# Check engine light comes on.



## Team_Yamaha (Nov 30, 2002)

On our shop truck a 94 F-150 4X4 300-6. One of the guys came back and said the chech engine light popped on. So I started driving it and noticed that the check engine light started coming on when the engine is warm (temp gauge on R on normal, where it always has been) and I let off the gas or it is idling it pops on, step on the gas and about 10 seconds later it goes out, let off and it comes back on. It will only do this when the engine is warm. Cold motor, no light, when idling or on the gas. I pulled the codes and here is what I got:

113- Intake Air Temperature (IAT) circiut open/below -40 deg. F
173- Right HEGO indicates rich
179- Adaptive Fuel lean limit reached at part throttle, right side rich.

The truck only has one O2 sensor, at the Y pipe where cylinders 1-3 and 4-6 meet. Any ideas??


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## wyldman (Jan 18, 2001)

Sounds like either a failed or disconnected IAT sensor.Find the sensor and short the two connectors inside the plug,and do another KOEO test,or key on engine off test.It should not give you the opposite code for the IAT,don't have it handy though.If it does,it's a bad IAT,if not,then you have a wiring or ECM problem.

Your EGO codes,and adaptive strategy codes are probably being cause due to the IAT problems.Open or low IAT will cause the system to go hard rich,setting the rich EGO codes.


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## bdsean (Dec 5, 2002)

take alook at the egr valve it might be the problem


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## Team_Yamaha (Nov 30, 2002)

Next question, where is the IAT sensor?? I have never even heard of it. Thanks guys!!


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## wyldman (Jan 18, 2001)

Hmmmm......been a while since I poked around a 4.9L,but it should be mounted (screwed) into the intake manifold somewhere.It has a 1 1/8" hex brass body with a grey or white top.Connector will be black with two contacts.You should be able to find it.If not,I'll look it up mon when I get back into the shop.


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## Team_Yamaha (Nov 30, 2002)

wyldman, thanks for all your help!! I am not sure if I found the right sensor or not. The one that I found is on the #1 cylinder intake, just like you described (2 wires, sensor looks like it takes a 1" wrench or there abouts) could this be it? Coming from the same wiring harness (not the same wires, just the same wire block) there is a sensor going to the thermostat housing. If you could find out if I got the right one or if I must start looking again. Once again thankyou for your help!!!


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## wyldman (Jan 18, 2001)

Looks like you fuound it.It sould look identical to the one on the thermostat housing,except that one has a green body i thing.Take it out and take a look at it.It should have 3 little windows on the end with a small red or black bulb (a thermistor),in the middle.If it looks like that,you found it.

You can also test without pulling the sensor.If you disconnect it it will give you a hard code for the sensor being low,or open.If you short the two little pins in the end of the connector on the harness,it will throw the opposite hard code,high,or shorted.


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## smokey (Dec 17, 2002)

Codes 173 and 179 are o2 sensor codes. I have a '93 f-150 4.9L and have had the same problem. Change the o2 sensor! It is located in the exhaust pipe right next to the tranny. It costs about $30-40 but it is worth it. The o2 sensor is an item that should be changed on a preventative maintenance schedule that most people overlook when doing oil changes and other tune up items, i.e. plugs,wires and cap and rotor.It is an easy fix that the dealer charges $100 for. Worst case senenrio is it will increase your fuel enconomy. Matt


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## wyldman (Jan 18, 2001)

smokey,you may be right with the oxygen sensor,but he's better off to clear up the IAT first,as it is probably causing the o2 codes. 

The o2 sensor sometimes takes out the threads when they sieze,so what may seem like a simple o2 replacement,becomes a nightmare.I wouldn't touch it unless I had to.


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## Team_Yamaha (Nov 30, 2002)

OK, got the new sensor and replaced it. The old one was soo full of crap that I couldn't tell that colors anything was, and until I scrapped a little I couldn't even tell that there was a probe in the center of the cage. Now I just have to clear the codes. I just have to disconnect the battery for a little while and then reconnect it, right??


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## wyldman (Jan 18, 2001)

You can also clear codes with a code reader.I am assuming you have one as you did get codes.Do a KOEO test,and after the first code flashes (which should be an 11 if everything is OK),then turn off the key.Codes are now gone.

Doing it this way will keep all the adaptive strategy intact.If you want to reset it all,then pull the battery terminal (negative),and let it sit for 10-15 mins.

Does it run any better now and did it fix your problem ? Might want to check your oil for fuel fouling,as it is very common when the system runs hard rich like that.If it smells anything like fuel,change it.It's a lot cheaper than bearings.


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## Team_Yamaha (Nov 30, 2002)

Yep the truck runs great, or as good as a 300-6 with 190,000 miles can run. It time for the oil change anyway, so I will have one of the boys do that tomorrow. Actually I got the codes by useing a jumper wire and a test light on the OBD port, and counting the blinks of the light. Thanks a ton for your help!!!


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## wyldman (Jan 18, 2001)

*Great*

Good to hear it's fixed.


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