P202C

Tank Heater Control Circuit High

P202C is a generic OBD-II powertrain diagnostic trouble code: Tank Heater Control Circuit High. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P202C
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
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What P202C means

P202C is a SAE generic powertrain diagnostic trouble code defined as Reductant Tank Heater Control Circuit High. It is set when the engine control module (ECM) detects a voltage level on the reductant (DEF/AdBlue) tank heater control circuit that is higher than the expected operating range. The reductant tank heater is a critical component of the Selective Catalytic Reduction (SCR) system, responsible for preventing the urea solution from freezing in cold ambient temperatures so that NOx aftertreatment remains functional.

A circuit-high fault typically points to a short to voltage somewhere in the heater control wiring, a failed open-circuit in the heater element causing the driver circuit to sense excessive back-voltage, a faulty heater relay supplying unregulated voltage, or an internal ECM driver fault. P202C is the high-side counterpart to P202B (Reductant Tank Heater Control Circuit Low); together they bracket the normal operating window. When the heater cannot be reliably controlled, the SCR system may be unable to maintain fluid temperature, triggering NOx-related derate strategies.

Because the SCR system is emissions-critical, most diesel vehicles with OBD-II compliance will illuminate the MIL immediately and may enter a torque-reduction or regeneration-inhibit mode after a defined number of failed drive cycles. The fault must be diagnosed and repaired promptly to restore full engine performance and avoid progressive SCR catalyst damage from unheated or incorrectly heated reductant fluid.

Common causes

Most-frequently reported root causes when P202C is logged.

  • 1
    Short to voltage (B+) in the reductant tank heater control wiring harness
  • 2
    Open circuit in the heater element causing abnormal back-voltage on the control line
  • 3
    Failed or shorted reductant tank heater relay supplying unswitched voltage
  • 4
    Corroded, damaged, or improperly seated connector at the reductant tank heater
  • 5
    Chafed wiring between the ECM and the heater assembly shorting to an adjacent power wire
  • 6
    Faulty reductant tank heater element with internal insulation breakdown
  • 7
    ECM/PCM internal driver fault for the heater control circuit

Symptoms drivers notice

MIL (check engine light) illuminated
SCR/AdBlue warning lamp or countdown message displayed on instrument cluster
Possible torque reduction or limp mode on affected diesel vehicles after repeated failed drive cycles
DEF fluid may freeze in cold conditions if heater is non-functional despite the circuit-high signal
Increased NOx emissions potentially triggering further SCR-related fault codes (e.g. P20EE, P207F)
No noticeable driveability symptom in warm climates where tank heating is not actively needed

How to diagnose P202C

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and confirm P202C is stored; note freeze-frame data (ambient temp, engine load, run time) and check for co-stored SCR codes such as P202B, P20EE, or P207F
  2. 2
    Perform a visual inspection of the reductant tank heater wiring harness and connector for chafing, corrosion, melted insulation, or loose pins; repair any obvious damage before electrical testing
  3. 3
    With the ignition ON and heater commanded OFF, measure voltage at the heater control circuit pin relative to chassis ground — a reading significantly above the ECM's specified command voltage confirms a short to voltage
  4. 4
    Check resistance of the heater element at the tank connector (typical values are vehicle-specific; an open or near-zero reading indicates element failure); compare against OEM specification
  5. 5
    Inspect and test the reductant heater relay: verify coil resistance, swap with an identical relay if available, and confirm the relay is not passing voltage when de-energised
  6. 6
    Clear the DTC and perform a monitored drive cycle covering cold-start conditions; if P202C returns with no wiring faults found, suspect an ECM driver circuit fault and consult OEM service data before replacing the module
  7. 7
    If all external wiring and components test within specification, perform an ECM pinout voltage/resistance check per OEM wiring diagram to rule out internal ECM fault

Vehicles where we've handled P202C

Platforms in our catalogue with confirmed P202C coverage.

AUDI A4 20D
VW TRANSPORTER
AUDI A6
2015
AUDI A6 30D
2015
AUDI A7
AUDI A7 30D

Related powertrain codes

Frequently asked questions

Can I drive with P202C stored?

Short distances in warm weather are generally possible, but the reductant heater may not function correctly. In cold climates the DEF fluid can freeze, disabling the SCR system and potentially triggering progressive torque-derate after several drive cycles. The code should be diagnosed promptly.

Is P202C the same as P202B?

No. P202B indicates a circuit-low (undervoltage) condition on the reductant tank heater control circuit, while P202C indicates a circuit-high (overvoltage) condition. Both affect the same heater circuit but point to opposite electrical faults — P202B typically suggests an open or short to ground, whereas P202C suggests a short to voltage or a failed-open element.

Will P202C cause an AdBlue warning or DEF countdown?

Yes. Because the heater is part of the SCR system, most diesel vehicles will display an AdBlue/DEF warning or a remaining-miles countdown alongside the MIL. If the underlying fault is not repaired, the vehicle may enter a speed-limited mode once the countdown expires, as required by emissions regulations.

What is the most common cause of P202C?

Based on the affected circuit, the most frequently encountered root cause is a wiring harness fault — specifically a short to voltage from chafed or damaged wiring near the reductant tank or its connector. Heater element failure (open circuit creating an abnormal voltage reflection) and faulty relays are the next most common causes, while ECM faults are relatively rare.

Disabling P202C in software

RaceTune can permanently disable P202C — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P202C disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C74 verified 2 software versions
  • Bosch EDC17CP44 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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