DOCUMENTED DIAGNOSTIC CONTEXT
Diagnostic code articles — Page 12.
Understand what the recorded fault means, which findings change the diagnosis, and what the cited documentation can confirm. Each article states the system or vehicle scope it covers.
Some sources leave details unresolved. Those articles include a visible source note; a code match alone does not establish applicability to your vehicle.
Generic OBD-II
P0638
bank-1 throttle duty-cycle demand exceeds its limit
PowertrainRead article
Generic OBD-II
P0639
bank-2 throttle effort and supply-voltage compensation
PowertrainRead article
Generic OBD-II
P063B
Generator voltage-sense fault reported through the PCM
PowertrainRead article
Generic OBD-II
P063C
Generator voltage-sense low input inside the ISG
PowertrainRead article
Generic OBD-II
P063D
ISG voltage measurements disagree with an internal high signal
PowertrainRead article
Generic OBD-II
P0640
Intake-heater control response while heating is commanded off
PowertrainRead article
Generic OBD-II
P0641
reference A can fail through instability as well as level
PowertrainRead article
Generic OBD-II
P0642
Reference A low is a qualified supply-rail measurement
PowertrainRead article
Generic OBD-II
P0643
Reference A high must be distinguished from high battery supply
PowertrainRead article
Generic OBD-II
P0645
A/C clutch-relay command disagrees with circuit feedback
PowertrainRead article
Generic OBD-II
P0646
A/C clutch relay control stays electrically low
PowertrainRead article
Generic OBD-II
P0647
A/C clutch relay control stays electrically high
PowertrainRead article
Generic OBD-II
P064A
Fuel-pump module diagnosis includes timing and command plausibility
PowertrainRead article
Generic OBD-II
P064C
Glow-plug controller performance can include supply problems
PowertrainRead article
Generic OBD-II
P064D
bank-one oxygen-sensor processing can fail internally
PowertrainRead article
Generic OBD-II
P064E
bank-2 oxygen-interface processor self-checks
PowertrainRead article
Generic OBD-II
P064F
invalid authentication table in ECU software
PowertrainRead article
Generic OBD-II
P0650
A failed warning-lamp circuit can store a code without illuminating the lamp
PowertrainRead article
Generic OBD-II
P0651
reference circuit 2 has a separate stability monitor
PowertrainRead article
Generic OBD-II
P0652
Reference B low identifies the second TCM reference rail
PowertrainRead article
Generic OBD-II
P0653
transmission sensor-reference supply 2 is high
PowertrainRead article
Generic OBD-II
P0657
an actuator-supply open-load result saved at shutdown
PowertrainRead article
Generic OBD-II
P0658
shared actuator supply is checked while switched on
PowertrainRead article
Generic OBD-II
P0659
An actuator supply high fault can be discovered during shutdown
PowertrainRead article
Generic OBD-II
P065A
Generator performance compares alternator and battery information
PowertrainRead article
Generic OBD-II
P065E
Intake-manifold feedback must agree with a stable command
PowertrainRead article
Generic OBD-II
P0660
An intake tuning valve output fault is an electrical check
PowertrainRead article
Generic OBD-II
P0661
The intake tuning valve reports an output-one ground-short status
PowertrainRead article
Generic OBD-II
P0662
Read the reported tuning-valve fault separately from its inconsistent heading
PowertrainRead article
Generic OBD-II
P0666
The transmission control module temperature-sensor circuit
PowertrainRead article
Generic OBD-II
P0667
TCM substrate temperature must agree with two other measurements
PowertrainRead article
Generic OBD-II
P0668
A very low TCM substrate reading is treated as a circuit fault
PowertrainRead article
Generic OBD-II
P0669
An implausibly high substrate reading requires a valid operating context
PowertrainRead article
Generic OBD-II
P066A
cylinder 1 glow circuit low and the inrush check
PowertrainRead article
Generic OBD-II
P066B
cylinder 1 glow output short toward power
PowertrainRead article
Generic OBD-II
P066C
cylinder 2 glow control ground-short monitoring
PowertrainRead article
Generic OBD-II
P066D
load-dependent high-voltage fault on glow circuit 2
PowertrainRead article
Generic OBD-II
P066E
cylinder 3 glow circuit low versus excessive initial current
PowertrainRead article
Generic OBD-II
P066F
cylinder 3 glow circuit high has two load-dependent cases
PowertrainRead article
Generic OBD-II
P0670
Ford 7.3L glow-plug control-line fault
PowertrainRead article
Generic OBD-II
P0671
cylinder 1 glow-plug open-load diagnosis
PowertrainRead article
Generic OBD-II
P0672
open circuit detected on cylinder 2 glow output
PowertrainRead article
Generic OBD-II
P0673
cylinder 3 glow circuit appears electrically unloaded
PowertrainRead article
Generic OBD-II
P0674
cylinder 4 glow output open-load monitoring conditions
PowertrainRead article
Generic OBD-II
P0675
cylinder 5 glow open-load monitor with a source-label conflict
PowertrainRead article
Generic OBD-II
P0676
cylinder 6 glow circuit open-load evidence
PowertrainRead article
Generic OBD-II
P0677
Cylinder7 glow-plug open monitoring requires an active heating command
PowertrainRead article
Generic OBD-II
P0678
Cylinder8 open-load evidence depends on glow heating mode
PowertrainRead article
Generic OBD-II
P0679
Touareg V10 cylinder 9 glow-plug circuit
