Ford P20BA Heater Fault: Winter NOx Dosing Test & Repair

P20BA on a Ford Transit or Transit Custom points to the AdBlue (DEF) heater circuit “A”. In cold weather the SCR system won’t dose until the fluid is warm enough, so a dead heater, broken wiring, or a lazy relay can block dosing and set P20BA. Leave it and you’ll often see efficiency codes like P204F or P20EE and, in the worst case, a no-start countdown. This guide gives you fast driveway checks, a winter dosing test, and the fixes that actually work on these vans.

If a countdown has started, see the step-by-step no-start countdown bypass. For a full on-site diagnostic list, head to AdBlue repair & diagnostics.

What P20BA means on a Transit/Custom

P20BA flags an electrical fault on the AdBlue heater circuit. On these vans, heaters live in the tank module and the delivery line. The PCM watches relay command, current draw, and temperature rise. If current stays at zero or temperature fails to lift, it stores P20BA.

• Symptoms: cold-start AdBlue warnings, “Service AdBlue”, poor SCR performance below 5 °C, rising AdBlue use, sometimes P204F alongside.
• Usual causes: open circuit in the tank heater, cooked connector at the module, blown fuse or weak relay, corroded earth, damaged line heater, or a harness rubbed through at the subframe.

Need a quick outcome today? Book a mobile visit. Most heater faults are diagnosed and sorted kerb-side.

Fast checks before you buy parts

These steps are safe and quick. They also give us a head start if you book a visit.

1) Confirm codes and freeze-frame

Scan the SCR/PCM and save freeze-frame. Note coolant and ambient temperatures when P20BA set. If it only triggers in cold weather then clears in mild temps, the heater circuit is the prime suspect.

2) Visual: plugs, earths, and loom rubs

Drop the tank shield and inspect the heater harness. Look for green crust, loose pins, and rub marks where the loom crosses a chassis rib. Confirm the earth point is tight and clean. Browned plastic around the connector = heat from resistance — repair it.

3) Power supply basics

Check the AdBlue heater fuse and relay. Don’t just eyeball; load test. If the relay matches another in the box, swap to prove. With a meter, confirm battery voltage at the heater feed when the PCM commands heat (cold engine, key on). Low voltage points to relay or wiring loss. See our full electrical checks on SCR system repair.

Winter NOx dosing test (15–25 minutes)

This routine proves whether the heater works and whether the system will actually dose in the cold.

1. Cold soak: Leave the van outside for 2–3 hours, ideally below 5 °C.
2. Command the heater: With a capable scanner, run the “DEF heater activation” test or watch heater status at key-on. You should see command = ON and current > 0 A. No current? Suspect fuse/relay/wiring or an open heater.
3. Watch temperature rise: Monitor “DEF tank temperature” and “line temperature”. You want a steady climb over 5–10 minutes. Flat line = dead heater.
4. Dose readiness: When tank temperature rises, the SCR will allow dosing. At 1,800–2,200 rpm on a short drive, watch “reductant injector duty” and downstream NOx ppm. If dosing starts and NOx falls, the heater fix restored function.

If the heater behaves but P20BA still sets, the PCM may be seeing current that’s too low or too high. A chafed harness can short to ground and blow the fuse only under load. Find that before condemning the tank module. If you’re stuck, our AdBlue repair & diagnostics covers on-site current and temp rise testing.

Common Transit/Custom faults that trigger P20BA

• Open tank heater element: Infinite resistance at the heater pins; zero current when commanded. Replace the tank module or the heater if it’s serviceable.
• Line heater split or short: Damage near clips or tight bends. Resistance way below spec or a fuse that blows on command.
• Burnt connector / loose pin: High resistance and intermittent current; plastic browning. Re-pin and clean, then load test.
• Relay drop-out: Relay clicks but voltage sags under load. Swap and retest with the heater active.
• Bad earth path: Heater current hunts or drops. Clean the earth and remove paint or rust under the lug.
• Low battery / voltage dips: Cold cranking drags voltage down, the PCM aborts heat, and checks fail. Fix the battery/charging first so the heater can do its job.

Pin-down tests (multimeter only)

Unplug the heater at the tank module. With key off:

• Resistance: Measure across the heater pins. Open circuit = failed element. Near-zero = shorted heater.
• Feed voltage: Back-probe the feed with key on and heater commanded. You should see near-battery volts. Under ~11 V at command points to relay, fuse, or wiring loss.
• Current check (clamp): A DC clamp shows live current when commanded. Zero = open circuit; wildly high = short or wrong element.

No clamp? A 21 W bulb on a fused lead is a simple load tester to prove the feed can hold current without collapsing.

Repairs that actually solve P20BA

• Heater module (tank or line) replacement: When resistance is open/short or temperature never rises. Refill with sealed ISO-22241 AdBlue only. If fluid has been contaminated, follow the drain/refill steps in our AdBlue top-up guide.
• Relay and fuse refresh: If either tests weak or shows heat marks. Cheap insurance on high-mileage vans.
• Connector and loom repair: Re-pin burnt sockets, solder/heat-shrink rubbed sections, and re-route away from sharp edges. Add split conduit for protection.
• Battery/charging fix: Replace a lazy battery and confirm 14+ V charging on cold start so heaters get the juice they need.
• Software reset/learn: After hardware repair, clear codes, run the SCR heater tests, and take a short drive so the system confirms dose readiness.

Once the heater works, lingering efficiency codes (P204F/P20EE) should fade as dosing resumes. If not, go deeper with SCR system repair to confirm injector flow, pump pressure, and leaks before the catalyst. If the van is older and a tank module isn’t worth the spend, read the software-only route on AdBlue removal Stoke on Trent and our NOx sensor solutions page for options.

When to call for a mobile fix

Book help if P20BA returns after basic checks, the feed shows battery voltage but current stays at zero, or the countdown has started. We’ll run cold-soak tests, command the heater, check current draw and temperature rise, and repair or replace the failed part at your yard. Most jobs wrap up kerb-side in under an hour — no tow truck. See what’s included on AdBlue repair & diagnostics or schedule a slot via mobile service.

Preventing winter come-backs

• Keep the battery healthy. Cold cranking pulls voltage down; heaters suffer first.
• Use sealed, fresh AdBlue. Watered-down fluid freezes sooner and slows warm-up.
• Inspect the loom at each service — especially where it crosses brackets and the subframe.
• Rinse road salt from the tank and lines to protect connectors.

Ready to get it sorted? Call 07503 134 362 or email info@adbluespecialist.co.uk. We cover Stoke-on-Trent, Newcastle-under-Lyme, Stafford, Crewe and nearby. If you’re fighting a live countdown, jump to no-start countdown bypass for quick steps while you wait.