P CODE FAMILY

Powertrain code references — Page 17.

Engine, transmission and powertrain diagnostic references. Exact vehicle and module context still matters.

Generic OBD-II

P0644

These are code references, not a list of confirmed faults on every vehicle. Read the source and applicability notes, then match the exact vehicle, year, engine and scan report.

Driver Display Serial Communication Circuit

A/C clutch-relay command disagrees with circuit feedback

A/C clutch relay control stays electrically low

A/C clutch relay control stays electrically high

Immobilizer Lamp Control Circuit

Speed Control Lamp Control Circuit

Fuel-pump module diagnosis includes timing and command plausibility

PTO Control Module

Glow-plug controller performance can include supply problems

bank-one oxygen-sensor processing can fail internally

bank-2 oxygen-interface processor self-checks

invalid authentication table in ECU software

A failed warning-lamp circuit can store a code without illuminating the lamp

reference circuit 2 has a separate stability monitor

Reference B low identifies the second TCM reference rail

transmission sensor-reference supply 2 is high

Engine RPM Output Circuit/Open

Engine Hot Lamp Output Control Circuit

Fuel Level Output Circuit

an actuator-supply open-load result saved at shutdown

shared actuator supply is checked while switched on

An actuator supply high fault can be discovered during shutdown

Generator performance compares alternator and battery information

Generator A Control Circuit Range/Performance

Generator A Mechanical Performance

Reductant System Malfunction Lamp Control Circuit

Intake-manifold feedback must agree with a stable command

Intake Manifold Tuning Valve Performance Bank 2

An intake tuning valve output fault is an electrical check

The intake tuning valve reports an output-one ground-short status

Read the reported tuning-valve fault separately from its inconsistent heading

Intake Manifold Tuning Valve Control Circuit/Open Bank 2

Intake Manifold Tuning Valve Control Circuit Low Bank 2

Intake Manifold Tuning Valve Control Circuit High Bank 2

The transmission control module temperature-sensor circuit

TCM substrate temperature must agree with two other measurements

A very low TCM substrate reading is treated as a circuit fault

An implausibly high substrate reading requires a valid operating context

cylinder 1 glow circuit low and the inrush check

cylinder 1 glow output short toward power

cylinder 2 glow control ground-short monitoring

load-dependent high-voltage fault on glow circuit 2

cylinder 3 glow circuit low versus excessive initial current

cylinder 3 glow circuit high has two load-dependent cases

Ford 7.3L glow-plug control-line fault

cylinder 1 glow-plug open-load diagnosis

open circuit detected on cylinder 2 glow output

cylinder 3 glow circuit appears electrically unloaded

cylinder 4 glow output open-load monitoring conditions

cylinder 5 glow open-load monitor with a source-label conflict

cylinder 6 glow circuit open-load evidence

Cylinder7 glow-plug open monitoring requires an active heating command

Cylinder8 open-load evidence depends on glow heating mode

Touareg V10 cylinder 9 glow-plug circuit

cylinder 4 glow ground-short and inrush-current assessment

cylinder 4 glow-plug high-circuit detection

cylinder 5 glow-plug low circuit and inrush faults

understanding the cylinder 5 glow-plug high fault

cylinder 6 low-circuit detection has a startup branch

cylinder 6 glow-plug output high-circuit fault

Cylinder 10 Glow Plug Circuit/Open

Cylinder 11 Glow Plug Circuit/Open

Cylinder 12 Glow Plug Circuit/Open

Ford glow-plug module communication check

Volkswagen CJAA glow-controller communication fault

powertrain relay open-circuit detection uses the off state

low power-relay control voltage can overlap an open fault

power-relay control high with an unresolved state conflict

J271 load-circuit failure can explain fan or no-start complaints

relay feedback remains low when the relay is on

A relay that de-energizes too early is a timing classification

ECM/PCM Power Relay De-Energized Performance - Too Late

Cylinder7 glow-control low is a short-to-ground classification

Zero current alone cannot distinguish every glow-plug fault

Cylinder8 glow-control low uses a different category from open load

Cylinder8 high-circuit evidence needs the driver’s classification

relay feedback stays high after an off command

cooling-fan relay control is unexpectedly low while off

cooling-fan control stays high during an on command

Fan2 circuit low can include an open relay circuit in this Ford monitor