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Why is my car running so cold?
and
Why is my heater putting out so little heat?


You've got one or more of the following symptoms:
  1. The temperature gauge takes forever to get to the half-way mark
  2. The gauge barely moves off the "Cold" mark
  3. My heater's output is terrible
  4. The transmission's torque converter won't lock up
  5. The transmission won't shift into top gear
  6. I just failed emissions on high HCs
  7. My gas mileage is lousy
Numbers 1 through 3 are symptomatic of a thermostat that has failed open.

Number 3 can also be a defective or seized heater control.

Numbers 4 through 7 may be related to the thermostat.

As stated here, the thermostat's job is to keep the engine running at the correct temperature, which is about 90C (194F) (depending on the model). Coolant temperature is an important sensor input to the ECU.

If the engine's temperature is reported as being cooler than  what the ECU is expecting as full-operating temperature, the ECU will direct a richer mixture, which will lower mileage, increase emissions, and put fuel into the oil, which will in turn greatly accelerate cylinder bore wear. As well, the ECU will often prevent an automatic transmission from upshifting into overdrive or lock up the torque converter until it sees a "full-warm" signal from the coolant temperature sensor.

An OEM thermostat is less than $30, is easy to change and can solve a multitude of small problems. It's worth doing in any case if the one in your car is more than five years old.


Poor heater output
Poor heater output can also be due to a seized heater control valve, or a bent or disconnected cable. The heater control valve is normally next to the firewall, where one of the two rubber heater hoses pass through the firewall. The hoses are about an inch in diameter, usually enter the firewall at the same spot, and can be very hard to see what with all the spaghetti that festoons modern engine bays.
Here's where mine is:
General heater control valve location I'm standing by the right front tire, facing towards a spot just behind the left front tire.

It's an absolute mess under there, the years having made things worse by adding rust and dirt, so everything looks the same.

The heater control valve looks like a square box with a little lever sticking out from it. It's under the fuel filter in this picture.
Closeup, way down in engine bay
 Here I'm holding the camera deep inside the engine bay, alongside the right shock tower, pointing pretty much towards the middle of the firewall.

See the rusty control lever? The yellow arrow is just touching the end of it. It's hard to see, but there's a small post attached to the end of the lever, and the cable end slips on that. It rotates the lever as you move the dash control.

On older cars, this lever can rust in place. You'll notice because the dashboard control will act oddly.  Once you find the valve, have a helper move the dash control and watch to see if the lever is swinging as it should.  If it doesn't, some 3-in-1 oil dripped on the shaft, and a bit of manual help for the lever should free it up. Don't use WD-40; there's not enough lubricant in that.

Some newer cars (not necessarily Hondas) have no heater control valve at all, and reply on "blend doors" inside the pasenger compartment. These can suffer the same sort of seizures and cable-bendings external valves can.  Many newer cars have vacuum valves that control the climate functions instead of cables. Fixing these is beyond the scope of this article.