94 Mazda b4000 no power from ecm

[Ttownmedia]

Ok brain trust, I have a Mazda b4000. 4.0 V-6 automatic. Took it into the shop to have a valve job done. Afterwards the shop states it fired right up then quit. They started throwing parts at it.. new fuel pump, fuel filter, all new fuel injectors, m airflow sensor, crankshaft position sensor and a host of other sensors and all relays, then replace twice the ECM, no luck so they ordered a specially programmed ECM programmed to the serial number. After four months in the shop I had it towed home. When I put my code reader on it the reader gets no signal or power to come to life , the reader is getting no power. This explains the no spark issue. Fuses are good, relays are new, diode is good, I am not good and going nuts. Please throw some ideas my way. The no power to the code reader when everything is hot makes no sense.

 

[lil_Blue_Ford]

I’m gonna move this over to the 4.0 section…

 

[SenorNoob]

My OBD1 code reader isn't powered by the truck. It requires batteries to power on. It only connects to the signal wires to/from the PCM.

 

[Ttownmedia]

My OBD 1 reader works off the truck. Plug it it and when you turn the key on it powers up with the control module…normally

 

[SenorNoob]

So it draws power from the fuel pump circuit? That'd be best thing to check 1st then. That and the PCM relay. I think power for the fuel pump relay actually comes from the PCM relay.

 

[Ttownmedia]

Turns out that the oxygen pressure sensor on the exhaust manifold had a low power voltage. Should read 5 volts but only read 3 volts. I unplugged it and the engine fired right up. Replaced the sensor and it runs great. Go figure