Kalos

Hi everyone, this will be my first ever post. I've spent literally days on this website reading other code 25 adventures and I've learned a ton, but my situation is a little different. I've done a lot of troubleshooting, so hopefully someone out there can help me put the puzzle pieces together.

I have a '93 pickup 3vze with 225,000 miles. It consistently gives a code 25 CEL but runs rich and fails emissions, especially the idle test. The main difference from my situation and most other posts I read is that I have no performance problems, just bad MPG (14 avg city/hwy). It idles very smoothly at ~850 rpm, no misses during acceleration and has decent power.

Here's what I've done:

New Denso O2 sensor, new plugs/cap/rotor, confirmed consistent compression in all cylinders, smoke-tested vacuum lines and exhaust w/ no leaks, new EGR modulator valve, confirmed EGR valve by applying vacuum to EGR inlet, confirmed timing @ 10*, confirmed cam pulleys and crankshaft correctly positioned on the belt itself, new engine coolant temp sensor, confirmed and reconfirmed all my vacuum lines are accurate.

Lately, I've been reading and troubleshooting possible electrical causes, so here are my results:

O2 sensor heater circuit has 13v
Black wire has continuity with Ox1 and Brown has continuity with E1
When sensor is plugged in, voltage reading at the pigtail stays consistently lean at around 0.14v and no oscillation
Bench tested with propane torch flame test, O2 sensor will react and oscillate
Vf1 reading (te1/e1 NOT jumped) is 0.04 at any rpm
Vf1 reading (te1/e1 jumped) is 4.94 at idle, but jumps to 0.04 with even the slightest application of throttle
I get zero signal when I test voltage at Ox1 (which surprises me because I have good continuity from black O2 pigtail wire to Ox1)

I even tested the O2 sensor with it unplugged from the harness (but still receiving 12v to the heater) and it read 0.95 consistently...more confirmation that my new denso sensor works and I don't have fresh air entering my exhaust.

So, after all this I am stumped. Perhaps ECU is the issue, although I've read so many statements that ECUs are very unlikely culprits. Any help interpreting this info would be SO helpful!

854x4

Sounds like you have done some work and tested the circuit, but before you replace the ECU do these checks. You should have 4 wires on the harness
Black - O2 signal
Brown - O2 ground also E1
White with Red stripe - O2 Heater power
Pink with Green stripe - O2 heater ground

I like to test these circuits with a meter and test light right at the O2 connector, check the heater circuit first -meter hooked to both heater wires power and ground, key on or start vehicle. If you have 12v then remove the meter and hook up the test light to the same wires and if it lights your done, the heater circuit is working. Now onto the signal ground, hook a meter from battery positive to the brown wire if you have 12v then hook your test light up the same way and if it lights then you know the ground is good.
OK now to test the O2 sensor signal, this as you know can be tested at the diagnostic box, hook you meter up to battery ground or the brown wire (E1) and to the O2 signal now start the vehicle, on a heated O2 you should have movement within a few minutes if not seconds. If there is no voltage movement or you are stuck lean then spray some propane or carb cleaner into the intake this should cause the O2 sensor to jump rich if it doesn't then you have a problem with the wiring, faulty sensor. If the voltage does go up then you have a actual lean condition AFM, vacuum leak clogged injector, low fuel pressure, etc. or a exhaust leak before the sensor. If you don't get a voltage reading at the diagnostic box then try the sensor itself, if you get a reading there then you have a broken wire.

scope103

I believe code 25 is a LEAN failure http://web.archive.org/web/201211190...85diagnosi.pdf . If I understand this correctly, the ECU THINKS it's running lean (0.14v from sensor) but the tailpipe test says it's rich. You haven't said this specifically, but I assume you cannot get the sensor voltage (either directly at the sensor, or the conditioned sensor voltage at VF1) to oscillate 8 times in 10 seconds. http://web.archive.org/web/201003261...01heatedox.pdf

What concerns me is the lack of signal at Ox1. If, perhaps, Ox1 had a short to ground (you would still find continuity, but everything would end up at ground), the ECU would see only the low (0v) lean signal and try to compensate (and throw the code).

Alas, your VF1 signal should sit at low voltage then, so I'm grasping.

You've done a lot; I'd try to figure out why you have no signal on Ox1.

Kalos

Figured it out this morning!! Scope, you were right on -- Ox1 had perfect continuity with E1. Seems so obvious in hindsight, however I was stuck thinking my low voltage reading at the Diagnostics box was due to corroded wires weakening the signal. And I had gotten that 0.14v reading at the pigtail, which doesn't make sense with a short, but perhaps it was intermittent or just a fluke. Turns out, Ox1 was shorted to ground. THANK YOU BOTH for going out of your way to help me troubleshoot this!!

I couldn't see where the short was occurring so I eventually just ran a clean wire from Ox1 pin at the ECU to my sensor wire on the O2 sensor. Now it oscillates like it's supposed to. Thanks again!!

scope103

Woo Hoo!

854x4

Glad you got it figured out.

siktruk@gmail.com

Hi, so my 92 runner is throwing this code, running lean? I have checked for vacume leaks and found that my muffler has a hole in it also its comng apart at the factory seam, could this make it run lean an trip the code 25?

Thank you.

854x4

Only if the exhaust leak is before the front oxygen sensor.