I havent' been able to find much on this error. It is the only one I get but I can't pass dumb CA smog!

I replaced the o2 sensor about a year ago and am not sure if 173 means it needs replacing again? I don't put on alot of miles, but it does kind of smell rich right now.

Can anyone tell me if they know what I should be looking for on this error? I can try an o2 sensor but wanted some opinions before I start throwing money at the computer!

This is a 1990 FullSize with the 5.8L. I've already replaced quite a bit including the TPS, Idle Air, MAPI sensor, gaskets, plugs, wires, rotor, etc.

 

Don't replace the computer! The failure rate is about 1 in 10,000 or more.

First question is why does your 1990 model give three-digit fault codes? Three-digit codes weren't part of the EEC-IV self-test until 1991.

If the codes generated are indeed three-digit for some reason, yes, replacement would be the first and most likely solution. Remember when replacing any O2/HEGO sensor to avoid touching it any more than necessary as the sensor uses both tip inside the exhaust pipe AND the body of the sensor on the outside of the pipe to compare O2 levels.

If, on the other hand, the ECM is indeed generating two-digit codes like all of the OTHER ECMs manufactured prior to 1991, Code 73 indicates a failure to perform the "goose" test during the KOER. Code 17 indicates that engine rpm's were below the necessary speed to conduct the KOER test (approx. 850 rpm).

A rich condition can be caused by an inoperative smog pump or broken Thermactor plumbing or check valves. These components are not monitored by the ECM and therefore cause things as vague as a rich condition.

 

Thanks - in my EEV manual, it says that some of the 90's use 3 digit codes. On the KOEO, it is 3 digit as well and passes them all. I haven't looked up the VIN, but maybe it was a late model 90?

Thanks for the pointers - I will start with a sensor to see if that clears things up and then look at the other items. I don't know if it's another clue, but the idle is not smooth and fluctuates by about 200 RPM when in drive at a stop light. Not sure if that is related, but I have had two mechanics look at idle and it still has problem where it does not purr but goes up and down by 200 RPM when at stoplight while in gear stopped.

 

It's quite likely your O2 is fine. Check fuel pressure and for the presence of fuel in the vacuum line attached to the fuel pressure regulator. Pressure should be about 40psi with vacuum removed from the FPR and engine idling and drop to about 35psi when vacuum is reconnected. If pressure is higher or there is fuel leaking from the regulator replace it.

 

I know the original post was sometime ago, but I thought I would make an addition with the faults I found on my vehicle in hopes that this helps some poor soul out there trying to figure out the problems with their vehicle.
The predicament continued to rise when time after time I would run the diagnostics on my F-150 and the fault codes would change or become new codes! I decided to investigate, and it turned out that 75% of the connectors in the engine bay were corroded or had a faulty connection.
I decided to disconnect all the plugs, found some decent electrical connection cleaner, a decent small sized brush and some dielectric grease. After opening the connectors I've cleaned both the front side and rear of the connectors with the cleaner and the brush. Then resealed with grease on the fitting to reduce the chance of moisture intrusion. Also found some handy liquid electrical tape, which I applied to wires that were bare and hanging out ready for the chance of grounding out, and the backside of the connectors. On the bare wires I first applied the liquid product, followed by regular tape (when the product was dry) and then re-wrapped all the wires with the proper sized loom (which is a plastic corrugate tube designed to protect the wires from the elements in the engine bay and out), and wrapped the loom with electrical tape..

This has greatly helped improve fuel economy as the ground connection to most of these connections were badly corroded and had I not checked I probably would have GIVEN UP and sent it to the shop only for them to charge for the replacement of the component...and fixing of the bad wire making me/them think it was the component.

Lessons learned, and passed on

 

i had a similar problem, the fuel pressure regulator was letting 90 psi into the fuel rails. And that caused a few of my injectors to leak.