Scott Wells

All-
New member here and was not able to answer this so far....

I am getting CEL codes 24 25 and 27 intermittently and am very confused.

24 Intake Air Temperature Sensor (THA) Signal.
25 Air/Fuel ratio lean. AFM? Fuel Filter? o2 sensor?
27 Sub-oxygen sensor signal.

I just replaced the one o2 sensor and cat to get the truck through emissions.
Since there is only one, does the "sub" mean a loose or crossed signal wire (and not a secondary sensor)?

I was unaware of the existence of an IAT sensor on this truck. Is it hidden?

Please assist. If there is a thread out there addressing any / all of these questions, please excuse the rookie-move.....

scope103

Welcome to YotaTech!

THA is in the VAF. How to test it? Look in the manual! http://www.ncttora.com/fsm/1990-1995...e/volumeai.pdf (Don't forget that code 24 means "open or short." So it is more likely a wiring issue than a sensor, so don't go replacing parts willy-nilly. You DO have a multimeter, don't you? http://www.harborfreight.com/7-funct...ter-98025.html)

27 is limited to California Trucks with the second downstream (sub) sensor. Could yours have been from Calif at one time? ECU replaced?

25 might come from the THA problem, but remember that you can look directly at the conditioned O2 Sensor output at the VF1 diagnostic connector. http://www.ncttora.com/fsm/1990-1995...e/heatedox.pdf The cheap multimeter I linked won't do it, because it won't react quickly enough to display "8 times in 10 seconds." But a digital multimeter with a "bar graph" will work fine.

Good luck!

Scott Wells

Another issue took hold first.
Drove the truck yesterday, no issues (other than the CEL) and parked. As I went into the house, I thought I smelled an electrical issue near the battery area as I walked by, but it wasn't strong enough for me to investigate immediately. An hour later, went to start the truck and it doesn't even try to crank. I have a 3-week-old battery and I see gauge activity when I twist the key, so I doubt it's a switch or battery. Checked the fuses inside and the ones I could access under the hood. No obvious stuff.
Are there any fusable links anywhere in that corner of the engine bay?

Scott Wells

Can't detect any signs of burnt wiring anywhere with either the nose or eyes.....
Bummed.

Herkdriver

Just wondering if you resolved the code 24. I too have that code...hoe did you check the wiring on the plug at the VAFM? I don't want to damage that rubber boot

jerry507

If you have a code 27, your ECU thinks you have a california truck. That ECU will require two O2 sensors on either side of the cat. Since you said "to get it through emissions" I assume you just got it?

THA is really easy to check. When you're looking at the AFM with it pointed up as it normally would be, you can measure resistance from the 3rd pin from the left to the furthest right pin. Measuring some odd number of kilo-ohms means it's good at room temp. Anything else means it's bad. If that's good, then you have a harness issue up to the ECU.

ldivinag

hmmmm.

i live and bought my 90 4runner new, in kalif.

i only have one o2 sensor and it is after the CC...

Gamefreakgc

There doesn't seem to be consistency with which second gen P/U's & 4Runner has two oxygen sensors and which ones have one. I live in Cali and the truck I bought had been in Cali, only one oxygen sensor though (has EGR though). Either these trucks get around a lot or not all models have two oxygen sensors despite being made for Cali.

I know that if your VAFM is bad, it won't start since it cannot read the air/fuel mixture and cuts out the fuel. It should still crank though... sound like you have a short circuit somewhere.

When you turn the key to the "ACC" position (not cranking the engine) does the CEL show on the dash? If it doesn't, your ECM is bad.

jerry507

Gamefreak, is your O2 sensor before or after the cat? That's interesting that some have one and some have two. Maybe they were experimenting and eventually decided on needing two.

It could be possible that his ECU is a california one, but there is a bug and even though it only requires the one O2 sensor it still throws the CEL when he has a problem with his IAT sensor. That IAT sensor is listed as one of the causes for all the codes he's getting so it seems like that is where he should start looking.

Scott Wells

I have just one O2 Sensor, before the cat and it is an all-original / unmolested Arizona truck. No ECU changes of any sort.

I have not yet checked on wiring (not sure where to start, but no obvious burns / tears / rips / scrapes anywhere and all ends look to be in damned good shape for a 21+ year old vehicle.

I found a replacement MAF unit but don't want to spend the money if it isn't the problem (not cheap).

-Scott

jerry507

Easy, check the IAF sensor. Between terminals 3 and 7 on the VAFM you should measure about 2-7 kilo-ohms depending on the ambient temperature. Looking down at the connector (on the VAFM side obviously), pin 1 is on the left and 7 is on the right. Time to invest in a multimeter if you don't already have one.