DBR Scan Tool .....

and Powertrain Diagnostics Procedure manual. The FSM for my '96 XJ refers to these several times. I'm troubleshooting a check engine light with DTC 51. Symptoms: on a hot day w/low humidity, veh stumbles after shifting out of low gear (auto tranny). The more throttle I give it, the worse it gets. I can keep going by slowly giving it throttle. On a cooler day or when humidity is higher (raining out), I have no problem. This veh is RHD and used to deliver mail. I'm in process of checking TPS, MAP and manifold temp sensors with volt/ohmeter using FSM specs to check against. Upstream OS2 sensor replaced 6K miles ago. Check engine light came on and I spent $160 at a shop for the diag and OS2 sensor. Don't remember any impairments with operation then, just the MIL. What is DBR scan tool, and who has it and what is the Powerstrain Diag Procedure manual ?

Actron makes a OBDII scan tool, the CP9135, I think it was about $150 or so at AA. I bought one about 4 months ago and it has popped the correct codes 4 times, once on one of our TJ's and three times on neighbors... Comes with a CD for the error codes too...

Thanks for the reply. But, I have the code. DTC 51 aka P0171. That code is all I'm apparently going to get out of the veh. computer. I could use some suggestions about what might be causing it, though. FSM mentions a DBR scan tool and powertrain diag procedure manual. Is that something only the stealership would have ?

Thanks !

If you have the FSM, then you should have the list of codes -- try looking in the Fuel System section.

Fault code 51 - O2 signal stays lean.

It does not necessarily mean that the sensor is bad. Could be wiring....or fuel delivery system.


DRB - Diagnostic Readout Box, what Chrysler calls their OBD readers?

The scan tool refered to is the DRB III, the factory scan tool. The code 51 or p0171 is a fuel system lean code. It indicates that the PCM has enriched short and long term fuel trims as far as it can, and the O2 sensor is still indicating lean. The most common cause is a bad O2 sensor. On higher mileage vehicles, the sensors should be replaced as a pair. The downstream sensor, which monitors catalyst efficiency, can drift over time, giving a lean indication. The upstream sensor, which effects short term fuel control, is given a biased target voltage based on the downstream sensor. IE: if the downstream sensor starts to drift lean, the computer will artificially set the threshold voltage of the upstream sensor toward the rich end of the scale to try to return the downstream to center. This enrichens fuel mixture, but the downstream usually in this case doesn't respond and the process keeps gong until fuel trims are maxed out rich and the fuel system lean code is set.

Ok, the upstream sensor was replaced just a year ago. That was $71. The downstream sensor has never been changed out and the veh has 141K. It's $128. at the local parts store. I'm trying not to have to just go out and spend money but based on the info in the last post, which I really appreciate, it looks like that's the cause. I was going to remove the MAP and TPS sensors and see if the mechanical portions of them needed cleaning, but I can't get by the security torx bolts !! I don't have the special torx bit that will fit them. There's enough info in the FSM about what voltages to expect from all the sensors. I need to go out and by fittings that I can use to get into the wiring, though. I don't know whether to pursue that to find out for sure what's not working correctly, or to throw down the $128 for the downstream sensor. Thanks again for the input.

Bob L.

The MAP sensor has only a vacuum hose connected to it. Nothing to service there. The TPS simply engages the end of the throttle shaft. Also nothing to clean or otherwise there. The downstream O2 sensor, while expensive, is the only thing I have ever replaced for this problem, along w/ the upstream if its old. Your upstream should be fine. If there were wiring shorted or open, there are specific codes that relate to that. You may not be able to access them w/out a factory tool, but the fuel sytem lean would not appear due to either of those.

My own experience w/ this came when pieces of my deteriorating cat. pummeled the downstream sensor till it was no longer accurate. Same code you got. Engine was running fine down the highway, but when I slowed down, it was running very rich, black smoke out the tailpipe, and rough idle.

Good enuff, then. I found that AutoZone has the Bosch downstream sensor much cheaper than Advance Auto Parts at $74. There's a universal one for $59. but it doesn't plug right up. You would need to do a little rewiring. They can reset the code for me, too.

Bob L.

Well, picked up the Bosch 13272 from Autozone for $74.99. It didn't fit ! The sensor head was too large. Took it back and stopped by Advance Auto. They had the correct sensor to be Bosch 13138 for $128. But, they also had a universal O2 sensor for $93. I bought that instead. It doesn't have the Jeep connector - you use the crimps provided. No problem, the wire colors are the same. After dinner, I'll install it.

Since the fuel trim values are learned, you may want to do a battery disconnect for several minutes to clear the adaptive memory. This will reset all learned values to default. It will normally idle high for a short period of time after this, just let it run, and the idle will relearn.

Well, it's in and code's reset. Check engine light's off and it runs normally. I won't know if this O2 sensor replacement has done the job until I get out there delivering mail again. I'll post something in this thread when I know something definite.

Bob L.

Do a google on o2 sensors. You'll find direct fit ones for between $50 and $60. I got Denso's for my 96, they were about $60 each.

JoBo

Well, I've made 3 trips since installing the downstream O2 sensor. No check engine light ! Yesterday, the outside air temp was high enough that I should have noticed some problem if it were still present. Thanks for all input.

Bob L.