably not. But it should last a few years. It will last longer if you add a fresh bottle every time you flush your engine coolant. If it stops working, you can still replace or bypass the heat exchanger.