O2 sensor, '98 volvo S70

Baer1964 · Post by **Baer1964** » 01 Sep 2010, 13:29

I am working on a 1998 Volvo S70 with a 2.4L nonturbo. I replaced the front oxygen sensor(customers request). The car was getting a po130 code and black smoke out the tail pipe. After replacing the front sensor and turning off the check engine light, the car ran ok. Soon afterwards engine light came back on with po131 code. This time I checked the live data and front o2 sensor was running about .310 volts and the rear sensor was reading 0 volts. I replaced the rear sensor and it is still getting 0 volts. I swapped the wires, connecting the wiring harness for the front sensor to the rear sensor, and rear harness to the front sensor. Now front sensor reads about .610 volts and the rear still reads 0 volts. This tells me both sensors are working, but the ecm is not seeing a reading on the rear harness. I unplugged the ecm and ohmed the wiring and all wires are good. I hate to just say bad ecm. Can some one advise me on a way to check this any further and why am I not getting any voltage reading on the rear O2 sensor harness.

Thanks
Rondy

Post by **misha** » 01 Sep 2010, 15:49

Do you still have black smoke from the exhaust after replacing sensor?

If you do,you can try with disconnecting negative battery terminal for about 15-20 min. in order to reset fuel trim.

FCPEURO · Post by **FCPEURO** » 01 Sep 2010, 18:24

I think it would be best to leave the battery disconnected for about 20 minutes either way to be honest.

Baer1964 · Post by **Baer1964** » 02 Sep 2010, 06:26

Have already disconnected battery for a couple hours. Battery went dead and was replaced. Yes it does have black smoke when operating on front sensor. If you connect front harness to rear sensor the black smoke goes away.

Post by **misha** » 05 Sep 2010, 19:00

Baer1964 wrote:If you connect front harness to rear sensor the black smoke goes away.

Strange thing.

I guess that somebody you bought the car from messed something up with o2 connectors.Maybe he just connected front o2 sensor where the rear o2 should be by mistake.

Hmmm,is car working properly when you connect front o2 sensor to the connector of the rear one & the rear o2 sensor on front o2 connector?

If it does then leave it like that,maybe that is the proper connection....like i said before...could be that previous owner connected them wrongly.

Baer1964 · Post by **Baer1964** » 06 Sep 2010, 15:30

I have traced the wiring out. O2 sesors were connected correctly. If yoou connect the harness for the front O2 sensor to either sensor I get a readig on the scan tool. If you connect the wiring harness for the rear O2 sensor to eitherr sensor I get a 0 volt reading. I have traced the wires to the ECM and wiring is good, no short to ground or opens. Also no short to other wiring.
The car is getting black smoke out of tail pipe when front harness connected to front sensor. When front harness is connected to rear sensor, black smoke goes away, but I am concerned about car running to lean.The only thing I can think of to repair this problem is to replace the ECM

Post by **misha** » 06 Sep 2010, 17:44

I usually don't recommend Volvo stealerships(dealerships),but you could visit them if you want to solve your problem.

I think they are the only one who probably can solve your problem,but that could be expensive.

Visit them and see what they have to say about it and how much the repair would cost.

It could cost you a lots of money...or it just can be very cheap repair.

Good luck!

ziggy55 · Post by **ziggy55** » 07 Oct 2010, 14:43

Im having the exact same issue with a 1998 Volvo S70. I tried another ECU and the same issue still. Did you solve yours?

t5ftw · Post by **t5ftw** » 07 Oct 2010, 16:06

Baer1964 wrote:I am working on a 1998 Volvo S70 with a 2.4L nonturbo. I replaced the front oxygen sensor(customers request). The car was getting a po130 code and black smoke out the tail pipe. After replacing the front sensor and turning off the check engine light, the car ran ok. Soon afterwards engine light came back on with po131 code. This time I checked the live data and front o2 sensor was running about .310 volts and the rear sensor was reading 0 volts. I replaced the rear sensor and it is still getting 0 volts. I swapped the wires, connecting the wiring harness for the front sensor to the rear sensor, and rear harness to the front sensor. Now front sensor reads about .610 volts and the rear still reads 0 volts. This tells me both sensors are working, but the ecm is not seeing a reading on the rear harness. I unplugged the ecm and ohmed the wiring and all wires are good. I hate to just say bad ecm. Can some one advise me on a way to check this any further and why am I not getting any voltage reading on the rear O2 sensor harness.

Thanks
Rondy

ECU may just not report rear O2 sensor voltage when it detects a problem (e.g., front O2 sensor). So, it does not necessarily mean that rear sensor is bad. Your problem sound similar to my problem, but not identical. My guess is that you need new front sensor and adapter cables for both front and rear O2 sensors. Check my thread. I have a link to a TSB about the problem that I experienced.

Post by **xtrattitude4u** » 07 Oct 2010, 17:15

Ive been told by several people that the rear O2 sensor is only for monitoring that the catalytic converter is functioning properly, and doesn't affect fuel trim. The ECU may only check this sensor periodically.