87 Accord failed smog, high CO at 25 mph

My 1987 Honda Accord LX-i sedan (fuel-injected) failed smog today, after previously passing it with no problem in 2018. Everything looked good except for CO at 25mph. Never had this problem before, the car has been running fine for the most part. Mechanic suggested the fuel injectors may need adjustment, or some related parts (which he did not specify) might need replacing. From my research, high CO means the engine is running rich.

https://i.imgur.com/hIroGxB.jpg

The fact that it only occurs at high speeds is puzzling to me, and the mechanic did not seem to know why either. I did some searching around and found that it could be: Coolant temperature sensor (I believe there are multiple on this car, I replaced a faulty one back in May 2018); O2 sensor; MAP sensor; leaky fuel injectors; clogged air filter (replaced only a few months ago); defective thermostat/thermostat stuck open. PGM-FI light is not on, so nothing obvious. Only other item of interest is that I noticed several months ago that the radiator cooling fan sometimes immediately comes on at full blast during a cold start (usually at night or early in the morning), but not always. Keeps the temp needle lower than usual when it does occur. Eventually fixes itself after 20-30 minutes of driving.

How many miles are on this car?

As far as l can tell, high CO is usually caused by general neglect. Pull your plugs and check their condition. If you haven't done it in awhile (or if you did it recently, but used crummy parts), perform a complete tune-up: plugs, wires, cap and rotor. Change the oil and filter. Change the air cleaner element. Always use top quality parts and fluids. Mobil 1, Wix, NGK, Belden and Bosch are all good names. Make sure your catalytic converter is working properly. Fill with a tank of premium gas and put in a bottle of injector cleaner. Drive out that tank, making sure to give the car a lot of spirited driving and a few good Italian tune-ups. Change the oil again and fill with premium again. Re-test, making sure you get the car good and hot before you go. Run it down the freeway for a few miles before you take it in and make sure they test it while it's hot.

I've written a general how-to on getting your car through a smog check. It's worth a read if you're having trouble.

Let us know how it goes.

I actually just changed the oil last week. I change it every 5,000 miles or once a year (every July for me). I tuned the car up with NGK parts when I bought it in May 2018, which was 10,000 miles ago, I suppose I’ll tune it up again now. Also the car has 160K miles on it and is driven mostly in the city.

Only thing I can add is to make sure the car is fully warmed up. It will run rich if the coolant temp is under 160 degrees.