P251B PTO Enable Switch Circuit Low

With P251B the module reads the PTO enable switch input as stuck at a low level, below what either switch position should produce. A wire chafed through to chassis ground, water in a connector or a switch shorted internally are the usual explanations. No maker publishes P251B with thresholds; the closest documented case is P251A on GM medium-duty Silverado trucks, where bulletin PIP6108 traces PTO faults back to ground points, which shows how sensitive this switch circuit is to its reference. CARB lets OBD systems suspend selected monitors while a PTO unit is active, so a false PTO state also affects readiness. Depending on how the input is wired, a low reading may mean PTO is permanently refused or permanently requested. P251A is the open circuit, P251C the high side.

Likely causes

Symptoms

• PTO will not engage, or engages without being selected
• PTO indicator lamp behaves incorrectly
• Readiness monitors may stay incomplete

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, static.nhtsa.gov.