Fuse keeps blowing, "wrench" light is illuminated, and poor start-up idle and acceleration

Question

A few weeks ago, the check engine light illuminated on my 2012 Ford E-350. My scan tool showed two codes: P2096 "Post Catalyst Fuel Trim System Too Lean Bank 1" and P2098 "Post Catalyst Fuel Trim System Too Lean Bank 2". (Note that according to the wiring diagram, this cutaway vehicle only has a single post-cat O2 sensor, so it makes sense that these come together; only the van/wagon has two post-cat sensors.) All other performance appeared to be normal, so I noted it down to take care of later when I had a chance. Using the scan tool, I determined that all other sensors looked like they were working properly with appropriate readings, other than high long-term fuel trims (bank 1 - 9%; bank 2 - 12%) which I expected to see based on having a "too lean" code.

A few days ago, I was accelerating from a traffic light and suddenly felt a short dropout in acceleration, accompanied by the "wrench light" next to the check engine light. Accompanying this, I had poor acceleration, and the vehicle almost stalls (then recovers) on initial start-up.

The scanner now gives me all of the following codes:

• P0102 - Mass Air Flow A Circuit Low
• P0104 - Mass Air Flow Circuit Intermittent
• P0135 - O2 Sensor Heater Circuit Bank 1 Sensor 1
• P0141 - O2 Sensor Heater Circuit Bank 1 Sensor 2
• P0155 - O2 Sensor Heater Circuit Bank 2 Sensor 1
• P0443 - Evaporative Emission System Purge Control Valve Circuit
• P0504 - Brake Switch A/B Correlation
• P0505 - Idle Air Control Circuit Fault
• P0573 - Brake Switch A Circuit High

Searching for combinations of these codes online, I found a YouTube video about multiple codes on a Chevy Malibu that suggested a wiring problem.

I got a friend to print me some wiring diagrams for my E-350 from ALLDATA, and I found that Fuse # 76 under the hood (what Ford calls the "Battery Junction Box") powers all of the following components:

• Brake Pedal Position Switch Circuit A (circuit B is Fuse 30)
• O2 Sensor Heater #12 (i.e. bank 1 sensor 2; on this vehicle the only post-cat sensor)
• Mass Air Flow Sensor
• O2 Sensor Heater #11 (i.e. bank 1 sensor 1)
• O2 Sensor Heater #21 (i.e. bank 2 sensor 1)
• Evap Canister Purge Valve

I checked the fuse, and it was blown. Just to test, I replaced the fuse and the new one blew immediately. The pre-cat O2 sensors on this vehicle are easily accessible from inside through the doghouse, so I tried unplugging them to eliminate them as the problem - and another fuse blew right away.

At this point, I know that I could just keep unplugging different components and trying again, but that's going to waste a lot of fuses. I suspect that my next step needs to be testing all the components with a multimeter for resistance and/or continuity to ground, but I'm not looking forward to doing that.

Before I start unplugging everything else or ripping wiring harnesses apart, is there anything specific I should look for? Perhaps any problem point with this model around this age?

I should also note that around a week before the wrench light came on, I had one of the rear brake lines blow out, just below the ABS pump, dripping down onto the steering gear. Could brake fluid spraying around in the engine compartment have precipitated this issue?

Also, this vehicle typically is used for 10-15 mile trips at speeds between 25-50 MPH, but two days before the light came on it had been used for an 80 mile trip at speeds of 60-70 MPH. Could the different/heavier load on the engine have caused this issue?

Answer 1

Following the suggestion by @Pᴀᴜʟsᴛᴇʀ2, I disconnected all of the components listed in the question. Using my multimeter, I confirmed that there was no longer a short on that circuit.

I then inspected each component individually, and here's what I found:

• The post-cat O2 sensor had insulation abraded from rubbing against a loose heat shield. I left this sensor unplugged until the replacement arrives, and I will resecure the heat shield after I replace the sensor.
• The brake pedal switch circuit A was stuck closed, and circuit B was stuck open. (Circuit B is the one that controls the brake lights, so it turns out I've been without brake lights for at least a few days.) A local parts store had this one in stock, so I was able to replace it right away.

I don't think the faulty brake pedal switch could have caused the fuse to blow (because it is just a switch), but the O2 sensor wires were almost certainly the cause of my issue.