P0411/P0153 1996 Mustang Cobra


New Member
Oct 25, 2011
I have a 1996 Mustang Cobra, stock, ~200K miles. Original exhaust system, motor, never raced or abused. The car runs well, but recently it has started throwing P0411 ((AIR) Incorrect Upstream Flow Detected, (AIR) Incorrect Flow Detected, Secondary Air Injection Incorrect - Upstream Flow Detected) and P0153 (O2 Circuit Slow Response (Bank 2 Sensor 1)) codes. In addition, at around the same time, the following anomaly: with the motor warm, after decelerating to a stop the idle will sit at ~1500 rpm, although it eventually settles down to normal idle if I wait long enough.

Most posts dealing with the P0411 code suggest a bad air (smog) pump (in my case, electric). With the pump installed on the car however, I'm unsure if it works properly: I never noticed fan noise with the motor running, but given its location maybe this isn't surprising. I removed the unit completely to test, and it spins up OK off the battery. The lead going to the unit tests 6.24V -- not sure if this should be closer to 12V (ignition off, engine not running). The fuse (30A, "thermactor") is intact and tests OK.

I've also checked the power lead to the air diverter valve (again ignition on, engine NR). This connection shows 0V, so that seems to indicate a problem. However, Haynes and Chilton manuals suggest only that "the circuit and PCM be checked at a dealer" -- not that helpful. Also according to these manuals, in addition to the pump itself and air diverter valve, the AIR system involves a check valve -- whose location isn't given, so to test I need to find where this is as well.

The P0153 code indicates O2 sensor problem. Replacement isn't that expensive, but from my reading they are a PITA to get off.

Perhaps some of these OBD-II codes are not really independent of one another: if the required secondary air flow isn't reaching the exhaust (for whatever reason), then this could trigger a fail in the O2 sensor (although I'm obviously guessing here). But before I go replacing anything, I thought I'd find out if anyone here has had similar problems, and could suggest what to do next. Thanks in advance for the help.