Tahoe Diesel Heat Issue

Sounds like a defective water valve could definitely be the culprit, especially with those symptoms. The low heat output and stored error code point to it. Before condemning the valve itself, though, have you checked the wiring and plug connection to it? I remember having a similar issue on my old truck, turned out to be a corroded connector causing intermittent problems. Saved me a bunch of money!

Thanks for the quick reply! Where exactly is the water valve located, and what should I be looking for on the connector? Is there a specific wire I should check, or is it more of a general visual inspection for corrosion?

The water valve location can vary a bit, but it's usually somewhere along the coolant lines going to the heater core – often near the firewall on the passenger side. As for the connector, look for any signs of corrosion (greenish or white powdery residue) or loose pins. Check that the wires are securely attached to the connector as well. If you're not comfortable with electrical diagnostics, taking it to a workshop might be the safest bet. A faulty valve could cause more issues if left unchecked.

Thanks a lot for the detailed advice! I took it to the shop, and you were right, it was the water valve. They said the connector was a bit dodgy too. Total cost was 95€, so not too bad. Appreciate the help!