OBD-II trouble code
P0813: Reverse Output Circuit
Here the module is not listening, it is switching. P0813 means a control module drove its reverse output and did not like what came back — which is why a burnt reverse bulb, or an LED replacement one, can set a powertrain trouble code.
Quick facts
- System
- Powertrain
- Category
- Transmission / Shift Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $20 – $450
- DIY difficulty
- Intermediate DIY
What does P0813 mean?
The word that separates this code from the one numbered immediately before it is output. P0812 concerns a signal arriving at the module. P0813 concerns a signal leaving it. That single difference in direction changes where the fault lives, how it is found, and what it usually turns out to be.
On vehicles that use this strategy, the module does not simply pass the driver's reverse selection along a wire to a lamp. It takes responsibility for the circuit: when reverse is selected, the module energises a driver transistor that supplies or grounds the reverse output, and that output may feed the reversing lamps, a relay, a reverse-inhibit or lockout device, an alarm on commercial vehicles, or another controller entirely. Because the module owns the circuit, it can also measure it. Modern output drivers carry a sense function that reports back roughly how much current is flowing, and the module compares that against what it expects a healthy load to draw.
That feedback is the whole story of P0813. Too much current means the circuit is shorted to ground or the load has failed short. No current at all means the circuit is open — a broken wire, a corroded socket, a blown fuse, a burnt-out bulb. Some strategies also test the output while it is switched off, looking for voltage that should not be there, which catches a short to power or a driver that has failed on. In other words, a module can detect a dead reversing lamp the same way it detects a broken wire, because from where it sits they are electrically identical.
This is also the code where a well-intentioned upgrade causes the problem. LED reversing bulbs draw a small fraction of the current of the filament bulbs they replace, and on a circuit with current-based load detection the module reads that reduced draw as an open circuit and sets the code. The lamps work perfectly, the owner is certain nothing is wrong, and the module is not wrong either — it is measuring exactly what it was designed to measure. Where the module is doing genuine work, such as driving a reverse lockout on a manual gearbox or a reverse-inhibit valve, the consequences are more than cosmetic, because the output failing to switch can mean reverse simply refuses to engage.
Common causes
- Burnt-out reversing lamp filament, which many modules read as an open output circuit
- LED or other low-current replacement bulbs drawing too little current for the module's load detection
- Corroded or water-damaged reverse lamp socket, common on tailgate and bumper-mounted lamps
- Open or chafed wire between the module and the load, often where the harness crosses to the tailgate
- Short to ground in the output wiring, which the module detects as excessive current
- Failed reverse relay, or a relay socket with a spread or burnt terminal
- Reverse lockout or inhibit solenoid failed open or short
- Blown fuse in the reverse output feed
- Trailer socket wiring faulty or a trailer plugged in with a shorted reverse circuit
- Failed output driver inside the module, which is uncommon and normally follows a short that was never repaired
Symptoms
- Reversing lamps not illuminating, illuminating dimly, or one side out
- Check engine light stored alongside an obvious lighting fault
- Code appearing shortly after bulbs were changed, particularly to LEDs
- Reverse refusing to engage on vehicles where the module drives a reverse inhibit or lockout
- Repeated blown fuse on the reverse circuit
- Reverse warning alarm inoperative on commercial and fleet vehicles
- Trailer reverse lamps not working while the vehicle's own lamps do
- Code setting only when a trailer is connected
Diagnostic steps
- 1.Start at the load, not the module. Select reverse with the vehicle safely immobilised and look at the reversing lamps. On this code the answer is visible from outside the car more often than on any other transmission code.
- 2.Replace any failed bulb and clear the code before going further. If the code stays away, the diagnosis is finished and it cost the price of a bulb.
- 3.Ask whether the bulbs have been changed recently and what they were changed to. If LEDs have been fitted, refit conventional bulbs temporarily and see whether the code clears — that test is definitive and takes minutes.
- 4.Check the fuse feeding the reverse output, and if it has blown, do not simply replace it. A blown fuse on this circuit means a short exists and the module has already told you so.
- 5.Measure voltage at the load with reverse selected. Battery voltage present at a lamp that does not light points at the socket or the ground; no voltage points upstream at the wiring, relay or driver.
- 6.Test the ground side as deliberately as the supply side. Rear lamp grounds are mounted in the most corrosion-prone part of the vehicle and are a frequent cause of a circuit the module reads as open.
- 7.Inspect the harness where it passes through the tailgate or hatch hinge if the lamps are mounted there. Repeated flexing breaks conductors inside intact-looking insulation.
- 8.Disconnect any trailer and clear the code. A shorted trailer reverse circuit will set this code on the towing vehicle and is invisible until the trailer is removed from the equation.
- 9.Only after the wiring, load, ground and fuse are proven should the module output be suspected. Command the output on with a bidirectional scan tool if the vehicle supports it and measure at the module connector.
Repair cost
$20 – $450
This is one of the few powertrain codes with a genuine do-it-yourself outcome at the bottom of the range: a reversing bulb is $5 to $20 and takes minutes on most vehicles. Reverting from LED bulbs to conventional ones costs about the same. A corroded lamp socket or pigtail is $25 to $80 in parts plus 0.3 to 0.8 hours. Wiring repair where the harness crosses the tailgate runs $120 to $350 depending on how much trim has to come out. A reverse relay is $15 to $60 fitted in under half an hour. The expensive end is a failed reverse inhibit or lockout solenoid on the gearbox, or in rare cases a module output driver, which pushes the job toward and past $450. Diagnosis is $90 to $150, though a burnt bulb may not need any.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with exterior lighting repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.