PowertrainRead article
Generic OBD-II
P067A
cylinder 4 glow ground-short and inrush-current assessment
PowertrainRead article
Generic OBD-II
P067B
cylinder 4 glow-plug high-circuit detection
PowertrainRead article
Generic OBD-II
P067C
cylinder 5 glow-plug low circuit and inrush faults
PowertrainRead article
Generic OBD-II
P067D
understanding the cylinder 5 glow-plug high fault
PowertrainRead article
Generic OBD-II
P067E
cylinder 6 low-circuit detection has a startup branch
PowertrainRead article
Generic OBD-II
P067F
cylinder 6 glow-plug output high-circuit fault
PowertrainRead article
Generic OBD-II
P0683
Ford glow-plug module communication check
PowertrainRead article
Generic OBD-II
P0684
Volkswagen CJAA glow-controller communication fault
PowertrainRead article
Generic OBD-II
P0685
powertrain relay open-circuit detection uses the off state
PowertrainRead article
Generic OBD-II
P0686
low power-relay control voltage can overlap an open fault
PowertrainRead article
Generic OBD-II
P0687
power-relay control high with an unresolved state conflict
PowertrainRead article
Generic OBD-II
P0688
J271 load-circuit failure can explain fan or no-start complaints
PowertrainRead article
Generic OBD-II
P0689
relay feedback remains low when the relay is on
PowertrainRead article
Generic OBD-II
P068A
A relay that de-energizes too early is a timing classification
PowertrainRead article
Generic OBD-II
P068C
Cylinder7 glow-control low is a short-to-ground classification
PowertrainRead article
Generic OBD-II
P068D
Zero current alone cannot distinguish every glow-plug fault
PowertrainRead article
Generic OBD-II
P068E
Cylinder8 glow-control low uses a different category from open load
PowertrainRead article
Generic OBD-II
P068F
Cylinder8 high-circuit evidence needs the driver’s classification
PowertrainRead article
Generic OBD-II
P0690
relay feedback stays high after an off command
PowertrainRead article
Generic OBD-II
P0691
cooling-fan relay control is unexpectedly low while off
PowertrainRead article
Generic OBD-II
P0692
cooling-fan control stays high during an on command
PowertrainRead article
Generic OBD-II
P0693
Fan2 circuit low can include an open relay circuit in this Ford monitor
PowertrainRead article
Generic OBD-II
P0694
Fan2 high-control monitoring reports a short-to-power category
PowertrainRead article
Generic OBD-II
P0695
Fan relay 3 low-side diagnostic can also indicate an open circuit
PowertrainRead article
Generic OBD-II
P0696
Fan relay 3 driver reports a high-circuit condition
PowertrainRead article
Generic OBD-II
P0697
reference circuit 3 faults include rapid variation
PowertrainRead article
Generic OBD-II
P0698
Reference voltage C is low, but the code does not name the attached sensor
PowertrainRead article
Generic OBD-II
P0699
A high five-volt reference needs a supply-level diagnosis
PowertrainRead article
Generic OBD-II
P069E
The ECM is forwarding a fuel-controller fault request
PowertrainRead article
Generic OBD-II
P06A0
Ford variable-displacement A/C diagnosis before compressor replacement
PowertrainRead article
Generic OBD-II
P06A1
Variable compressor valve control circuit low
PowertrainRead article
Generic OBD-II
P06A2
Variable compressor valve control circuit high
PowertrainRead article
Generic OBD-II
P06A3
reference D means the fourth monitored ECM reference
PowertrainRead article
Generic OBD-II
P06A4
transmission speed-sensor supply 1 undervoltage
PowertrainRead article
Generic OBD-II
P06A5
Reference D high can be a 10.5-volt speed-sensor supply check
PowertrainRead article
Generic OBD-II
P06A6
Sensor reference A can invalidate several otherwise unrelated monitors
PowertrainRead article
Generic OBD-II
P06A7
Sensor reference B is a separate monitored supply group
PowertrainRead article
Generic OBD-II
P06A8
Sensor reference C needs its own circuit identification
PowertrainRead article
Generic OBD-II
P06AC
Powerup temperature plausibility depends on two independent differences
PowertrainRead article
Generic OBD-II
P06AD
A low powerup-temperature signal has load and speed prerequisites
PowertrainRead article
Generic OBD-II
P06AE
An extreme powerup-temperature reading is a circuit diagnosis
PowertrainRead article
Generic OBD-II
P06AF
engine shutdown code can reflect controller pattern monitoring
PowertrainRead article
Generic OBD-II
P06B1
speed-sensor supply switch current or temperature fault
PowertrainRead article
Generic OBD-II
P06B2
Motor A sensor power is a 15-volt rail in this implementation
PowertrainRead article
Generic OBD-II
P06B4
Sensor supply B low may originate from a protective switch report
PowertrainRead article
Generic OBD-II
P06B5
Motor B has a separate high sensor-supply diagnosis
PowertrainRead article
Generic OBD-II
P06B6
the knock-monitor test circuit has its own diagnostic
PowertrainRead article
Generic OBD-II
P06B7
The knock-processor test is not a measurement of engine knock
PowertrainRead article
Generic OBD-II
P06B8
NVRAM monitoring differs from processor resets and missing solenoid data
PowertrainRead article
Generic OBD-II
P06B9
Cylinder1 glow resistance is checked after basic electrical faults
PowertrainRead article
Generic OBD-II
P06BA
Cylinder2 performance can fail after an open-circuit check passes
PowertrainRead article
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.