Hello folks, My first post. I have a 2000 C3500 fleet truck 5.7L. I deleted the EVO solenoid when the PS pump failed several months ago. DTC C0503 is present on a scan with NO CEL and will not clear. I tied the solenoid connector back to the harness when I deleted it. Didn't read that a jumper was needed to prevent a code but maybe I missed it. All works good and with no CEL the C0503 being there is a nuisance but I would like to understand. Maybe the pump failure caused the code but I am beyond limits of my experience and curious. Any suggestions?
C0503 is for ABS, specifically the front left wheel speed sensor. Has NOTHING to do with power steering/EVO/PS pump.
You probably need a new wheel speed sensor, but the problem could be in the wire harness closer to the ABS unit, or potentially a fault in the ABS unit itself. Initial diagnosis would be to connect a multimeter, set to "low" AC volts, (my meter's low range is 0--40 volts, other meters may be different. 0--12 volts would be fine.) direct to the wheel speed sensor plug where it would connect to the vehicle harness, spin the wheel, and see if you get a volt or five (GM specs are in the service manual) of AC voltage. The faster you spin the wheel, the higher the voltage and the faster the frequency but most AC voltmeters don't measure frequency/Hz. No voltage? Replace the sensor. Got voltage? Inspect the wire harness farther "up" and closer to the ABS unit. Weak voltage? MAYBE clean the rust off the mounting surface of the hub, so that the sensor gets a little closer to the tone wheel. But often, it's a matter of replacing the sensor.
The usual problem is that the wire lead from the sensor gets torn, abraded, or otherwise defective. USUALLY the vehicle harness is OK, as is the ABS unit.
Photo of LF wheel speed sensor and it's wire harness I've replaced on my '97 K2500. This truck is an 8-lug, so the hub has to come off to access the sensor. A 5- or 6-lug can have the rotor pulled off for access, I think.
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Harness abraded by rubbing the inside of the brake rotor.
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I don't know how the 2WD sensors are serviced, but it'll be in the service manual. 4WD 8-lug are a real PITA. By the time I got the hub off, I figured I never wanted to have that side apart again...so I also replaced the lower ball joint, and maybe should have replaced the hub/bearing assembly as well.
