Frustrated Chasing OBD Gremlin

[Verhoevc]

Hey guys. Could really use some help identifying the next step to try chasing this OBD code gremlin. So I bought an ‘06 V8V and a week before the final date for my emissions test it started throwing P0135. We clear it and after awhile it comes back. Reading that these cars love to throw codes when battery is low I check that and OMG it’s like 11.5v. I immediately put new battery and clear code and keep car on a CTEK... but it quickly returns. So I bite the bullet and order the new O2 sensor and install it (pre-cat passenger side NTK). We clear the code and drive around. The OBD reader says the code hasn’t thrown but it is pending. We drove around way more than it usually takes to throw the code and it just stays pending. Then I shut off the car, swap with my buddy who helped install, he starts the car and BAM! Code throws immediately.
We used his reader then to check voltage and find that despite the new battery, the voltage stays VERY low for the first 10-ish minutes after starting the car before eventually making its way up to 14v. I’m thinking this low-voltage period at start-up is what’s causing the code to artificially get thrown... out it’ll stay pending if it only gets noticed once? Hence why it full-on threw the code when we shut down and restarted while the code was pending?
Either way, that’s all the data and postulating I’ve got. No idea what to try next and I’ve somehow gotta fix this and re-test before May (with all this coronavirus junk going on and all).
Thanks in advance!

[Jon39]

Verhoevc said:

... and I’ve somehow gotta fix this and re-test before May.

Cannot tell where you live, but in the UK, I think the validity of MoT tests have been extended by 6 months.

[yellow_tang]

Hope this helps, looks like a circuit continuity issue on the O2 sensor heater circuit. Are the contacts clean on the connector?

[JonnyCJ]

I reckon OP is in SoCal

Alternator not charging the battery sufficiently, but just enough to not trigger the alternator light ?

[Verhoevc]

I’m in Maryland. When we changed out the O2 sensor the contacts seemed fine. Unless you’re referring to some other contacts?

Oh yeah! Just remembered that every now and then the red engine light will come in and say ‘electrical system service urgent’ and stay there for a few minutes before disappearing. This could point to alternator being the issue...?

[JonnyCJ]

Slipping alternator belt is my guess - someone will be along shortly to advise whether these can be tightened or will need replacing.

[Verhoevc]

Thanks guys! I’ll check that first thing after quarantine is over and I can actually get folks and parts and stuff over haha.

[JonnyCJ]

Looks like the electrical system needs servicing is a Volvo system comment https://www.volvoforums.org.uk/showthread.php?t=13... and is alternator specific/approporiate

There should be a Volvo/Denso equivalent alternator available if it is that.

[Verhoevc]

Jonny, that explains THAT red engine light pretty well. But what do you think of the theory that it’s actually electrical gremlins that is causing the emissions light?

[JonnyCJ]

No idea I'm afraid - do you have a multi meter ? Should be easy to check the output voltage when the engine is running. If it's less than 14 and a bit volts that could well be your problem.