It could be something simple like a crack in the overflow tank, an overflow tank hose not sealing properly, or a slight leak in the radiator, etc. no need to assume the worst at this point. troubleshoot the issue. Lots of threads on this.

 

Well- I'm not even a novice. Used to do almost all my work....but near 77 now- I just let the shop do it all.

But-there should be coolant in the reservoir. There might be a line across the tank denoting normal level. It is normal to lose a little over time. When the engine is hot the level should be a little higher than when cold.

These Hondas seem to run so well that many of us get lax on checking stuff- and we just forget about it till something goes "ding".

(typo corrected)

 

As the engine coolant heats up it expands, the radiator cap lifts allowing a small flow to the overflow tank, sucks it back when the engine cools. Add fluid to the tank to the low mark and see what happens.
P0420 is a generic code, could be a vacuum hose leak, exhaust leak, plugged cat, etc.

 

Since it’s empty, I’d pull it out myself and give it an inspection to see if it’s still intact. Under hood plastics don’t last forever, my washer bottle is losing its lid and a buddys washer bottle is cracking and breaking to bits, his V lived in Arizona a lot of its life.
anyway, pull the bottle and see if it’s intact or leaking, maybe take a moment to use a Sharpie marker on the near invisible low and full marks, and then put it back in the V and keep fluid in it,it needs some in it all the time

 

It appears the previous person who did the coolant change did not top off the reservoir tank; it actually had maybe a couple tablespoons or so of coolant, which I must have missed with my light. I put my endoscope camera down the reservoir and confirmed there was a teeeeny tiny bit left, but nowhere near the 0.6L at the max line.

Anyway, I did a flush and refill, only to be met with an overheating engine and the radiator fan, which normally comes on at 210F-ish, did not come on even at 230F ECT on my OBD tool.

I was panicking thinking I killed something somehow.
Well, it was AIR in the radiator tank.

I burped it out by squeezing the top radiator hose a couple dozen times, and left the car sitting for 30m, came back, saw the reservoir tank went down, so the heat soak eventually did open the thermostat and let the radiator fill up completely.

I used this for troubleshooting: https://www.crvownersclub.com/posts/127635/

Problem solved, and I learned something new about how the CRV coolant system works!

 

Re check the reservoir tomorrow.

 

It’s a bit frustrating the burping needed on these radiators, but glad you solved your puzzle and yes, check it again after a nice driving cycle

 

Hi.

Pardon me for the dumb question as I am a novice mechanic...

When I open the radiator cap on my dad's 05 CRV, I see green coolant, but is it normal for the coolant overflow tank to be completely empty? There's not even a teaspoon of coolant in it, checked it by shining a light on the tank from below!

He's driven it that way for who knows how long.
Somehow I have a bad feeling about this...

On my '16 F150 as well as 07 Volvo, there is a coolant reservoir tank, where the fluid is sitting in between the min and max levels. I assume it's gotta be the same for most cars.

Where could all that original coolant have gone? gulp... I hope it's not a head gasket leak. It currently has an engine that idles rough, and has a P0420 permanent code. Oil looks dirty, but I only changed it 2 months ago!

I'd be more concerned with the green coolant, that is definitely the wrong type and with the dissimilar metals in these Hondas will lead to catastrophic cooling system and engine damage over time

 

Most green is universal, but change if you like.

Prestone® All Vehicles Antifreeze + Coolant

Optimized for all makes, models, years & fluid colors. Protects engines from freezing, buildup & corrosion while improving temperature control.

prestone.com

 

honda coolant is blue