Faulty 03 PCM?

[520Terminator]

I have heard and read about the faulty PCMs in the 03 Cobras, and mine does stall randomly every once in a while, and today started running like crap. Pulled the codes and it has 8.

P0443 Evap Purge Control Circuit
P0446 Evap Vent Control Circuit
P0135 o2 heater B1S1
P0141 o2 heater b1s2
P0155 02 heater b2s1
P0161 02 heater b2s2
P0403 EGR Circuit malfunction
P0133 o2 slow response b1s1

O2 sensors are relatively new, and all these codes popped up recently. If you clear them, they come back almost immediately, and sometimes if you hook up the scanner and read the codes, then simply cycle the key and hook up the scanner again, some of the codes aren't there anymore. WTF? Do the faulty PCMs cause these symptoms as well?

 

[03SonicVert]

I would print out or look through the EVTM and just make sure there are no common connectors for these component, other than PCM but absolutely check the fuses for the o2s, CVV, CVP and EGRVR. Did this appear out of the blue or any recent mods/work?

All of the codes could be caused by a simple power/fuse concern too. That is why I say check for power at the canister purge, canister vent, o2s heater circuits and the EGR vacuum regulator. You could have a burnt wire on the exhaust which would take out the power to these components.

If that all checks good....
The 4 heater circuit codes are pretty common for a damaged PCM but I would still do all the diag first. The 4 heater circuits are supplied 12v so make sure that fuse is good or voltage drop the power to each HEGO/O2S. The PCM grounds the heather circuits on first start up to bring them up to operating temp quicker resulting in faster closed loop and more accurate fuel control. When the PCM does ground each of the 4 drivers it measures the amp draw to verify the circuits and sensors are functioning. It is very unlikely you have 4 bad O2S nonetheless all four with heater circuit portion of the sensor damaged. That is why I say all 4 heater circuits with hard faults typically point to a PCM issue.

Granted I would still load test and voltage drop the PCM powers and grounds since a faulty ground could cause this too. There is no way to tell which of the PCM grounds is used internally for the heater circuits so always test powers and grounds to the PCM as outlined above. Don't fool yourself with LED test lights that you could light up by rubbing your feet on a carpet. Use an old headlight to load the circuit then a DVOM to go between the power and ground and then directly to the battery power or ground. In a perfect world you would get zero loss or "voltage drop".
If the HEGOs were not heating up then they would be slow/lazy resulting in the P0133.

 

[520Terminator]

It was a fuse :bash:

 

[520Terminator]

Well, it blows the new fuse as soon as I turn the key on, so I guess I get to go digging for wiring faults or a faulty 02/EGR/Evap.

 

[03SonicVert]

Right on!
You most likely have a short to ground on that fused power circuit.
Get under the car and look at all 4 o2s and the wiring. Most common is wires touching the exhaust and burning up. If you had the trans out recently then there could be misrouted wires touching too.

Just hook up a DVOM and check for continuity to ground while wiggle testing if you don't see anything on the visual. It could be a shorted sensor on the circuit but not too common. Any stereo work lately or modifications in the trunk. The CVV circuit runs in the back so any work underneath, in the trunk, KB BAP, etc where you rerouted wires or natal led any screws could pierce a wire.

Fuses don't open for no reason so the fact there is a hard short is good for you and much easier to diag vs waiting anxiously for it to occur again.

 

[520Terminator]

One of the downstream O2s was shorted to ground internally. Easy fix.

 

[03SonicVert]

I love happy endings..... In stories!

 

[testorossa1989]

I love happy endings..... In stories!

^ Nice save lol


Yeah generally speaking PCMs really dont go bad per say. Commonly they can be updated/reflashed/ ect