It all started when my 1999 Mustang Cobra kicked out the following Stored Codes
P0190 – Fuel Rail Pressure Sensor A Circuit
P1151 – Lack Of HO2S21 Switches – Sensor Indicates Lean (see P2197)
Well first of all, I had one of the best devices I have ever bought.
Xseries Auto™ WIFI Wireless OBD2 Auto Scanner Adapter Scan Tool to Check Engine Light & Diagnostics $16.49 on Amazon. Then all I had to do was buy the iPad app DashCommand for $10. I so not recommend trying to connect to your PC, I couldn’t get it to work. This device lets you check and clear codes and monitor and log a tone of stats from my PCM. For an additional $10 I was able to buy a special Ford code pack. So, for $36 bucks I can investigate more info on my Mustang than I know what to do with!
Back to the Saga … the P0190 code was easy to fix. I replaced my 14 year-old fuel pump with a BBK 300 LPH high performance direct replacement fuel pump, probably could have gone cheaper with a Walbro. However, I liked how the BBK said “direct Replacement”. And I also replaced the Fuel Filter.
The P1151 code is an enigma. The cause can be a laundry list of issues. Everything I read about P1151 said it is rarely the Oxygen Sensors, even though the messages seems to point to them. Here’s the info I got from
Symptoms You will likely not notice any drivability problems.
Causes A code P1151 may mean that one or more of the following has happened:
Electrical: Short to VPWR in the harness or HO2S Water in the harness connector Open/shorted HO2S circuit Corrosion or poor mating terminals and wiring Damaged HO2S Damaged PCM
Fuel System: Excessive fuel pressure Leaking/contaminated fuel injectors Leaking fuel pressure regulator Low fuel pressure or running out of fuel Vapor recovery system
Induction System: Air leaks after the MAF Vacuum Leaks
PCV system: Improperly seated engine oil dipstick
EGR System: Leaking gasket Stuck EGR valve Leaking diaphragm or EVR
Base Engine: Oil overfill Cam Timing Cylinder compression Exhaust leaks before or near the HO2S(s)
Read more at: http://www.obd-codes.com/p1151-ford
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So I worked my way through the list above. Looked for any bad or wet electrical connections including all 4 O2 Sensors. Everything looked fine.
I have a Vacuum Pressure gauge and checked for leaks with it. Vacuum was a nice steady -11.4 psi at 2000 RPM. Checked all the vacuum hoses I could fined for leaks. Found one connection at the back of the intake that was hanging half off. Thought maybe that was it, but no dice.
Checked my PCV system and replaced the PCV valve. Dipstick looked like it was seating well and no leak there.
Through all this the code was tripping everyday and then I would reset it. Seemed to happen at low RPM cruising speeds, so I thought maybe it is my Battery or Alternator. I check the output of my Alternator and it was fine at about 13V at idle. I cleaned my battery connections and the P1151 code did not trip for 3 days. Then it tripped. My battery was about 4 years old, so I figured maybe it needs to be replaced so I did. It went a few days before tripping again and then it went back to tripping multiple times a day.
Through all this I was monitoring different stats from my PCM with my OBD2 Scanner and the DashCommand iPad App. I noticed that I was only getting voltage from one Upstream O2 Sensor. No voltage from the other Upstream or the 2 Downstream Sensors. I monitored the voltage at start-up and when the car was all warmed up. I have never monitored these voltages before and I figured it could be a problem with my scanner or something. I learned that the most important are the 2 Upstream sensors, because they are used by the PCM to control Fuel Trims and the 2 Downstream sensors only monitor how well your Cats are working. I also learned that the Upstream O2 sensors will bounce from Lean to Rich and back again multiple times a second. This is how the system works.
As I said before, throughout this whole saga I did read on many forums that a P1151 Code is rarely an O2 Sensor. However, this was about the last thing to try. So I found all 4 NTK Oxygen Sensors on RockAuto.com for about $27 a piece. Great deal on OEM Ford sensors (I verified this when I pulled the Ford Sensors. They were stamped Ford and NTK).
SUCCESS! The code has not tripped for a month and my Cobra is running great. I’m still not getting voltage from one of the Downstream O2 Sensors, but no codes have tripped.
If you have any questions or comments Let Me Know!
Joe