I own a 1995 Volvo 850 GLT Sedan 2.4 N/A. About a year ago I traveled about three hours away, the first time I actually made a "road trip" in my car. Ran great but was getting poor gas mileage. Later on the trip the exhaust broke off the end of the cat. I drove it home after patching it the best I could. Ran fine with same gas mileage. Eventually a code for "Slow response O2 Sensor Bank 1 Sensor 1" and "No signal O2 Sensor Bank 1 Sensor 2" came on. After investigating I found that the wiring of the second sensor had broken off when the exhaust broke. I replaced both O2 sensors with Bosch Direct Fit sensors but had an exhaust leak at the flex pipe. Both codes went off but now ever since I have had P0131 & P0137 "Low voltage O2 Sensor Bank 1 Sensor 1 & 2" Finally i got the exhaust leak fixed and have cleared the codes several times as well as disconnecting the battery for a whole day at one point. But as soon as the key is turned to II before starting the CEL comes on instantly and when it is started the light stays on with the same two codes. There is no delay like you would think, letting the computer detect the low voltage condition before throwing the code. It comes on with the car no matter what I do. I've tried every mechanic I know and the only specialist in my area and no-one has been able to offer a solution. Can anyone help? The car is perfect other then that. There are no other codes present and never have been. just the O2 codes from the exhaust breaking off and now the low voltage codes. There is suppose to be two different O2 sensors for the front based on whether or not you have an air pump or not. The one that I put in was the one for cars without the air pump because I could not find one. It is suppose to be under the battery shelf and air box. I removed both today only to see the transmission underneath. As well as a small vacuum pump mounted to the underside of the battery shelf. But, it looks nothing like the air pump I looked up to see what they actually looked like. The car is completely stock, no wiring modified or added to. The only other thing wrong with the car is the rear main leaks a little because the PCV system might be clogged slightly or was at one time. That's the only other thing I can think of that would affect anything. Please help guys. Thank you.

 

Do you have OBDII? I have a 95 daily driver, but I have the earlier computer. If you have OBDII, you could read sensor millivolts at temperature and you' know if the computer is seeing any signal normally. that would be interesting to know.


Since the light comes on immediately, when the sensors are cold, that is odd. They aren't supposed to do much when cold.

 

you may have a connector issue - try measuring voltage by unplugging the O2 sensor voltage. there are four wires - two are "returns" (ground) one goes to the sensor's heating element (not sure if its 5V or 12V), the other is for the sensing circuit (just a guess but most likely its a lower voltage (ie .1 to .9V)

My other thought is to not assume its tied to the O2 sensor issue and start looking for vacuum leaks. It may be coincidental or you may have knocked a line when working on the car.

 

Sorry I haven't replied lately. Just replaced the radiator and tranny cooling lines on the 850. I had a K&N oiled filter in as an upgrade. Turns out the oil stuck to the mass air flow sensor and caused a lot of dirt to build up on the element. I cleaned the sensor and put a regular paper filter back in. The light stayed off for two days and came on last night while I was driving. Same 2 codes (P0131 P0137) But there are no other codes anywhere in the system, just those two. Today I used a smoke machine and ran every vacuum line I could find. I thought maybe the PCV system was clogged but smoke came pouring out of the connection on the side of the air intake elbow. I found no leaks at all. I looked up specs on how to diagnose the MAF. Said to check voltage between wire 3 and ground for .1 to .2 volts. I read 0 volts with ignition off which says indicates open ground between the MAF and main relay. I have no idea how to begin opening that harness to find a short. But, it still gives a reading in the datastream on a OBDII scanner of 4.2-4.5 g/s at idle??? The front O2 sensor starts out around .4 volts then drops to .02 volts after idling for a while. I tached the car up to 2500 and the MAF read 14.5 g/s and a maximum of .45 volts on the O2 sensor. This car has been good to me but is killing me on gas. REALLY don't want to get rid of it but I dont know how to solve this problem on my own...

 

Also, the mechanic at the shop I work at said it is possible that the broken leads on the rear o2 sensor could've backfed voltage into the ECU and fried that part of the unit. I really hope that's not the case or that I have a ground issue between the MAF and the main relay. Is there any way to tell if the ECU is malfunctioning? I feel like if that were the case there would be more then just the two "Low voltage" codes for the O2 sensors. All the other threads I've read have included other codes with P0131 and P0137 that all affect each other. My car only has those two codes, Maybe something silly that got overlooked? Has anyone had this problem with just the O2 sensors blowing lean and nothing else?

 

**UPDATE**


Did some diagnostic work today. The test I did on the MAF with the ignition off was a typo in the paperwork concerning the diag on the sensor. The MAF sensor is grounded and working properly.


I have P0131 & P0137 with no other codes:


- MAF producing correct signal (Did full diagnostic today)
- No vacuum leaks (Smoke tested whole system)
- No exhaust leaks
- PCV system not clogged
- 2 new Bosch OEM Direct Fit O2 sensors within last 6 months (140k miles)
- Fresh OEM Volvo spark plugs
- OE or close to it Distributor Cap and Rotor/Wires
(getting to doing a tune up :O )
- Correct fuel pressure with new fuel filter less than 2 months ago
(I use 93 octane fuel)


The ASE mechanic I work with said either the front O2 sensor is bad or has a bad/weak ground back to the CU